Modeling ruminal digestibility of carbohydrates and microbial protein flow to the duodenum.

Carbohydrates are the major source of energy for dairy cows and for microbial protein synthesis in the rumen. The prediction of ruminal carbohydrate digestibility and of the flow of microbial protein to the small intestine is difficult because of the variability among various feeds in the kinetics of digestion and passage of neutral detergent fiber and starch. Disappearance of fiber and starch in vitro or in situ and gas production in vitro have been extensively evaluated, improved, and reviewed. Similarly, markers and models to measure ruminal passage rate have been extensively researched and improved. Sources of variation and decreased accuracy for these techniques are discussed. Variation and potential errors also remain for the prediction of microbial protein flow to the duodenum using in vivo procedures. However, when in vivo results were accumulated into a database, microbial N flow to the duodenum over a wide range of conditions could be predicted accurately by intake of net energy for lactation or by dry matter intake and percentage of neutral detergent fiber in the diet. Although evaluation of feeding interactions and specific dietary limitations for microbial protein production in the rumen are possible with some models but not with this regression approach, mechanistic models need further validation and more accurate rate constants for improved accuracy over a wide range of conditions.

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title: Evaluation of chemical and physical properties of feeds that affect protein metabolism in the rumen.
abstract: The goal of the NC-185 Cooperative Regional Research Project is to provide the information needed to improve the nutrition and feeding of dairy cattle, a major factor determining composition of milk and cost of milk yield. Emphasis is placed on understanding how energy and protein nutrition of lactating cows can be manipulated to increase the quantity and improve the profile of AA passing to the small intestine and to improve yield of milk and milk protein. To achieve this goal, one of the major objectives of this project has been to evaluate quantitatively the chemical and physical properties of protein and energy sources that determine AA availability to lactating cows. Reliable measurements of microbial protein synthesis and protein degradation in the rumen are critical in the evaluation process. Therefore, one of the ongoing areas of investigation of this research project has been to determine the most appropriate methods for estimating microbial protein synthesis and dietary protein degradation in the rumen. Other areas have been investigated, using continuous culture fermenters and ruminally and duodenally cannulated cows, including factors that alter microbial metabolism of N in the rumen and subsequently protein supply to the small intestine, such as sources of carbohydrate, protein, and fat and interrelationships of protein and carbohydrate. Findings of the NC-185 Cooperative Regional Research Project Committee and other investigators are summarized in this review.

title: Design and interpretation of nutrient digestion studies.
abstract: A variety of factors influence the results of digestion studies designed to measure ruminal disappearance of nutrients and flow of nutrients to the intestine. Feed intake, marker usage, sample collection, statistical design, and computation methods are among the factors discussed in this review. Guidelines are provided for presentation and interpretation of data from digesta flow studies. The scientific value of such studies often is limited because 1) replication, relative to expected variation, is inadequate and 2) experimental animals are not maintained under conditions similar to those used in normal production. Because studies conducted with inappropriate methods or inadequate replication are of little value and waste resources, validation of techniques is critical, and individual researchers should strive to verify their techniques.

title: Considerations for gastrointestinal cannulations in ruminants.
abstract: The complexity of ruminant digestion necessitates a greater variety and complexity of experimental methods than with any other species. The fact that dietary ingredients are first subjected to microbial fermentation requires elaborate measures to ascertain nutrients presented for absorption. Numerous approaches have been attempted to obtain representative samples of digesta at sites throughout the gastrointestinal tract. The choices of a researcher before an experiment include animal(s), site(s) for cannula placement, style of cannula, cannula material, and numerous other more subtle factors that may contribute to the success of an experiment. This review compares the advantages and disadvantages of various approaches, cannula types, and cannula materials that should be considered before experiments are conducted.

title: Interlaboratory variation in a diaminopimelic acid assay: influence on estimated duodenal bacterial nitrogen flow.
abstract: Samples of ruminal bacteria and duodenal digesta were collected from two dairy cows fed a 65% forage diet. Samples were sent blind to four laboratories for diaminopimelic acid analysis. Analyzed values differed among laboratories within sample type, and concentrations ranked as follows: laboratory D greater than laboratory A greater than laboratory B greater than laboratory C. Consideration of differences in actual procedures used among laboratories resulted in several hypotheses to explain some of the interlaboratory variation. Using diaminopimelic acid values from each laboratory to estimate duodenal bacterial nitrogen flow showed that laboratory D estimated a 17% higher flow than the average for laboratories A, B, and C, which were similar.

title: A net carbohydrate and protein system for evaluating cattle diets: I. Ruminal fermentation.
abstract: The Cornell Net Carbohydrate and Protein System (CNCPS) has a kinetic submodel that predicts ruminal fermentation. The ruminal microbial population is divided into bacteria that ferment structural carbohydrate (SC) and those that ferment nonstructural carbohydrate (NSC). Protozoa are accommodated by a decrease in the theoretical maximum growth yield (.50 vs .40 g of cells per gram of carbohydrate fermented), and the yields are adjusted for maintenance requirements (.05 vs .150 g of cell dry weight per gram of carbohydrate fermented per hour for SC and NSC bacteria, respectively). Bacterial yield is decreased when forage NDF is < 20% (2.5% for every 1% decrease in NDF). The SC bacteria utilize only ammonia as a N source, but the NSC bacteria can utilize either ammonia or peptides. The yield of NSC bacteria is enhanced by as much as 18.7% when proteins or peptides are available. The NSC bacteria produce less ammonia when the carbohydrate fermentation (growth) rate is rapid, but 34% of the ammonia production is insensitive to the rate of carbohydrate fermentation. Ammonia production rates are moderated by the rate of peptide and amino acid uptake (.07 g of peptide per gram of cells per hour), and peptides and amino acids can pass out of the rumen if the rate of proteolysis is faster than the rate of peptide utilization. The protein-sparing effect of ionophores is accommodated by decreasing the rate of peptide uptake by 34%. Validation with published data of microbial flow from the rumen gave a regression with a slope of .94 and an r2 of .88.

title: A net carbohydrate and protein system for evaluating cattle diets: II. Carbohydrate and protein availability.
abstract: The Cornell Net Carbohydrate and Protein System (CNCPS) has a submodel that predicts rates of feedstuff degradation in the rumen, the passage of undegraded feed to the lower gut, and the amount of ME and protein that is available to the animal. In the CNCPS, structural carbohydrate (SC) and nonstructural carbohydrate (NSC) are estimated from sequential NDF analyses of the feed. Data from the literature are used to predict fractional rates of SC and NSC degradation. Crude protein is partitioned into five fractions.  A is NPN, which is trichloroacetic (TCA) acid-soluble N. Unavailable or protein bound to cell wall (Fraction C) is derived from acid detergent insoluble nitrogen (ADIP), and slowly degraded true protein (Fraction B3) is neutral detergent insoluble nitrogen (NDIP) minus Fraction C. Rapidly degraded true protein (Fraction B1) is TCA-precipitable protein from the buffer-soluble protein minus NPN. True protein with an intermediate degradation rate (Fraction B2) is the remaining N. Protein degradation rates are estimated by an in vitro procedure that uses Streptomyces griseus protease, and a curve-peeling technique is used to identify rates for each fraction. The amount of carbohydrate or N that is digested in the rumen is determined by the relative rates of degradation and passage. Ruminal passage rates are a function of DMI, particle size, bulk density, and the type of feed that is consumed (e.g., forage vs cereal grain).

title: A non-linear compartmental model to describe forage degradation kinetics during incubation in polyester bags in the rumen.
abstract: A new model for describing forage degradation kinetics during incubation in the rumen using polyester bags is presented. Attention is given to dealing with the problem of deviations from exponential behaviour in the early stages of degradation by devising a function capable of representing exponential or sigmoidal trends. This is achieved by allowing part of the fractional degradation rate to vary with time of incubation, thus enabling responses other than those expected under simple first-order kinetics to be described. Seven sets of data consisting of 620 curves were analysed to study the performance of the new model compared with a commonly used exponential model. The proportion of significantly better fits varied from set to set. The new model deals successfully with sigmoidal behaviour and, thus, provides a means of analysing the degradation profiles of low-quality forage feeds.

title: Evaluating constraints on fiber digestion by rumen microbes.
abstract: Factors affecting fiber digestion in ruminants were evaluated with the use of simple mathematical models. These models were constructed to define the dynamic processes involved so that constraints on fiber digestion may be elucidated. The fraction of fiber that is resistant to digestion and the rate of digestion and passage of potentially fermentable fiber were identified as constraints on fiber digestion in the rumen. Fermentation lag was shown to have no direct effect on fiber digestibility. Fiber that is resistant to fermentation by rumen microbes represents a significant fraction of forage fiber and accumulates in the rumen relative to potentially fermentable fiber. The digestibility of fiber that is potentially fermentable is a function of the rate at which the fiber is digested and its retention time in the rumen. Selective retention of potentially fermentable fiber in the rumen is necessary for the maximization of fiber digestion.

title: Characteristics of plant cell walls affecting intake and digestibility of forages by ruminants.
abstract: Even under the intensive concentrate feeding systems of ruminant animal production in the United States, forages continue to represent the single most important feed resource. Cell-wall concentration and digestibility limit the intake potential and energy availability of forage crops in beef and dairy production. Identification of cell-wall characteristics that should be targets of genetic modification is required if plant breeders and molecular biologists are to successfully improve forages for livestock feeding. As the forage plant cell develops, phenolic acids and lignin are deposited in the maturing cell wall in specific structural conformations, and in a strict developmental sequence. Lignin is the key element that limits cell-wall digestibility, but cross-linkage of lignin and wall polysaccharides by ferulic acid bridges may be a prerequisite for lignin to exert its affect. Lignin composition and p-coumaric acid in the wall are less likely to affect digestibility. Voluntary intake of forages is a critical determinant of animal performance and cell-wall concentration is negatively related to intake of ruminants consuming high-forage diets. Cell walls affect intake by contributing to ruminal fill. A simple model of cell-wall digestion and passage in which ruminal fill is a function of rates of digestion and passage, as well as the indigestible fraction of the cell-wall indicates that cell-wall concentration and rate of passage are the most critical parameters determining ruminal fill. Plant factors that affect rate of passage include those that affect particle size reduction by chewing and those that affect particle buoyancy in the rumen. The latter is primarily affected by 1) the ability of the particulate matter to retain gases, which is probably related to plant anatomy and rate of digestion of the plant tissue, and 2) the rate at which the gas is produced, which is affected by the potentially digestible fraction of the particulate matter and the rate of digestion of this fraction. Increasing rate of digestion should increase rate of passage by diminishing the gas produced and increasing density over time. A reduction in the indigestible cell-wall fraction is beneficial because this will decrease fill and increase digestibility. Animal production and economic benefits from reduced cell-wall concentration and increased digestibility are significant. Because of the high cell-wall concentration and large digestible cell-wall fraction of grasses, reduction in cell-wall concentration would probably be of greater value than improving digestibility in these species. Legumes represent the opposite situation and may benefit more from improvements in the digestibility of their cell walls.

title: Maximizing microbial protein synthesis in the rumen.
abstract: Microbial protein supply to the duodenum should be maximized for efficient use of feed protein and energy. High producing ruminants often are fed significant concentrations of cereal grains and fat in their diets. Increasing starch in the diet decreases ruminal pH, which often decreases extent of ruminal fiber digestion and also may decrease efficiency of microbial protein synthesis because of energy-spilling reactions. In contrast, higher grain feeding increased efficiency of microbial protein synthesis in some studies because ruminal passage rate was increased. Ruminal degradation of carbohydrates and protein must be synchronized for optimal microbial efficiency, but the microbes appear to withstand transient periods of asynchronous nutrient supply in many cases. Protozoa extensively prey upon bacteria, and a higher proportion of protozoa than bacteria lyse within the rumen, recycling significant amounts of protein. Feeding moderate amounts of unsaturated fat appears to reduce, especially on relatively low forage diets, protozoal numbers and the extent of intraruminal recycling. Indirect and direct calculations have been derived from models to estimate microbial protein transactions within the rumen; however, models need further definition and more detailed inputs from published literature to further their predictive ability.

title: Effects of feeding nonforage fiber sources on site of fiber digestion.
abstract: Although many nonforage fiber sources have high extents of neutral detergent fiber (NDF) digestion, most have rates of digestion similar to or slower than the rates of forage NDF digestion. Rates of NDF digestion vary considerably among and within sources of by-products. Digestion kinetics also vary because of the technique used (in vitro versus in situ) and because of high amounts of dietary concentrate. Based on available data for passage rate and specific gravity measurements, rates of passage of nonforage fiber sources from the rumen of high producing cows appear to be faster than those of forages. Therefore, the potential to shift NDF digestion to the hindgut has been discussed. To account for variability in ruminal and total tract digestibility of NDF, multiple regression analysis was used to indicate that nonforage NDF percentage in the diet had about two-thirds the positive response on total tract NDF digestion that forage NDF percentage did. Although the loss of potentially digestible NDF may occur, DMI does not appear to decrease much until forage NDF is below 14 to 16% of dietary DM. Conversely, replacement of starch with nonforage NDF appears to increase digestibility of fiber, mostly in diets with high concentrations of nonfiber carbohydrates, apparently because of reduced negative associative effects. Increasing the concentration of total NDF above 35% also can decrease DMI with little improvement in NDF digestibility. Increased knowledge of the kinetics of digestion and the passage of various nonforage fiber sources used to replace forage or concentrate should increase the accuracy and precision of dynamic models, thereby increasing the flexibility and utility of nonforage fiber sources in dairy rations.

title: In Situ Degradation Kinetics: Evaluation of Rate Determination Procedure
abstract: In situ digestion profiles from dry matter, acid detergent fiber, neutral detergent fiber from alfalfa hay, and dry matter from orchardgrass hay were selected to compare methods to describe kinetically degradation rates. The four methods were 1) least squares regression of logarithmic-transformed residuals without correction for ruminally undegradable residue, 2) method 1, with correction for undegradable residue, 3) curve peeling, and 4) nonlinear iterative least squares regression. Not correcting data sets for undegraded residue resulted in rate constants for all chemical components of forage approximately one-third as fast compared with results correcting each fermentation point. Neutral detergent fiber of alfalfa hay and dry matter from orchardgrass hay exhibited single component linear degradation profiles; however, alfalfa hay dry matter and acid detergent fiber each exhibited two distinct linear component profiles.  the following rate constants for alfalfa hay dry matter and acid detergent fiber: 7.5, 22.3, and 17.4%/h and 4.4, 25.2, and 8.8%/h, respectively. The nonlinear method tended to underestimate lag and potential extent of degradation to achieve a best fit curve for alfalfa acid detergent fiber and tended to overestimate potential extent of degradation when one linear component existed. One specific method may not be adequate to describe all degradation profiles.

title: Estimation of Ruminal Digestibility of NDF from in Sacco Degradation and Rumen Fractional Outflow Rate
abstract: The relation between effective degradation (ED) and ruminal digestibility of NDF was tested with 6 cows in two 3 × 3 latin square experiments, one with three different roughages fed alone and one with the same roughages fed together with concentrate. In vivo ruminal digestibility of NDF could be estimated (R 2=0.77) with the parameters from a degradation model including lag time and with rate of passage determined as rumen outflow of NDF divided by rumen NDF pool size. The largest deviations between ruminal digestibility and ED were found with medium to high contents of starch in the rations owing to a more profound depression of NDF degradation in nylon bags compared to in vivo. This was caused most probably by an underestimation of rate of degradation and an overestimation of lag time in the nylon bags.

title: Development of buffer systems for pH control and evaluation of pH effects on fiber digestion in vitro.
abstract: An in vitro buffering system capable of pH control between pH 5.8 and 6.8 was developed to examine the effect of media pH on disappearance of NDF at various times of fermentation and to assess initially the effect of media pH on kinetics of NDF digestion. The pH conditions selected for evaluation of these buffer systems were 5.8, 6.2, and 6.8. Use of McIlvaine's solution with sodium bicarbonate was not successful because of rapid drifting of pH downward during fermentation. To evaluate the effectiveness of citric or phosphoric acids as components of phosphate-bicarbonate buffer systems, alfalfa silage and a mixture of alfalfa silage and corn grain (1:1 mixture, dry basis) were fermented for 0, 12, 24, 48, and 72 h. The pH of each flask was measured at 0, 4, 12, 24, 48, and 72 h postinoculation, and pH was readjusted with bicarbonate solution when necessary. Drifting of media pH downward was more noticeable when phosphoric acid was used to adjust the buffer pH than with citric acid. Citric acid had no adverse effects on NDF digestion compared with phosphoric acid when used to adjust a phosphate-bicarbonate buffer system. Alfalfa hay, bromegrass hay, and corn silage were incubated for 0, 12, 24, 48, 72, or 96 h at pH 5.8 or 6.8 using the phosphate-bicarbonate buffer system adjusted with citric acid. Estimation of kinetics of NDF digestion indicated that a decrease in media pH from 6.8 to 5.8 resulted in a marked reduction in NDF digestion; the largest apparent difference was extended digestion lag time.

title: Effect of Glucose on Fiber Digestion and Particle-Associated Carboxymethylcellulase Activity In Vitro,
abstract: 1 were to delineate effects of glucose addition on digestion of cellulosic and hemicellulosic fiber sources in vitro as either pH-dependent or pH-independent. Two buffer media (regular and strong), three fiber sources (cellulose, soyhulls, and corn bran), and four glucose concentrations (0, 10, 25, and 50m M ) were used in an in vitro digestion system in a 2 × 3 × 4 factorial arrangement of treatments. Strong, but not regular, buffer medium maintained pH >6.2 when glucose was added. Strong buffer medium and 50m M glucose increased NDF digestion lag time. Digestion rate of cellulose was decreased when 25 or 50m M glucose was added compared with effects of no glucose in both buffer media. Digestion rate of soyhull NDF was decreased when 25 or 50m M glucose was added compared with no glucose in cultures containing regular, but not strong, buffer medium. Corn bran NDF digestion rate was decreased when 50m M glucose was added with strong, but not regular, buffer medium compared with effects of no glucose. Thus, effects of glucose addition on cellulose and corn bran digestion rates were, in part, pH-independent, but effect on soyhulls was pH-dependent. Effects of adding 25m M glucose at 0 or 24h of incubation in strong buffer medium on microbial colonization and digestion of fiber were studied in trial 2. Glucose added at 0h, but not at 24h, of incubation decreased NDF digestion rate compared with effects of no glucose. Glucose added at 0 or 24h of incubation decreased carboxymethylcellulase activity (index of colonization of fibrolytic microbes). Corn bran digesta had lower carboxymethylcellulase activity than did digesta from cellulose or soyhulls. Glucose decreased NDF digestion rate, apparently by decreasing colonization of fibrolytic microbes on cellulosic and hemicellulosic fiber sources.

title: Effects of amount and availability of starch on amylolytic activity of ruminal solid-associated microorganisms
abstract: Three ruminally cannulated dry cows were used in an experiment designed as a Latin square to investigate the effects of amount and availability of starch on the amylolytic activity of ruminal solid-associated microorganisms (SAM). Animals were fed twice daily a diet (7 kg DM day-1) consisting of wheat straw, cocksfoot hay and ground barley in the ratios 10: 90: 0, 10: 60: 30 and 10: 30: 60. Nylon bags containing rapidly (barley) or slowly (maize) degradable cereal grains were incubated in the rumen. Enzymes of the SAM were extracted from rumen contents and bag residues, and amylolytic (amylase and α-D-glucosidase) activities were measured. The in situ degradation of starch was determined. Increasing the level of barley in the diet induced a linear increase in amylase activity of SAM in the rumen contents. In the bags, amylase activity and starch degradation rate were not modified by supplementation. High activity was observed more rapidly in bags containing barley, where starch is rapidly available, than in bags containing maize, where starch is protected by a resistant endosperm. These results suggest that amylolytic activity of SAM may depend on the amount of starch available to microorganisms.

title: In vitro starch disappearance procedure modifications
abstract: Four in vitro experiments evaluated the effects of ruminal fluid inoculum:artificial saliva ratios, grinder type, grind size, and diet of ruminal fluid donor on in vitro starch disappearance. Experiment 1 examined rates of starch disappearance and coefficients of determination obtained by linear regression of starch disappearance using five grain sorghum (Sorghum bicolor (L.) Moench) lines, a corn (Zea mays L.) control, and a wheat (Triticum aestivum L.) control. Grains were incubated for 4, 8, 12, 16, and 20 h with inoculum varying in proportion of ruminal fluid and artificial saliva (1:1, 1:2, 1:3, and 1:4). In vitro rates of starch disappearance and coefficients of determination were similar for the 1:2, 1:3, and 1:4 inoculum dilutions and were higher (quadratic, P 0.10) by grain type fed; however, absolute rates of digestion varied among inoculate sources. Grinder, grind size, and ruminal fluid inoculum:artificial saliva ratios affected rate of starch disappearance in samples digested in vitro. Diet of ruminal fluid donor affected the rate of starch digestion, but not the relative ranking of the grains. If rates are to be compared across in vitro runs for different grains, these processing and dietary factors must be kept constant.

title: In vitro and in sacco methods for the estimation of dietary nitrogen degradability in the rumen: a review
abstract: In vitro and in sacco methods for estimation of dietary protein degradability in the rumen are reviewed. Several variables associated with in sacco experimentation were evaluated and recommended guidelines were proposed: bag porosity must be large enough for the degradation process to take place normally inside the bag and sufficiently small to limit losses of undegraded fine particles; fineness of grinding sample must mimic the effect of mastication; it must be defined not by the grinding screen aperture, but by the mean particulate size; other variables were studied as sample size to bag surface ratio, bag incubation sequence, relative location of nylon bag within the rumen and explanations for variations were proposed. Microbial contamination of bag residues and modelling of nitrogen degradation kinetics were the subjects of recent publications and are discussed in this paper. The variability of results were studied and recommendations (use of standard feed) were made to limit the variations.

title: Technical note: forage in vitro dry matter digestibility as influenced by fiber source in the donor cow diet.
abstract: Objectives of this study were to determine the influence of five donor cow diets that differed in source of fiber on true in vitro DM digestibility of eight forages. Test forages included two alfalfa (31.2 and 38.3% NDF), corn silage (50.2% NDF), oat forage (48.2% NDF), perennial rye (62.1% NDF), two reed canarygrasses (55.9 and 68.1% NDF), and timothy (68.2% NDF). Sources of fiber in donor cow diets were alfalfa haylage or alfalfa haylage plus either corn cobs, cottonseed hulls, oat hulls, or soy hulls. In addition, the effect of filtering through sintered glass crucibles or filter paper (two replicates each) was evaluated. There were differences (P < .01) among feeds in in vitro DM digestibility, but there were no interactions (P > .05) between test forages and either source of fiber in donor diets or filtration method. There was an interaction between source of fiber in the donor diet and method of filtration (P < .01). Samples inoculated with ruminal fluid from cows fed diets with oat hulls or soy hulls had lower (P < .01) in vitro DM digestibility when filtered on crucibles than on filter paper. Filtration method did not affect (P > .05) in vitro DM digestibility of samples inoculated with ruminal fluid from other diets. The in vitro DM digestibility of samples inoculated with ruminal fluid from cows fed alfalfa haylage was less than the in vitro DM digestibility when other inocula were used. The source of fiber in the donor cow diet and filtration method can affect in vitro DM digestibility, but relative ranking of forages was unaffected by these variables.

title: Effect of cellulose fine structure on kinetics of its digestion by mixed ruminal microorganisms in vitro
abstract: The digestion kinetics of a variety of pure celluloses were examined by using an in vitro assay employing mixed ruminal microflora and a modified detergent extraction procedure to recover residual cellulose. Digestion of all of the celluloses was described by a discontinuous first-order rate equation to yield digestion rate constants and discrete lag times. These kinetic parameters were compared with the relative crystallinity indices and estimated accessible surface areas of the celluloses. For type I celluloses having similar crystallinities and simple nonaggregating particle morphologies, the fermentation rate constants displayed a strong positive correlation (r2 = 0.978) with gross specific surface area; lag time exhibited a weaker, negative correlation (r2 = 0.930) with gross specific surface area. Crystallinity was shown to have a relatively minor effect on the digestion rate and lag time. Swelling of microcrystalline cellulose with 72 to 77% phosphoric acid yielded substrates which were fermented slightly more rapidly than the original material. However, treatment with higher concentrations of phosphoric acid resulted in a more slowly fermented substrate, despite a decrease in crystallinity and an increase in pore volume. This reduced fermentation rate was apparently due to the partial conversion of the cellulose from the type I to the type II allomorph, since mercerized (type II) cellulose was also fermented more slowly, and only after a much longer lag period. The results are consistent with earlier evidence for the cell-associated nature of cellulolytic enzymes of ruminal bacteria and suggest that ruminal microflora do not rapidly adapt to utilization of celluloses with altered unit cell structures.

title: Digestion kinetics of fiber: influence of in vitro buffer pH varied within observed physiological range.
abstract: In vitro buffer pH reflective of the diurnal variation in ruminal pH was evaluated for its impact on digestion kinetics of NDF from three forage sources. Alfalfa hay, bromegrass hay, and corn silage were incubated in phosphate-bicarbonate buffer solution adjusted to pH 6.8, 6.5, 6.2, 6.0, 5.8, or 5.5 using 1 M citric acid. Ash-free NDF was measured after 0, 6, 12, 18, 24, 48, 72, and 96 h of fermentation. The experiment was replicated three times; kinetic parameters of fiber digestion were estimated by logarithmic transformation, and by linear and nonlinear regression procedures. Lag in NDF digestion increased as pH fell from 6.8 to 6.5 and again when pH decreased below 6.0. Decreasing buffer pH below 6.2 dramatically reduced NDF digestion rate for alfalfa hay and corn silage but had no significant effect on NDF digestion rate of bromegrass hay. Lag in NDF digestion increased below pH 6.0 for both alfalfa and corn silage but increased only when pH fell below 6.2 for bromegrass. Results suggest that lowered pH exerts its negative effect on NDF digestion between pH 6.2 and 5.8, as evidenced by increased lag and decreased rate, and that critical pH and specific, affected component of digestion varied among forages.

title: Quantification of intraruminal recycling of microbial nitrogen using nitrogen-15.
abstract: Studies were done to derive a technique to quantify intraruminal recycling of microbial nonammonia nitrogen (NAN). After dosing 15NH4+ into the rumen, ruminal NH3N and NAN pools were sampled over time for analyses of 15N enrichment. Compartmental analysis of both pools was used to quantify the percentage of microbial NAN that recycles in the rumen. Microbial NAN does not need to be fractionated from total NAN, assuming that only microbial NAN becomes labeled with 15N and that the ratio of microbial NAN:nonmicrobial NAN remains constant over time. Based on the data obtained from eight nonlactating cattle fed 85% corn silage diets at 1.5 to 1.0% BW, percentage of intraruminal recycling of microbial NAN was related to efficiency of bacterial protein synthesis (grams of CP/100 g of OM truly digested) in a curvilinear manner (y = 100 - .073x2). From this function, recycling was predicted to be approximately 75% for dairy cows consuming 50:50 forage:concentrate diets at 3.5% of BW. However, more data are needed to evaluate intraruminal recycling of microbial NAN using this technique under different dietary conditions.

title: Kinetics of hydration and effect of liquid uptake on specific gravity of small hay and silage particles.
abstract: Kinetics of hydration of ground hay and silage particles (2-mm screen), determined by a pycnometric technique, was best described by a two- and one-pool exponential model, respectively. Fractional rates of hydration of the large pool, detected in hay particles only, and of the small pool present in both hay and silage particles averaged .135 and .021 min-1, respectively. When hydration was complete, liquid associated with particles averaged 1.16, 1.90, and .83 g/g of insoluble DM for bromegrass hay, alfalfa hay, and alfalfa silage, respectively. Functional specific gravity, which accounts for the effect of associated gas volume, averaged 1.54, 1.46, and 1.54, but unit specific gravity, calculated to include the effect of gases and liquid of hydration, averaged 1.22, 1.14, and 1.26 for bromegrass hay, alfalfa hay, and alfalfa silage, respectively. Preservation of forage as silage not only lowered gas volume, but also reduced water-holding capacity, both of which contribute to greater unit specific gravity and faster rate of escape from the rumen. In addition, estimates of unit specific gravity of approximately 1.2 indicate that even in the absence of associated gas, hydrated forage particles would tend to escape the rumen at a slower rate than that achieved by more dense particles.

title: Cellulose Degradation by Ruminal Microorganisms
abstract: AbstractThe rumen is a highly developed digestive organ in which feed material, particularly plant fiber, is efficiently digested by a complex microbial fermentation. Cellulose, the major component of plant fiber, is digested in the rumen by an assemblage of anaerobic bacteria, fungi, and protozoa. While pure cellulose itself is completely digestible under the proper environmental conditions, the matrix interactions among structural biopolymers in natural plant material prevent the complete digestion of the cellulose component. Ruminal bacterial are probably the most active cellulose degraders in the rumen, but the fungi also play an important role, particularly in the physical disruption of recalcitrant tissue to render the cellulose more accessible to degradative enzymes. Cellulose degradation in the rumen is enhanced by attachment of microbial cells to the substrate and by synergistic interactions with noncellulolytic microbes, but is inhibited by factors that produce a low ruminal pH. Recent advances ...

title: Microbial attachment and feed digestion in the rumen.
abstract: Direct microscopic examination of the rumen and its contents shows microbial populations largely attached to feed particles in the digesta. Most feeds contain a surface layer that is resistant to attachment and therefore to digestion. Infiltration of these recalcitrant epidermal layers through damage sites or through focused enzymatic attack is essential for initiation of the digestive process. Proliferation of primary colonizing cells produces glycocalyx-enclosed microcolonies. Secondary colonizers from the ruminal fluid associate with microcolonies, resulting in the formation of multispecies microbial biofilms. These metabolically related organisms associate with their preferred substrates and produce the myriad of enzymes necessary for the digestion of chemically and structurally complex plant tissues. Upon accessing the internal, enzyme-susceptible tissues, microbial "digestive consortia" attach to a variety of nutrients, including protein, cellulose, and starch and digest insoluble feed materials from the inside out. Substances that prevent microbial attachment or promote detachment (e.g., condensed tannins, methylcellulose) can completely inhibit cellulose digestion. As the microbial consortium matures and adapts to a particular type of feed, it becomes inherently stable and its participant microorganisms are notoriously difficult to manipulate due to the impenetrable nature of biofilms. Properties of feed that place constraints on microbial attachment and biofilm formation can have a profound effect on both the rate and extent of feed digestion in the rumen. Developments in feed processing (i.e., chemical and physical), plant breeding, and genetic engineering (both of ruminal microorganisms and plants) that overcome these constraints through the promotion of microbial attachment and biofilm formation could substantially benefit ruminant production.

title: Kinetics of Cellulose Digestion by Fibrobacter succinogenes S85
abstract: Growing cultures of Fibrobacter succinogenes S85 digested cellulose at a rapid rate, but nongrowing cells and cell extracts did not have detectable crystalline cellulase activity. Cells that had been growing exponentially on cellobiose initiated cellulose digestion and succinate production immediately, and cellulose-dependent succinate production could be used as an index of enzyme activity against crystalline cellulose. Cells incubated with cellulose never produced detectable cellobiose, and cells that were preincubated for a short time with thiocellobiose lost their ability to digest cellulose (competitive inhibition [K(infi)] of only 0.2 mg/ml or 0.56 mM). Based on these results, the crystalline cellulases of F. succinogenes were very sensitive to feedback inhibition. Different cellulose sources bound different amounts of Congo red, and the binding capacity was HCl-regenerated cellulose > ball-milled cellulose > Sigmacel > Avicel > filter paper. Congo red binding capacity was highly correlated with the maximum rates of metabolism of cellulose digestion and inversely related to K(infm). Congo red (250 (mu)g/ml) did not inhibit the growth of F. succinogenes S85 on cellobiose, but this concentration of Congo red inhibited the rate of ball-milled cellulose digestion. A Lineweaver-Burk plot of ball-milled cellulose digestion rate versus the amount of cellulose indicated that Congo red was a competitive inhibitor of cellulose digestion (K(infi) was 250 (mu)g/ml).

title: The endogenous metabolism ofFibrobacter succinogenes and its relationship to cellobiose transport, viability and cellulose digestion
abstract: Fibrobacter succinogenes S85 digested ballmilled cellulose at a rapid rate (0.10 h−1), but there was a long lag time if the culture was not transferred daily. WhenF. succinogenes was starved for 100h, a large fraction of the cells (>30%) still bound to cellulose, but the lag time was 150h. The lag time was similar for either cellulose- or cellobiose-grown inocula, and lag times were highly correlated (r2 = 0.91) with a decrease in viable cell number. The number of viable cells declined from 108 to 106 in the first 30h of starvation, and by 72h the viable cell number was less than 103/ml. Cells growing exponentially on cellobiose had a large pool of polysaccharide, and continuous culture experiments indicated that polysaccharide accumulation was not significantly influenced by the growth rate of the culture (approximately 0.7 mg polysaccharide mg−1 protein). When the cellobiose was depleted, cellular polysaccharide decreased at first order rate of 0.09 h−1. The rate of endogenous metabolism was initially 0.08mg polysaccharide mg−1 protein h−1, and there was little decline in viability until the rate of endogenous metabolism was less than 0.01 mg polysaccharide mg−1 protein h−1. When the rate was less than 0.01 mg polysaccharide mg−1 protein h−1, the cells could not maintain a sodium gradient, transport cellobiose or grow. The endogenous metabolic rate needed for cell survival was 20 fold less than the maintenance energy of cells growing in continuous culture (0.01 versus 0.232mg carbohydrate mg−1 protein h−1).

title: Interactions between Fibrobacter succinogenes, Prevotella ruminicola, and Ruminococcus flavefaciens in the digestion of cellulose from forages.
abstract: The synergistic and inhibitory interactions observed between Fibrobacter succinogenes A3c, Prevotella ruminicola H2b, and Ruminococcus flavefaciens B34b in the digestion of forage cellulose were studied in detail. Orchardgrass and alfalfa hays, both at two maturity stages, were used as substrates. Sequential inoculation procedures were developed whereby a second inoculation was made after the initial fermentation was killed. Total cellulose digestion from sequential addition of the organisms was then compared to values obtained in simultaneous co-culture. When the noncellulolytic P. ruminicola was co-cultured with either of the two cellulolytic species (F. succinogenes or R. flavefaciens) forage cellulose digestion numerically increased over that of the cellulolytic species alone. In contrast, decreases from co-culture values were noted with sequential addition of the organisms. When F. succinogenes and R. flavefaciens were co-cultured, cellulose digestion was reduced compared to F. succinogenes alone. However, no such reduction was observed when the organisms were added sequentially. Further experiments indicated that this inhibitory activity is only produced when the organisms are co-cultured and is stable to autoclaving at 121 degrees C for 20 min. Inhibition of this type could be the result of bacterocin production by one of the organisms; however, most bacterocins are destroyed by autoclaving.

title: Are ruminal bacteria armed with bacteriocins?
abstract: The production of toxic compounds or antibiotics is a common component of intermicrobial competitive interactions, and many of these toxins have been adopted and adapted for the control of microbial populations. One class of these toxins, the bacteriocins, is a heterogeneous group of proteinaceous antibiotics that often display a high degree of target specificity, although many have a very wide spectrum of activity. To date, only limited information is available concerning the occurrence of bacteriocins among ruminal isolates or the sensitivity of ruminal microorganisms to exogenous bacteriocins. A survey of 50 strains of Butyrivibrio spp. isolated from a variety of sources (sheep, deer, and cattle) for bacteriocin production indicated a high incidence of bacteriocin-like activity (50%). Many of these inhibitory compounds appear to have a broad spectrum of activity, which suggests that bacteriocins may have a significant impact on both the competitive fitness of individual microbial strains within the rumen and on the overall structure of the microbial population within the rumen. Selected bacteriocins from lactic acid bacteria also were shown to have activity against Butyrivibrio spp. and may have application in ruminant systems. Bacteriocins may provide an alternative group of antibiotics for the manipulation of ruminal microbial populations. Bacteriocins have significant advantages over other antibiotics in target specificity, susceptibility to proteolytic digestion, possibility of genetic transfer and manipulation, and, in the case of some bacteriocins derived from lactic acid bacteria, a long history of safe use.

title: Why don't ruminal bacteria digest cellulose faster?
abstract: The bacteria Fibrobacter succinogenes, Ruminococcus flavefaciens, and Ruminococcus albus generally are regarded as the predominant cellulolytic microbes in the rumen. Comparison of available data from the literature reveals that these bacteria are the most actively cellulolytic of all mesophilic organisms described to date from any habitat. In light of numerous proposals to improve microbial cellulose digestion in ruminants, it is instructive to examine the characteristics of these species that contribute to their superior cellulolytic capabilities and to identify the factors that prevent them from digesting cellulose even more rapidly. As a group, these species have extreme nutritional specialization. They are able to utilize cellulose (or in some cases xylan) and its hydrolytic products as their nearly sole energy sources for growth. Moreover, each species apparently has evolved to similar maximum rates of cellulose digestion (first-order rate constants of 0.05 to 0.08 h-1). Active cellulose digestion involves adherence of cells to the fibers via a glycoprotein glycocalyx, which protects cells from protozoal grazing and cellulolytic enzymes from degradation by ruminal proteases while it retains-at least temporarily-the cellodextrin products for use by the cellulolytic bacteria. These properties result in different ecological roles for the adherent and nonadherent populations of each species, but overall provide an enormous selective advantage to these cellulolytic bacteria in the ruminal environment. However, major constraints to cellulose digestion are caused by cell-wall structure of the plant (matrix interactions among wall biopolymers and low substrate surface area) and by limited penetration of the nonmotile cellulolytic microbes into the cell lumen. Because of these constraints and the highly adapted nature of cellulose digestion by the predominant cellulolytic bacteria in the rumen, transfer of cellulolytic capabilities to noncellulolytic ruminal bacteria (e.g., by genetic engineering) that display other desirable properties offers limited opportunities to improve ruminal digestion of cellulose.

title: Site of digestion of starch in the gastrointestinal tract of dairy cows and its effect on milk yield and composition.
abstract: Physical and chemical processing of feed ingredients and feeding management strategies are major instruments of manipulating amount and site of starch digestion in the gastrointestinal tract. Generally, as rumen escape of starch increases, postruminal starch digestion increases, and there does not appear to be a limitation to intestinal starch digestion. However, the efficiency with which postruminal starch is digested decreases, which represents a limitation that warrants investigation. Even though digestible dietary starch is presented to the intestine, there is no net glucose absorption at the portal vein, and plasma glucose levels remain relatively unaffected. This result may be associated with the large metabolic requirement for postruminally absorbed glucose, which is preferentially used for oxidative metabolism at the visceral tissue level. In addition, peripheral glucose concentration is highly regulated. A possible implication is that the exogenous glucose supply may spare endogenously synthesized glucose for gut metabolism, allowing more to be directed to the mammary gland. Amino acids also may be spared (less metabolism of dietary and tissue amino acids in the gut). Current production studies yield no clear evidence as to the benefits of postruminal digestion of starch to enhance milk yield or to change its composition. However, studies suggest that starch digested postruminally is used more efficiently for milk synthesis than that digested in the rumen.

title: Effects of processing method on degradation characteristics of protein and carbohydrate sources in situ.
abstract: Two ruminally cannulated Holstein cows in early lactation were used to determine the effects of heat treatment and particle size on fractional degradation rates and ruminal degradabilities of DM, CP and total nonstructural carbohydrate of dry shelled corn and soybeans. A randomized complete block design with cows as blocks was used. Carbohydrate sources were cracked corn, chick cracked corn, finely ground corn, and steam-flaked corn. Protein sources were soybeans that were roasted at 144 degrees C, cracked, or ground through a 4-mm screen and raw soybeans that were cracked or ground through a 4-mm screen. Reduction in particle size increased both degradability of total nonstructural carbohydrate in the rumen. Heat treatment decreased degradability of CP and increase degradability of total nonstructural carbohydrate in the rumen for both protein and carbohydrate sources. Roasting of soybeans decreased degradability of all AA except Lys. However,reduction in particle size had a stronger effect than heat treatment on ruminal degradability of CP and AA of soybeans, but had the opposite effect on total nonstructural carbohydrate degradability from corn. Methionine, Leu, and Thr had the lowest ruminal degradability across all soybean sources, and Lys and Val were the most degradable. The AA profile of the RUP fraction of soybeans differed from that of the original feed-stuff and was affected by processing method.

title: The effect of preservation method on the neutral detergent soluble fraction of forages.
abstract: Fermentation of neutral detergent solubles (NDS) was assessed using a 3 x 3 x 3 factorial arrangement. Three forage species (alfalfa, bromegrass, and orchardgrass) were collected at three maturities and preserved either by freeze drying, oven drying at 50 degrees C, or by ensiling. Each feed sample and its isolated NDF were fermented in vitro and gas production was monitored. Gas yield from NDS was determined as the difference between gas from the unfractionated forage and from its respective NDF. The forages ranged from 23 (immature alfalfa) to 68% NDF (mature orchardgrass). The silages were well fermented with a final pH of < or = 4.5. Increasing maturity decreased the final gas volume but did not change the rate of gas production from the NDS fraction. There was little difference in gas production between freeze-dried and oven-dried forage samples. Ensiling decreased gas yield from the unfractionated forage. The rate of gas production from the NDS fraction of the ensiled forages decreased an average of .05 h-1 compared with the freeze-dried sample. Gas yield from the NDS fraction decreased (from the freeze-dried sample) between 7 and 36% upon ensiling. The curve substraction approach can be used to evaluate the effects of ensiling on the neutral detergent-soluble fraction of forages.

title: In situ and Other Methods to Estimate Ruminal Protein and Energy Digestibility: A Review
abstract: Methods to estimate ruminal protein and carbohydrate availability were reviewed. Solubility of carbohydrate and protein is dependent on the chemical characteristics of the buffer. Protein solubility is highly correlated to short-term (1 to 3 h) ruminal digestion of protein, however, not to extent of ruminal digestion. Enzymatic digestion techniques for both protein and carbohydrate offer precision in ranking feeds; however, absolute values may be questioned. Combining several enzymes may offer more accurate estimates. Continuous culture techniques offer enhanced simulation of the ruminal environment compared with batch culture and is more applicable to examination of total rations than individual feedstuffs. Several variables associated with in situ experimentation were evaluated and recommended guidelines were proposed. This methodology offers a better way to simulate the rumen environment within a given feeding regimen. However, its usefulness will be dependent on the standardization of inherent variables associated with its conduct (i.e., bag porosity, sample size, feed particle size). Although in vivo methodologies (postruminal cannulation) are usually the standards by which other techniques are compared, they are plagued with variation associated with microbial and particulate or liquid flow markers and animal variation.

title: Alternative techniques for measuring nutrient digestion in ruminants.
abstract: Because in vivo measurement of nutrient digestion in the rumen and small intestine requires ruminally and intestinally cannulated animals that are expensive, labor-intensive, and subject to error associated with markers and inherent animal variation, alternative techniques have been developed. Researchers have proposed various in situ or in vitro procedures for estimating ruminal and small intestinal nutrient digestion. This review summarizes these alternative techniques.

title: Validation of in sacco method: influence of sampling site, nylon bag or rumen contents, on fibrolytic activity of solid-associated microorganisms
abstract: Three ruminally cannulated cows, fed twice daily with a 70:30 forage:concentrate diet, were used to investigate the differences in fibrolytic activity of solid-associated microorganisms between nylon bags and rumen contents. Two different grass hays (regrowth and late harvested) were incubated in ruminal nylon bags. After 2 h or 23 h incubation time, pH was measured in bags and rumen contents, and enzymes of solid-associated microorganisms were extracted from bag residues and surrounding digesta by grinding, freezing, defrosting and sonication. Xylanase, avicelase, β-D-xylosidase and β-D-glucosidase activities were measured. Activities were lower in bag residues than in rumen digesta, and differences were greater after 2 h than after 23 h incubation time. Causes of these differences are discussed. For each incubation time and each enzyme, the differences in solid-associated microorganisms activities between rumen and bags contents were independent of the quality of hay in the bag. Thus the lower fibrolytic activity inside the bags may account for an underestimation of in vivo ruminal fiber degradation by the in sacco method, but this underestimation may be similar whatever the nature and content of forage cell walls.

title: Comparison of in sacco and in vitro techniques for estimating the rate and extent of rumen fermentation of a range of dietary ingredients
abstract: Fifteen plant-derived ingredients (crude protein (CP) 28–390 g kg−1 DM (dry matter) and neutral detergent fibre (NDF) 55–902 g kg−1 DM), were incubated in sacco (2, 4, 8, 12, 18, 24, 48 and 72 h) and in vitro (2, 4, 8, 24 and 48 h). Fermentabilities (F) were fitted to the asymptotic equation F = a + b(1 − e−ct) where a represents material disappearing immediately, b the fraction which does not disappear immediately but which does so eventually, and c the rate of disappearance of the b fraction, where t is time. Curve parameters from the porous synthetic fibre (psf) bag technique were not readily predicted from feed chemistry or in vitro estimates. The only useful prediction was that for the completely unfermentable fraction (1 − a − b) from Christian lignin content (r2 = 0.70; n = 15; P < 0.001). This relationship was particularly strong within the Gramineae (r2 = 0.98; n = 8; P < 0.001), where lignin protects around three times its mass from fermentation. Comparison of asymptotes (a + b) estimated from the two techniques showed that completely unfermentable material was leaving psf bags and that this was a particular problem with soya hulls and maize germ. At shorter incubation times (2–8 h), unfermented material that leaves bags (particularly with wheat, sweet potato and maize germ) is likely to be fermented by 24 h in any case. The overestimation of fermentability by the in sacco technique, relative to in vitro, was greatest with feeds of low acid detergent fibre (ADF) content, being particularly marked at less than 250 g ADF kg−1 DM. The discrepancy accelerated from 2 to 8 h as the feed structure opened up and thereafter declined to a mean overestimation of around 0.1 g g−1 organic matter at the asymptote. The study casts serious doubt on the applicability of the psf bag technique for studies of concentrate ingredients with a high content of either soluble constituents or unfermentable ‘fines’ which will pass out of bags.

title: Effects of forage species and particle size on bacterial cellulolytic activity and colonization in situ.
abstract: Two ruminally cannulated Holstein heifers were used to evaluate effects of three forage species and two particle sizes in a 3 x 2 factorial arrangement on in situ fiber disappearance and carboxymethylcellulase activity (CMCase). A second objective was to compare CMCase and 15N as relative bacterial colonization markers. At time zero, 15N-enriched ammonium sulfate was dosed into the rumen. Residues in nylon bags incubated for 3, 6, 9, 12, 18, 24, 30, 36, 42, 48, 60, and 72 h were analyzed for NDF, ADF, CMCase, and 15N. Rates of NDF and ADF disappearance were faster (P < .05) for red clover (RC) than for gamagrass (GG) or orchardgrass (OG) and for forages ground to pass a 2-mm screen (P < .05) than for those ground to pass a 5-mm screen. Cumulative area under the curve (AUC) of CMCase activity was related to both NDF disappearance (r2 = .93) and 15N AUC (r2 = .99), indicating that CMCase can be used to estimate relative colonization of cellulolytic particle-associated bacteria (PAB). During the first 18 h of incubation, RC had greater (P < .05) CMCase and 15N enrichment of in situ residues than did OG or GG. The RC residues also had a greater (P < .05) ratio of CMCase activity:15N enrichment; this was presumed to indicate that, during this time, RC supported a greater total PAB population and had a greater proportion of cellulolytic PAB than did either GG or OG. Differences in the colonization rate of forage particles by cellulolytic PAB relative to total PAB may account for differences in rate of fiber digestion.

title: Relationship between fermentation acid production in the rumen and the requirement for physically effective fiber.
abstract: The content of ruminally fermented OM in the diet affects the fiber requirement of dairy cattle. Physically effective fiber is the fraction of feed that stimulates chewing activity. Chewing, in turn, stimulates saliva secretion. Bicarbonate and phosphate buffers in saliva neutralize acids produced by fermentation of OM in the rumen. The balance between the production of fermentation acid and buffer secretion is a major determinant of ruminal pH. Low ruminal pH may decrease DMI, fiber digestibility, and microbial yield and thus decrease milk production and increase feed costs. Diets should be formulated to maintain adequate mean ruminal pH, and variation in ruminal pH should be minimized by feeding management. The fraction of OM that is fermented in the rumen varies greatly among diets. This variation affects the amount of fermentation acids produced and directly affects the amount of physically effective fiber that is required to maintain adequate ruminal pH. Acid production in the rumen is due primarily to fermentation of carbohydrates, which represent over 65% of the DM in diets of dairy cows and have the most variable ruminal degradation across diets. The non-fiber carbohydrate content of the diet is often used as a proxy for ruminal fermentability, but this measure is inadequate. Ruminal fermentation of both nonfiber carbohydrate and fiber is extremely variable, and this variability is not related to the nonfiber carbohydrate content of the diet. The interaction of ruminally fermented carbohydrate and physically effective fiber must be considered when diets for dairy cattle are evaluated and formulated.

title: Studies on in situ degradation of feeds in the rumen: 1. Effect of species, bag mobility and incubation sequence on dry matter disappearance
abstract: Implicit to the newer more complex predictive models for estimating nutrient requirements for ruminants is a need for an accurate determination of feed degradation kinetics. The rumen in situ technique has been adopted as the standard method. However, recent ring tests and reviews have highlighted the need for improved standardisation of the in situ procedure. The aim of this experiment was to investigate differences between host species and to determine the effect of anchor weight and incubation sequence on dry matter (DM) disappearance of hay, soyabean meal (SBM) and fish meal (FM) in situ. Three mature rumen cannulated non-lactating cows and wether sheep fed a grass silage, barley diet (80:20, DM basis) were used. The experimental design was based on a latin square model with nine experimental periods, two major treatments and three levels of each treatment. Sheep and cow designs were run in parallel. Duplicate bags of each substrate were incubated (complete exchange) for 0, 2, 4, 8, 16, 24, 48 and 66 h and were allocated to treatment according to the latin square design. Treatment levels were: half normal, normal and twice the normal anchor weight for each species (0.5, 1 and 2 kg for cows and 75, 150 and 300 g for sheep) and three incubation sequences (i) 8, 16, 2, 4, 66, 48, 24 h; (ii) 8, 24, 66, 4, 2, 16, 48 h; (iii) 24, 4, 2, 66, 8, 48, 16 h. No species differences were observed ( P > 0.05) with overall mean losses of 55.8 and 55.6% for sheep and cows respectively. The degradation profiles of hay, SBM and FM were found to be different ( P P > 0.05) effective degradability values. Mean effective degradability (0.08 h −1 outflow) measurements for incubation sequence (i, ii and iii) were 52.9, 53.1 and 53.3% and for anchor weight (0.5 ×, 1 × and 2 × normal) were 52.9, 53.2 and 53.2%. It was concluded that mature ruminant species degrade hay, SBM and FM DM similarly and that anchor weight and the sequence of incubation has no effect on the degradability profiles of these feeds.

title: A collaborative study of in situ forage protein degradation.
abstract: An in situ procedure was used in four collaborative trials to evaluate the variation in estimates of ruminal escape protein among eight laboratories. A standard smooth bromegrass hay was established and evaluated in all four trials, and Trials 3 and 4 also included the standard hay material that was retained on a 75-microns sieve. Polyester bags (10 cm x 20 cm) containing 5 g of sample were soaked in water (39 degrees C) for 20 min and incubated ruminally for 16 h in a bag made of mesh material. Bag rinsing after in situ incubation consisted of a primary phase in which the mesh bags containing sample bags were washed with water (39 degrees C) in 19-L white plastic buckets. Repeated cycles of filling, agitating, and dumping were performed until the rinse water was clear. Secondary rinsing was performed on individual bags by rinsing the exterior with water (39 degrees C) followed by rinsing through the unsealed opening the interior residue to the bottom of the bag for 1 min (Trials 1 and 2). Secondary rinsing for Trials 3 and 4 was the same as for Trial 1, but with only enough water (39 degrees C) and time to rinse the residue to the bag bottom. Mean estimates of escape protein (EP/CP) were: Trial 1, 27.4 +/- 3.08; Trial 2, 30.8 +/- 3.21; ; and Trial 4, 27.7 +/- 3.06 (non-sieved). Reasonable control of error was accomplished by technician training, including the use of a videotape to clarify the procedure.(ABSTRACT TRUNCATED AT 250 WORDS)

title: Digestion kinetics of alfalfa and wheat straw assuming heterogeneity of the potentially digestible fraction.
abstract: Although it is recognized that the potentially digestible fraction of substrates in the rumen is heterogeneous in composition, most deterministic models that describe ruminal in situ digestion assume existence of only one or two homogeneous fractions of potentially digestible material. Alternatively, a stochastic model based on heterogeneity of substrate has been developed, although its validity has not been determined. To evaluate such a model, alfalfa hay and wheat straw were physically fractionated by dry sieving. Dacron bags containing these fractions were incubated in the rumen of a steer and the residue that remained was analyzed sequentially for DM, NDF, ADF, ADL, and acid detergent insoluble ash. Digestion kinetics of chemical fractions were determined by simultaneous analysis of the digestion profiles of DM, NDF, ADF, ADL, and acid detergent insoluble ash. The weighted mean and variance of the fractional digestion rate constant were .083 h-1 and 4.7 x 10(-4) h-2 for alfalfa and .021 h-1 and 2.4 x 10(-5) h-2 for wheat straw, respectively. In contrast, fitting DM data to a stochastic model assuming heterogeneous rates, the mean and variance of the fractional digestion rate were .078 h-1 and 8.3 x 10(-11) h-2 for alfalfa and .018 h-1 and 5.3 x 10(-12) h-2 for wheat straw, respectively. Because the estimates of the variance approached zero for the stochastic model, it is implied that this model reduced to a model involving a single fractional digestion rate. It is concluded that parameter estimates obtained from a stochastic model do not necessarily have a biological meaning.

title: Influence of rumen fluid and substrate concentration on fermentation kinetics measured with a fully automated time related gas production apparatus
abstract: A new fully automated time related gas production apparatus is presented to study fermentation kinetics in rumen fluid. The newly designed apparatus makes use of electronic pressure transducers in combination with electric valves to release overpressure during the incubation. In this very sensitive system, no gas accumulation and pressure build up takes place. Each valve opening represents a certain amount of gas, set at about 0.7 ml. By recording the number of valve openings in time the kinetics of degradation can be studied. The new equipment was used to study the influence of substrate concentration and source of rumen fluid on fermentation kinetics of grass, corn cob mix (CCM) and corn silage (CS). With the equipment, highly accurate cumulative gas production curves were obtained. In some cases differences between different curves could be made more comprehensive by plotting the rate of gas production against time. The gas production curves were fitted with a multi-phasic model. The shape of the gas production curve was influenced by the chemical composition of the substrate, the ration of the donor animal, the adaptation of the donor animal to a ration, the time of obtaining the rumen fluid from the donor animal and the concentration of the substrate.

title: Computerized monitoring of gas production to measure forage digestion in vitro.
abstract: The techniques reported in this paper were developed to facilitate the study of the kinetics of forage digestion in vitro by measuring gas production. Fiber disappearance as a measure of the reaction rate has been replaced by the use of computerized pressure sensors to monitor the gaseous products (CO2, CH4) of microbial metabolism. The recording system described requires a computer, pressure sensors, an interface card, and appropriate software to monitor gas production continuously. Several variables, including sample size, inoculum size, vessel size, and type of pressure sensor, have been investigated to determine ranges within which gas production can be measured accurately, and the reproducibility of the results has been established. Because this technique uses small (100-mg) samples, a modified NDF method has been introduced that allows determination of extent of digestion at the end of an incubation in which gas production has been monitored. A strong linear relationship existed between NDF disappearance and gas production.

title: Multiphasic analysis of gas production kinetics for in vitro fermentation of ruminant feeds
abstract: Recently developed time-related gas production techniques to quantify the kinetics of ruminant feed fermentation have a high resolution. Consequently, fermentation processes with clearly contrasting gas production kinetics can be identified. Parameterization of the separate processes is possible with a suitable multiphasic model and modelling method. A flexible, empirical, multiphasic model was proposed for parameterization of gas production profiles. This equation was fitted, using a two-step method, to four gas production profiles and the maximum fractional rate of substrate digestion (RM) was calculated for each phase. In the first step, the number of phases and starting values for parameters of the multiphasic model were derived from a combination of either (1) the measured cumulative gas production profile and its first (rate of gas production) and second (change in rate of gas production) derivative or (2) a fitted monophasic curve and the residuals between this curve and the observations. In the second step, the starting values were used to fit di- or triphasic models. Using Approach 1 was complicated by frequent fluctuations in the gas production rate. However, the combined graph of the fitted monophasic curve and residuals (Approach 2) gave good results for every profile. The multiphasic model fitted the profiles well. The robustness of the model declined when the number of phases was increased, underlining the importance of accurate estimations of starting values. It is argued that the model parameters and the calculated RM are useful for feed quality evaluation. The results show that mathematical description of gas production profiles requires a two-step approach and a multiphasic model.

title: Prediction of in situ rumen dry matter disappearance of Ethiopian forages from an in vitro gas production technique using a pressure transducer, chemical analyses or in vitro digestibility
abstract: In each of four experiments, forages (Experiment 1, 20 hays from Napier and Pigeon grass cut at different intervals; Experiment 2, eight leaves and branches from tree lucerne harvested at different intervals; Experiment 3, six hays from Reed Canary grass cut at different intervals; Experiment 4, 15 crop residues from sorghum, tef, wheat and barley) were assessed for in situ rumen dry matter (DM) disappearance, in vitro gas production using a pressure transducer technique (PTT), chemical composition (nitrogen and fibre fractions) and in vitro digestibility (DMD). In situ DM disappearance data were fitted to the model of Orskov and McDonald, i.e. P = ao + bo(1 − exp(− cot)), where P is the DM disappearance at time t, ao and bo are fitted constants and co (h−1) is the fractional rate of DM disappearance. PTT data were fitted to the model of France et al., i.e. 
Y=A(1 − e−b(1−T)−c(t−T)) 
 
 
where Y is gas production after time t, b and c are rate constants, A is the asymptote and T is lag time. In situ DM disappearance parameters were linearly regressed against in vitro gas production parameters, chemical composition and in vitro DMD. Overall co of forages was correlated (P < 0.01) with fractional rate of gas production μ (μ = b + c2√t). co was also correlated with nitrogen, fibre fractions and in vitro DMD. In all experiments, effective in situ DM disappearance (EDMD: DM degraded in the rumen at a given rate of passage) was significantly (P < 0.01) correlated with extent of gas production (at the assumed passage rate from the rumen). EDMD was also correlated (P < 0.01) with in vitro DMD in all experiments, and with chemical composition (N, NDF, ADF, KMnO4 lignin). It is concluded that in situ DM disappearance parameters of forages such as used in the present study, may be predicted from in vitro gas production parameters, and to some extent, also from chemical composition and in vitro digestibility. However, the accuracy of prediction is likely to vary, depending on the type of forages used.

title: Kinetics of fiber digestion from in vitro gas production.
abstract: In vitro gas production, measured by computer-interfaced pressure sensors, was used to follow the digestion of a crystalline processed cellulose, a bacterial cellulose, and mixtures of these substrates by mixed ruminal bacteria. A first-order, substrate limited model (simple exponential with lag) and two bacterial growth models (logistic, Gompertz) were tested to fit these data. No single pool model gave an optimal fit to all substrates, but dual pool versions of both the logistic and Gompertz models fitted the data extremely well. Derivations of these models in the context of gas production are presented. The dual pool version of the exponential model commonly used to analyze fiber digestion was not able to reproduce the slope variations seen with mixed substrates. A modified dual pool logistic equation, with a single lag value, was selected to model the in vitro digestion of these substrates. The model was able to predict adequately both the input composition and the kinetic parameters for a defined mixture and gave a good fit (r2 > .995) to data from all the single and mixed substrates tested. This model may be useful for interpreting gas accumulation from natural feedstuffs.

title: External and internal markers for appraising site and extent of digestion in ruminants.
abstract: Digesta markers are used routinely to calculate fecal output and to estimate kinetics within the digestive tract. A marker suitable for estimating fecal output may not be a suitable kinetic marker because of problems with marker migration, phase separation, inhibition of digestion, osmotic effects within the gut, and quantitation. Marker validity should be checked when possible based on alternative methods (e.g., fecal output and ruminal evacuation). Certain parameters, such as pool size and passage rate, that can be estimated with markers cannot be measured by other noninvasive procedures. Nevertheless, only when marker results are verified by other methods can one evaluate the magnitude of error associated with assumptions inherent in marker mathematics (steady state, instantaneous mixing, and first-order kinetics). When marker results do not meet expectations, marker failure, rather than inadequate knowledge of gastrointestinal function or analytical difficulties, is blamed. No marker is ideal, but research to compare markers is useless if results are not related to direct flow or output measurements. Marker results often are adjusted for marker recovery based on the premise that analysis is precise and that losses are constant. Such adjustment is appropriate only if the error is analytical and proportional. Despite imprecision in marker procedures, inherent variation may be small relative to other sources of variation (e.g., gut physiology, diet, environment, and feed intake). Even though absolute values may be imprecise and inaccurate, marker-based estimates usually provide reliable information about the direction and extent of kinetic changes induced by treatments.

title: The relative resistance to escape of leaf and stem particles from the rumen of cattle and sheep
abstract: In a study of the effect of animal species on the threshold particle size leaving the rumen, two grasses cut at two stages of growth and one mature legume were separated into leaf and stem fractions and fed to cattle and sheep. Samples of rumen digesta and faeces were used to determine the validity of using a 1·18 mm porosity screen to separate the rumen particles into large and small pools when studying escape of particles from the rumen. Samples of rumen digesta and faeces were collected for the determination of particle size by wet sieving and the calculation of resistance of particles to passage from the rumen relative to small particles retained on a 0·15 mm sieve. Particles 0·5 mm had a mean relative resistance to passage of 2·0 and 2·6 for cattle and sheep respectively, compared with resistance values of between 10·9 and 31·2 for particles between 1·18 and 2·36 mm. It is suggested that there is no justification for using different threshold particle sizes for sheep and cattle and that a 1·18 mm sieve may be used to divide the rumen contents of both cattle and sheep into two pools of particles with high and low relative resistance to passage from the rumen.

title: Removal of digesta components from the rumen of steers determined by sieving techniques and fluid, particulate and microbial markers.
abstract: When 103Ru-labelled Tris (1,10-phenanthroline) ruthenium II chloride (103Ru-P) particulate marker in aqueous solution was added to the rumen of four steers given 5.5 kg grass hay/d at two-hourly intervals, the distribution of 103Ru-P marker among rumen particles of various sizes was the same at 4 h, 3 d and 7 d after administration, the concentration of 103Ru-P/g dry matter (DM) was inversely related to particle size and 0.30 of the 103Ru-P was associated with the DM of particles too large to be moved from the rumen at a meaningful rate. Thus, fractional outflow rate (FOR) of 103Ru-P would reflect, but was not a direct measure of, the FOR of the small particle pool in the rumen. When rumen digesta were labelled with 103Ru-P, placed in nylon cloth bags and incubated in vitro with unlabelled digesta, 59% of the 103Ru-P disappeared from the nylon bag in 24 h, and 74% in 48 h. Similar results were obtained when large particles (retained by a 3.2 mm mesh screen during wet sieving) from rumen digesta were subjected to this procedure. In a further experiment, the steers were given the hay in either the long or ground form and drinking water to which 10 g sodium chloride/l were, or were not, added. The FOR of 51CrEDTA in centrifuged rumen fluid was increased (P less than 0.05) from 1.78 to 2.10/d by grinding of the hay diet, but was not influenced by the intake of an additional 257 g NaCl/d. The FOR values of 103Ru-P in mixed rumen digesta and organic 35S in micro-organisms were linearly correlated (P less than 0.05) and were not affected (P greater than 0.05) by grinding and salt treatments. On average, the FOR of organic 35S in micro-organisms was 0.41 of that of 51CrEDTA in centrifuged rumen fluid and 0.85 of that of 103Ru-P in rumen digesta respectively. Grinding of the hay did not (P greater than 0.05) change the proportion of rumen DM (0.476-0.515) or faecal DM (0.107-0.153) retained by the 3.2 mm mesh and larger screens. FOR from the rumen of a given size group of particles was calculated as the ratio, estimated daily flow from the rumen of the size group: rumen pool of the group.(ABSTRACT TRUNCATED AT 400

title: The effect of pelleting on the voluntary intake and digestibility of leaf and stem fractions of three grasses.
abstract: 1. Leaf is eaten in greater quantities than stem of similar digestibility. To determine whether this difference is caused by physical or chemical factors, leaf and stem fractions from Digitaria decumbens, Chloris gayana and Setaria splendida were fed ad lib, to sheep in the chopped and pelleted forms. Pellets were made from leaf and stem which had been ground through a screen with 3 mm holes. All sheep received a protein and mineral supplement. 2. Voluntary intake of chopped leaf was 34 percent higher than that of the chopped stem fraction (40-3 and 30-0 g/kg body-weight 0-75 respectively, P smaller than 0.01) although dry matter digestibility ratios were similar (0-478 and 0-450 respectively, P greater than 0-05). The higher intake of leaf was associated with a larger surface area (13 400 and 5200 mm2/g for chopped leaf and stem respectively), lower bulk density (60 and 180 kg/m3 respectively) and lower neutral-detergent fibre (706 and 724 g/kg respectively), acid-detergent fibre (383 and 413 g/kg respectively) and lignin (42 and 59 g/kg respectively) contents. Chopped leaf was retained in the reticulo-rumen for a shorter time than the stem fraction (19.9 and 26.4 h respectively). 3. Grinding and pelleting increased the voluntary intake of the leaf fraction by 88 percent and the stem fraction by 60 percent. This increased voluntary intake caused by grinding and pelleting was not accompanied by any significant changes in the chemical composition of the diet. Grinding and pelleting reduced the time that the food was retained in the reticulo-rumen and this change appeared sufficient to account for the observed increases in voluntary intake. 4. It was concluded that the higher intake of the leaf fraction of grasses is caused by differences in retention time of food in the reticulo-rumen. These differences in retention time are caused by differences in physical properties and not chemical composition.

title: Effects of feed intake on particle distribution, passage of digesta, and extent of digestion in the gastrointestinal tract of cattle.
abstract: Four nonlactating Holstein dairy cows (means +/- SD BW = 692 +/- 49 kg) were used in a 4 x 4 Latin square design experiment to determine changes in distribution of particles within the ruminoreticulum (RR) and total digestive tract in cattle fed a forage-based diet at four intake levels. Relationships between chewing activities and rates of particle breakdown, passage, and digestion were also determined. Percentage of large particles increased linearly in the dorsal rumen (P = .001), ventral rumen (P = .004), reticulum (P = .007), duodenum (P = .007), and feces (P = .006) as DMI increased. Particles of sizes less than or equal to 2,000 microns, eligible to pass from the RR, constituted 60 to 92% of particle DM in the RR. Increased feed intake resulted in a longer (P = .003) eating but a shorter (P = .008) ruminating time per kilogram of DM intake, whereas total chewing time per kilogram of DMI was not affected (P = .12). Passage rate constants of NDF from the RR increased (P = .03) with intake. There was no change (P = .87) in the rate constant for particle breakdown in the dorsal rumen with increased intake. Neither fractional passage rate of NDF nor the rate constant for particle breakdown was related to time spent eating (P + .12; P = .34) or time spent ruminating (P = .11; P = .55). It was concluded that rate of passage of small particles from the RR was a determinant of passage from the RR and that changes in rates of passage and breakdown of particles could only be partially explained by changes in chewing activities.

title: Altered ruminal mat consistency by high percentages of soybean hulls fed to lactating dairy cows.
abstract: Three ruminally fistulated dairy cows in early lactation were used in a 3 x 3 Latin square design with 3-wk periods to investigate the effects of alteration of ruminal mat consistency on ruminal function and soybean hull rate of passage. The control diet consisted of 60% forage (alfalfa:corn silages, 1:1, dry basis) and no soybean hulls. Replacement of 40% of the silage mixture with soybean hulls decreased ruminal mat consistency by 57% at 6 h postfeeding and reduced rumination activity 52%, ruminal pH 6%, and acetate:propionate ratio observed over 24 h by 10% compared with the control diet. Replacement of 75% of the silage with 25% soybean hulls combined with 20% coarsely chopped alfalfa hay increased ruminal mat consistency by 49% at 2 and 6 h postfeeding, increased the percentage of large particles (> or = 9.50 mm) retained in the dorsal rumen, and significantly increased ruminal pH 1.5% and acetate:propionate ratio by 17% compared with the diet of 25% soybean hulls without alfalfa hay. Increasing the ruminal mat consistency with coarsely chopped alfalfa hay did not significantly decrease the rate of escape of soybean hulls from the rumen, but addition of alfalfa hay increased 4% FCM 4.8 kg/d compared with FCM of diets containing 25% soybean hulls without alfalfa hay.

title: Influence of moisture content of forage on ruminal functional specific gravity and passage of digesta.
abstract: Four crossbred beef steers (392 kg BW) fitted with ruminal cannulas were used in a 4 x 4 Latin square experiment with treatments arranged in a 2 x 2 factorial to evaluate effects of forage species (F) and reconstitution (R) on apparent digestibility, ruminal kinetics, and digesta functional specific gravity (FSG). Dietary treatments were alfalfa (Medicago sativa L.) and timothy (Phleum pratense L.) hays (90% DM) and the same hays reconstituted with water to 38% DM. Apparent DM digestibility was unaffected by R or F. Proportion of ruminal digesta with a FSG of .9 to 1.1 or 1.1 to 1.2 decreased linearly (P < .05) after feeding, whereas the proportion with FSG > 1.2 increased linearly (P < .01). Reconstitution decreased (P < .10) the proportion of ruminal digesta with FSG of 1.1 to 1.2 and increased (P < .10) the proportion with a FSG > 1.2 compared with the hay diets. Reconstitution decreased digesta passage rate for timothy forage; no influence on passage rate for alfalfa was detected (F x R, P < .10). Ruminal DM pool was increased by reconstitution (P < .10). Inert particles of 1-mm length passed from the rumen at a faster rate (P < .05) than 3-mm particles. Inert particles with SG of 1.32 passed from the rumen at a faster rate (P < .05) in the reconstituted alfalfa than in the dry alfalfa diet but at a slower rate in the reconstituted timothy than in the dry timothy diet. These results suggest that factors other than SG of the digesta can have a profound effect on passage of forage particles from the rumen.

title: Interactions among forages and nonforage fiber sources.
abstract: Source, amount, and physical characteristics of dietary forage can interact with nonforage fiber sources and influence ruminal and total tract fiber digestion, passage, and performance of dairy cows fed diets containing substantial nonforage fiber in place of forage. Dietary NDF from forage can be reduced to < = 60% and still provide sufficient amounts of effective fiber for FCM production that is similar to or superior to that with high forage diets. Because of small particle size and high specific gravity, increased ruminal rate of passage may be responsible for lower ruminal NDF digestibility of nonforage fiber sources fed at high dietary amounts. As the amount of soybean hulls increased from 50 to 95% of a pelleted mix for dairy cows, passage rate increased by 8%. In five studies, the digestion of soybean hull diets was improved by the addition of coarse forage. Fiber digestibility might have improved because coarse hay increased ruminal retention time of nonforage fiber sources and allowed more complete digestion. Addition of coarsely chopped alfalfa hay to diets based on silage containing 25% soybean hulls increased ruminal mat consistency by 49% and tended to slow the ruminal escape rate of soybean hulls by 16%. When high percentage of nonforage fiber are fed, the amount of dietary forage is necessarily low, and forage particle size should be adequate to stimulate rumination and entrap small feed particles. The amount and particle size of forage in the diet interacts with the substituted nonforage fiber source to determine the net impact on the rate of ruminal digestion and passage of nonforage fiber.

title: An electronic device to measure the size and degree of packing of the rumen mat in cattle
abstract: An electronic device and method for determining the size and degree of packing of the rumen mat in ruminants are described. The mat meter outputs to a chart recorder the distance travelled and the time elapsed for a small weight to ascend through the rumen contents under the constant force of an external weight. From the chart recordings, the rate of ascent (min 100 cm−1) of the weight can be calculated as an index of the degree of packing of the rumen mat in selected strata of the rumen contents.  Rumen mat, mat meter, mat index, voltage divider, cattle

title: Buoyancy separation of particles of forage, feces, and ruminal contents and nuclear magnetic resonance examination.
abstract: The relationships between buoyancy of soaked particles and chemical composition of bromegrass hay stems and ruminal and fecal particulate matter from a steer fed brome hay were studied. Ruminal fluid from one steer fed at maintenance was put in a cylinder and particles previously soaked in physiological saline were added to the top of this fluid. The cylinder was then emptied from the bottom with each 200-mL stratum being collected separately. The dry matter content of each stratum was determined and used to construct buoyancy curves. The shape of the particles in the different strata was examined and their chemical composition determined by high resolution solid state 13C-NMR. The particles from ground bromegrass hay stems, rumen and feces which sank most slowly were long and thin and contained a high proportion of proteins. In the rumen, the large particles (> 3.35 mm) from the upper stratum, which is the floating mat, appeared to contain more peptides and proteins, and less lignin. The feces particles ...

title: Changes in ruminal and fecal particle weight distribution of steers fed coastal bermudagrass hay at four levels.
abstract: Samples of digesta from the ruminal upper strata (RUS) and feces (F) were taken from four ruminally cannulated steers fed Coastal bermudagrass hay (78% NDF) in the long form to evaluate the effects of feeding level and time postfeeding on particle breakdown. The experimental design was a 4 x 4 Latin square with 18-d periods. Treatments based on previous intakes were set at 50, 70, 90 and 110% (3.1, 4.4, 5.5 and 6.7 kg/d, respectively) of feed consumed per animal and fed at 12-h intervals. Samples were taken at 2, 6 and 12h postfeeding and wet-sieved. Dry matter weight distribution of total recovered particles was used to partition RUS and F digesta among percentages of large (greater than 4.0 mm), medium (greater than 4.0 and less than or equal to 1.0 mm), small (less than 1.0 and greater than or eual to .125 mm) and fine (less than .125 and greater than or equal to .0027 mm) particles. With increasing feeding level, the percentage of medium RUS particles increased linearly (P less than .02), whereas the percentage of fine RUS particles decreased linearly (P less than .01). Increased time postfeeding resulted in a linear decrease in the percentage of large RUS particles (P less than .01), a linear increase in the percentage of small RUS particles (P less than .01) and in a quadratic increase in the percentage of fine RUS particles (P less than .01). Percentage of medium RUS particles remained unchanged. Increasing feeding level resulted in linear increases in mean RUS (P less than .01) and F (P less than .02) particle sizes. Percentages of RUS and F material passing through a 1.0-mm sieve averaged 52.8 and 88.8%, respectively. Animal-to-animal variation in proportions of RUS and mixed reticuloruminal particles was not entirely removed by rumination and was still reflected in the percentages of F particles. Factors such as particle entrapment in the fiber mat, reticular sedimentation, changes in specific gravity, swelling and hydration capacity, reticuloruminal motility and amount of digesta exiting per contraction appear to be more important than particle size reduction in the regulation of the passage of digesta from the reticulorumen.

title: Specific gravity of various feedstuffs as affected by particle size and in vitro fermentation
abstract: Experiments were designed to compare true and functional specific gravity of feed particles and to study the effects of particle size and in vitro fermentation on functional specific gravity. True specific gravity was estimated by centrifuging size fractions of various feedstuffs in ethanol-carbon tetrachloride density gradients. Functional specific gravity was determined in pycnometers. True specific gravity ranged from 1.3 to 1.5 but did not consistently differ with feedstuff or particle size. In contrast, functional specific gravity was affected by feedstuff, particle size (for forages), and in vitro fermentation. Functional specific gravity increased as forage particle size decreased and, for different forages, changed similarly over the in vitro fermentation period. A significant forage by particle size interaction suggested that differences in functional specific gravity between forages were not maintained as particle size changed. Part of the reduction in ruminal retention time achieved by decreasing the particle size of a feedstuff may result from its effects on functional specific gravity. Manipulating the diet to alter the functional specific gravity of particles in the rumen may help to obtain the desired effects on digestion in and passage from the rumen.

title: Fragmentation and flow of grazed coastal Bermudagrass through the digestive tract of cattle.
abstract: Samples of forage fragments were obtained from the upper (RUS) and lower (RLS) strata of the reticulorumen and feces (F) of four Brahman X Jersey steers grazing Coastal bermudagrass (CB) of two maturities with dry matter digestibilities (DMD) of 54.8 and 64.3%. Forage fragments were separated by particle size and evaluated histochemically for tissue type and fragmentation pattern. Fragmentation pattern was similar to that previously observed due to ingestive mastication. There was longitudinal separation of vascular bundles (VB) and severing at VB ends. Microscopically, similar size fragments from RUS were indistinguishable from those of RLS. The major difference between RUS and RLS was the distribution of different size particles. Larger particles were associated with the RUS in cattle consuming immature and mature CB. More large particles were associated with mature compared with the immature CB in the RUS and RLS. The distribution of different size particles in the F was similar for both maturities, suggesting that similar particle size reduction was required regardless of maturity. Smaller particles in the rumen and F appeared to contain more lignin (determined histochemically) and were composed of indigestible fragments of cuticle and lignified vascular tissue. Cattle grazing mature CB had higher ruminal fills (2.40 vs 2.02 kg dry matter/100 kg body weight), reduced rates of passage and lower voluntary intake (2.50 vs 3.14 kg DM/100 kg body weight). Lower intake of mature CB may have resulted from a reduced rate of particle size reduction. Similarities in fragmentation patterns due to ingestive and ruminative mastication were interpreted to indicate that mastication was responsible for most of the particle size reduction of CB and that mastication facilitated digestion of potentially digestible tissues.

title: Ruminal plant cell wall digestibility estimated from digestion and passage kinetics utilizing mathematical models
abstract: Methods to describe the digestion and passage kinetics for estimating ruminal neutral detergent fibre digestibility (RNDFD) with simple mathematical models were evaluated in cattle fed a diet of hay, barley and urea (600, 396 and 4 g dry matter (DM) kg−1 total DM) at two levels of intake. The parameters of digestion kinetics were determined by in situ incubation and rumen evacuation technique. Digesta passage kinetics was estimated from duodenal and faecal Cr and Yb concentrations either from exponential decline in marker concentration or by using two-compartmental models with gamma age dependency in the first compartment. 
 
Increasing the feeding level from 40 to 80 g DM kg−1 live weight0.75 decreased (P < 0.05) the digestibility of DM (0.775 vs. 0.749), DNF (0.758 vs. 0.707) and cell solubles (0.838 vs. 0.824). The amount of total digesta, digesta DM content and the amount of DM in the rumen increased as feed intake increased. The relative increase in rumen pool size of digestible NDF was greater than in that of indigestible NDF (0.72 vs. 0.34). 
 
Using in situ digestion kinetics data and the exponential decline in marker concentration in a simple rumen digestion model yielded RNDFD estimates which were much lower than those based on rumen evacuation data, and only about 0.70 of the total in vivo NDF digestibility. This was partly because of the slower (P < 0.01) rate of digestion observed with the in situ method than estimated from rumen evacuation (0.0458 vs. 0.0607), and partly because of an underestimation of rumen residence time by the passage model. Incorporation of the selective retention of feed particles in the digestion model yielded RNDFD estimates which were 0.13 units higher than those based on exponential marker concentration decline curves. The kinetics parameters for this model were calculated from duodenal marker concentrations using two-compartmental passage models. The RNDFD estimates were 0.94–0.96 of the total in vivo NDF digestibility when the passage kinetics parameters, estimated with the two-compartmental model, were used in combination with the rumen evacuation derived rate of digestion. The values for RNDFD were higher when Cr and faecal sampling were used rather than Yb and duodenal sampling to estimate passage kinetics.

title: Comparison of methods, markers, sampling sites and models for estimating digesta passage kinetics in cattle fed at two levels of intake
abstract: Six ruminally and duodenally cannulated Friesian bulls were used in a switch-back experiment to compare passage models, markers and sampling sites, and duodenal or faecal marker excretion curves and rumen evacuation technique for estimating kinetics of digesta passage. Cr-mordanted and Yb-labelled hay were used as particulate markers and CoEDTA as a liquid-phase marker. The cattle were fed on a dry matter (DM) basis (g kg −1 ) a diet of hay (600), barley (396) and urea (4) at either low or high DM intake (40 and 80 g DM (kg 0.75 live weight) −1 ). Post-duodenal digesta kinetics were estimated by administering the markers (CoEDTA, Cr-mordanted and Yb-labelled duodenal digesta particles) into the duodenum and collecting faecal samples. Digesta kinetic parameters were calculated by linear regression of natural log transformed marker concentrations in ruminal fluid, duodenal digesta or faeces, and by non-linear models with either two age-independent compartments (G1G1) or gamma time dependency in the first compartment (GnG1, n = 2–4). Using the models with gamma age dependency improved the curve fitting which suggested an age-dependent flow. In most cases the error mean squares were smallest with the G3G1 model. Total compartmental retention time (CMRT), transit time (TT) and total mean retention time (TMRT) of the markers decreased as DM intake increased. Regardless of marker, sampling site or model, the relative differences in digesta kinetics between the diets' kinetics were similar. Shorter post-duodenal retention time of markers at high compared to low DM intake resulted from both shorter CMRT and TT. Similar post-duodenal kinetics of Co, Cr and Yb indicate that there was no differential passage of liquid and solid digesta in the caecum and proximal colon. Average CMRT was shorter for Yb than for Cr when derived either from duodenal (57.4 vs. 66.8 h) or faecal sampling (61.1 vs. 73.5 h). Faecal sampling resulted in 3.7 h (Yb) and 6.6 h (Cr) longer CMRT than duodenal sampling. Post-duodenal CMRT was only 0.223 (Yb) and 0.146 (Cr) of the retention time in the fast age-dependent compartment derived from faecal sampling. This suggests that preduodenal sites accounted for the major proportion of the retention time in this compartment. The passage rate estimates derived from rumen evacuation were much slower than those calculated from the exponential decline in duodenal and faecal Cr and Yb concentrations, mainly because rumen evacuation derived passage rate includes the retention time in both ruminal compartments. However, when a simple first-order passage rate was calculated from the passage rate from the two ruminal compartments and rate of neutral detergent fibre (NDF) digestion, the values for indigestible NDF, NDF and digestible NDF obtained from the two methods were fairly similar; rumen evacuation: 0.0165, 0.0126 and 0.0095 h −1 ; Cr: 0.0159, 0.0111 and 0.0085 h −1 ; Yb: 0.0179, 0.0123 and 0.0099 h −1 .

title: Periparturient changes in intake, ruminal capacity, and digestive characteristics in beef cows consuming alfalfa hay.
abstract: Four multiparous, ruminally fistulated Angus x Hereford cows (average BW = 568 kg) were bred to the same bull and used to monitor periparturient changes in DMI, ruminal capacity, and digestion and fermentation characteristics. Cows were individually fed alfalfa hay (16.0% CP, 51.3% NDF) at 130% of the average DMI of the previous 5-d period. Ruminal capacity and DM fill were measured once every 2 wk by removing the ruminal contents from each cow and filling the empty rumen with water. Dry matter intake was measured daily. Ruminal VFA, pH, NH3, and total tract DM digestibility (DMD) were measured during three 7-d periods (average of 58 d before calving, 16 d before calving, and 25 d after calving). Ruminal capacity and DM fill varied in a quadratic manner (P < .01); both were least 6 d before calving. Forage DMI (quadratic effect; P = .02) increased moderately during the prepartum period but increased dramatically after parturition. Although postpartum (d 22) ruminal capacity was only 5% greater than at 61 d before calving, postpartum DMI was 69% greater than DMI measured 61 d before calving. Indigestible ADF (IADF) passage rate changed quadratically with period (P = .01); the greatest IADF passage rate was observed 6 d before calving. Ruminal VFA (mM) also tended to change quadratically (P = .08); the highest concentrations occurred 25 d after calving. The acetate:propionate ratio declined linearly with period (P = .01). Ruminal fluid dilution rate, pH, NH3, and DMD did not change significantly over time.(ABSTRACT TRUNCATED AT 250 WORDS)

title: Variation in and relationships among feeding, chewing, and drinking variables for lactating dairy cows.
abstract: Twelve Holstein cows (63 DIM; 6 primiparous) were offered a common diet and monitored for 21 d (11 d of adaptation, 10 d of collection) with a data acquisition system to measure continuously feed and water intakes and chewing behavior. Objectives were to examine relationships among feeding behavior variables for noncompeting cows producing various quantities of milk and to determine experimental designs with adequate power to detect reasonable treatment differences in future experiments. Coefficients of variation across cows ranged from 5 to 41% for the variables studied. Milk production was correlated positively with DMI and water intake within and across parities. For multiparous cows, production was related positively to meal size (r = .78) and length of eating bouts (r = .75) and unrelated to meal number and eating rate. For primiparous cows, production tended to be related positively to meal number (r = .55) and eating rate (r = .87) and unrelated to meal size. Rumination and total time spent chewing per unit of DMI were correlated negatively (r = -.58) with milk production within and across parities. These correlations suggest that differences exist among cows for chewing efficiency. Reasons why high producing cows consume and chew more effectively deserve further study. Contrast differences of 10% of means for variables examined had an 80% probability of detection with a Latin square design utilizing 12 cows monitored for 5 d.

title: Ytterbium Binding among Particle Size Fractions of Forage Cell Walls
abstract: Cell walls from corn silage, orchardgrass silage, chopped alfalfa hay, and alfalfa haylage were used in two experiments to determine the relative binding of ytterbium among forages and sieve size fractions within each forage. In Experiment 1, ytterbium was added at the rate of 1mg/g cell wall to 4760, 2380, 1190, 600, and μ sieve size fractions of each forage in 30ml distilled water at 25°C for 48h. Mean recovery of ytterbium-169 from cell walls of each forage was 56.2, 34.7, 80.3, and 78.4% and for each sieve size across forage was 48.9, 50.5, 66.3, 72.9, and 73.3%. In Experiment 2, cell walls of each forage were incubated as in Experiment 1, then washed with water and acetone and dry sieved using the same screen sizes as in Experiment 1. Recovery of ytterbium-169 was approximately 20 times lower for corn silage and orchardgrass silage than for alfalfa hay and haylage. Mean specific activity for each sieve size was 7858, 5124, 7142, 9156, and 13506cpm/g cell wall. The increase in specific activity with decreasing sieve size resulted in low estimates of logarithmic mean particle size using accumulative activity as compared to using accumulative weight in a logarithmic normal distribution. Ytterbium should not be used as a marker in experiments of particle size reduction and its use could bias passage rates in experiments of rate of passage.

title: Ytterbium acetate as a particulate-phase digesta-flow marker.
abstract: The ability of ytterbium acetabe (Yb acetate) to fulfil the requirements of a particulate-phase digesta-flow marker in a dual-phase marker system, and of the indigestible acid-detergent-fibre fraction of the feed (IADF) to act as a digesta flow marker, were examined using six mature wether sheep given a diet of dried grass (1 kg dry matter (DM)/d). CrEDTA was continuously infused (240 mg chromium/d) into the rumen of all sheep and Yb acetate was also continuously infused (100 mg Yb/d) into the rumen of three of the sheep. At this level of infusion the equilibrium concentration of Yb in rumen, duodenal and ileal digesta and in faeces could be reliably measured by atomic absorption spectrometry. Estimates of faecal DM excretion based on either Yb or IADF did not differ (P greater than 0.05) from that determined by total collection, whereas estimates based on Cr were significantly (P less than 0.05) lower. Urinary excretion accounted for 3.1% of the infused Cr but no Yb was detected in urine. Estimates of ileal DM flow, assuming total marker recovery, were similar (P greater than 0.05) with all three markers, whereas the estimate of duodenal DM flow based on IADF was lower (P less than 0.05) than the estimates based on either Cr or Yb. Compared with the infusion of Cr alone, the infusion of Cr and Yb had no effect (P greater than 0.05) on nutrient flows at the duodenum, ileum and in faeces nor on microbial degradative activity volatile fatty acid production and N metabolism in the rumen. Polyester bag and in vitro studies showed that pre-labelling the dried grass with up to 285 mg Yb/g DM did not affect its susceptibility to microbial degradation. The Yb in rumen, duodenal and ileal digesta was predominantly (greater than 90%) associated with the particulate matter but was not uniformly distributed and its concentration increased as particle size decreased. The use of CrEDTA and Yb acetate as a dual-phase marker system proved more reliable in estimating 'true' duodenal flow than the use of the individual markers when the digesta sample was unrepresentative.

title: Evaluation of rare-earth markers using an in vitro ruminal fermentation system and effect of processing method on ruminal turnover of sized corn particles.
abstract: A two-stage in vitro ruminal fermentation experiment was conducted to determine the extent of marker migration and the effects of marker addition on digestibility. Fermentation studies were terminated either at the end of stage I, the fermentation phase, or stage II, the pepsin digestion phase. Treatment diets contained ytterbium-marked corn plus chopped Coastal bermudagrass hay and nonmarked corn plus chopped hay. After stage I of the in vitro ruminal fermentation, 90% of ytterbium was recovered. Of this, 8.8% of the ytterbium had migrated to the hay, 2% was recovered in the supernatant and 89.2% remained bound to the corn. After completion of both stages, 72% was recovered, of which 7.9% was bound to corn, 33.6% was bound to the hay, 1.1% was recovered in the stage I supernatant and 57.4% was associated with the acid pepsin supernatant. Dry matter digestion was not affected after stage I but after stage II, digestibility was decreased 7% due to marker addition. A ruminal-cannulated Holstein steer weighing approximately 680 kg was used to determine turnover rates from the rumen of liquid and five different particle sizes of corn. Corn from each of two processing methods was fractionated into five particle sizes by dry-sieving. The corn particles included steam-rolled corn (SRC) retained on a 4-mm sieve and marked with dysprosium, SRC retained on a 2-mm sieve and marked with erbium, cracked corn (CC) retained on a 4-mm sieve and marked with yttrium, CC retained on a 2-mm sieve and marked with ytterbium, and CC retained on a 1-mm sieve and marked with samarium.(ABSTRACT TRUNCATED AT 250 WORDS)

title: Automated Long-Term Total Collection Versus Indicator Method to Estimate Duodenal Digesta Flow in Cattle
abstract: Four steers, fitted with re-entrant duodenal cannulas, were used to compare digesta flow as measured by automated total collection (ATC) with flow estimations based on chromium oxide (Cr z O s ) and lignin concentrations of representative samples of 24-h collection of digesta. In two successive periods, each steer was fed about 4 kg/d of either an all-forage or an 80% sorghum grain diet. Duodenal digesta samples were automatically taken and pooled every 2 h, during the 3to 6-d total collections. For animals adapted to the collection procedures, there was no consistent evidence of flow inhibition in the first 24 h of a long-term total collection. Flow rates of duodenal digesta and dry matter (DM), estimated by Cr20 s and lignin were 15% greater and more variable than those measured by ATC. Digesta flow rates measured by ATC averaged 65 iiters/d in steers fed the forage diet and 42 liters/d in the same steers receiving the grain diet. Apparent ruminal DM disappearance, calculated from direct measurements by ATC was 44% for the grain diet and 37% for the forage diet. Ruminal DM disappearance, based on Cr 20 s and lignin, was about 25% lower than the ATC values. The data indicate that the use of total collection procedures should include replication over days. Duodenal flow rates suggested a 24-h cyclic pattern of digesta flow for the grain diet, with a peak about midnight followed by a period of low flow before the onset of the light hours. Chromium oxide showed a similar flow pattern, but flow rate decreased more rapidly following the midafternoon peak. With the forage diet, 24-h patterns were only observed for Cr~O s and lignin concentrations in duodenal digesta, which appeared different from the Cr: O s concentration pattern with the grain diet. Diurnal variation of digesta flow was greater than day-to-day variation. Because of the diurnal variation, spot-sampling techniques must be considered very carefully, since inadequate sampling could easily misrepresent total collection samples and flow estimates. The data also suggest caution in the use of lignin as a marker in grain diets, and of Cr z O s as a marker in forage diets for partitioning digestion in the duodenum of ruminants. (

title: Particulate Rumen Turnover Rate Measurement as Influenced by Density of Passage Marker
abstract: Alterations of estimates of ruminal turnover rate by density, alfalfa fiber chromium concentration, or sampling site were evaluated. Alfalfa fiber density was raised by mordanting with increasing chromium concentrations. Three Holstein cows were fed for ad libitum intake equal portions of a total mixed diet three times a day. After a pulse dose of marker, fecal and ruminal grab samples were collected from cows at scheduled intervals for 5 days. Linear regression analysis was used to measure slopes of declining phase of marker excretion curves. Cell wall densities (g/ml) and ruminal turnover rates (h -1 ) based on ruminal and fecal sampling sites were: 1.21, .0116, .0236; 1.33, .0525, .0306; and 2.08, .0392, .0485 for alfalfa cell wall with chromium concentrations of 7.01, 38.67, and 56.76 mg/g. Particle density and chromium concentration, but not sampling site, influenced estimates of ruminal turnover rate.

title: Evaluation of Different Chromium-Mordanted Wheat Straws for Passage Rate Studies
abstract: Samples of untreated and ammonia-treated wheat straw were mordanted with sodium dichromate after being chopped, ground, or ground and washed in neutral detergent solution. The NDF content of the mordanted samples increased to 97 to 100% DM in those ground or ground and neutral detergent-washed, whereas the increase was lower (92% DM) in those chopped. Chromium content after mordanting was 4.0% DM in the samples washed in neutral detergent solution and 2.4 to 2.6% DM in those simply chopped or ground. There were no major differences due to ammonia treatment. Particle size distribution of ground samples showed significant differences in proportions of particles larger than 1mm with control samples less than ground and mordanted samples less than ground, neutral detergent-washed, and mordanted samples. There also were variable and significant effects of these treatments on functional specific gravity. Rumen DM disappearance after 48-h incubation in fiber bags was reduced greatly by mordanting but was negligible only with the previous neutral detergent treatment. The estimated rumen passage rates of mordanted samples of untreated straw differed significantly among treatments with chopped samples less than ground samples, which were less than ground and neutral detergent-washed samples, reflecting their differences in particle size and functional specific gravity. It was concluded that the mordanting procedure can affect greatly the physicochemical properties of feedstuffs, thus altering their passage rates.

title: Kinetics of hydration and functional specific gravity of fibrous feed by-products.
abstract: Five studies were conducted to evaluate the kinetics of digestion, hydration, and functional specific gravity (FSG) of various feed by-products (FBP) in vitro. The water-holding capacity (WHC) of alfalfa and orchardgrass (1.428 and 1.005 g/g of insoluble DM [IDM], respectively) was higher (P < .05) than the WHC of FBP, which ranged from .175 for distillers grains to .481 g/g of IDM for brewers grains pellets. Rate of hydration was the highest in brewers grains pellets and beet pulp (.215 and .252 min-1, respectively), whereas the lowest hydration rates were observed in orchardgrass, corn cob pellets, and soyhulls (.055 to .066 min-1). Loss of associated gasses from feed particles fermented in vitro increased (P < .05) their FSG when the contents of incubation tubes were transferred to pycnometers, compared with that when the incubation was carried out directly in the pycnometers (1.17 vs 1.13) to determine their FSG. Gas produced during fermentation delayed the increase in the FSG of all sources of brewers grains and beet pulp, corn gluten feed, distillers grains, orchardgrass, alfalfa, and wheat middlings but not of corn cob pellets, cottonseed hulls, and soyhulls. Averaged across hours of incubation, the FSG of FBP (except beet pulp) was either higher (P < .05) or tended to be higher than that of alfalfa and orchardgrass. Particle size of FBP did not influence FSG during fermentation in vitro. The WHC and FSG of feeds may be helpful in predicting the rate of passage of feeds through the rumen.

title: Effect of microbial fermentation on functional specific gravity of small forage particles.
abstract: Two experiments were designed to determine the effect of gas production during in vitro digestion on functional specific gravity (FSG) of forage particles. In Exp. 1, FSG of ground alfalfa hay decreased from 1.123 to 1.049 between 3 and 9 h of incubation and increased thereafter to reach a plateau at 1.309 after 30 h of incubation. Gas production peaked at 6 h, but gas associated with particles increased until 9 h of incubation. Gas associated with solid residue was correlated to gas production (r = -.67) but also was influenced by gas holding capacity and rate of escape from the particles. In Exp. 2, measurements were performed on ground alfalfa hay, alfalfa silage, and bromegrass hay containing 42.6, 35, and 66.4% NDF, respectively. Gas production seemed to be related to the amount of readily available substrate. Although at 9 h of incubation more gas was produced by alfalfa silage (.235 mL.min-1.g of DM-1) than by bromegrass hay and alfalfa hay (.087 and .187 mL.min-1.g of DM-1, respectively), gas associated with particles was greater for alfalfa hay (.416 mL/g of DM) than for bromegrass hay and alfalfa silage (.256 and .281 mL/g of DM, respectively). The increase in FSG was more rapid for alfalfa silage than for the hays. After 27 h of digestion, gas associated with particles (milliliters per gram of DM) and FSG were .164, 1.226; .147, 1.235; and .001, 1.467 for bromegrass hay, alfalfa hay, and alfalfa silage, respectively. Gas produced during fermentation delayed the increase in specific gravity of forage particles.(ABSTRACT TRUNCATED AT 250 WORDS)

title: A quantitative model of reticulo-rumen particle degradation and passage.
abstract: Labelled particles were prepared by mordanting low concentrations (0.1 or 5 g/kg dry matter) of chromium to neutral-detergent-extracted stems (1-2 mm or 10 mm in length) of bromegrass (Bromus inermis). These were used in the study of reticulo-rumen particle kinetics of four steers given bromegrass hay and from the results a quantitative model of particle digestion and passage was developed. At the 0.1 g Cr/kg concentration there was minimal interference with digestibility of the feedstuff. The ratio, dry weight of the reticulo-rumen large-particle pool (greater than 3.35 mm): small-particle pool (less than 3.35 mm) was 2:1. It was derived from the model that volatile fatty acids (VFA) and carbon dioxide in the rumen were produced mainly from large particles, and that between 500 and 700 g/kg hay dry matter was digested in the reticulo-rumen. It was also derived from the model that a major portion, 200 (SE 110) g/kg, of the hay dry matter was rapidly solubilized and that the material leaving the reticulo-rumen was composed of small particles (500-840 g/kg), large particles (100-160 g/kg) and an unknown portion of soluble dry matter of hay (0-400 g/kg). Disappearance from the large-particle pool in the model involving the lowest Cr level was directed to formation of VFA and CO2 (0.68 (SE 0.04) of total flow) to the small-particle pool (0.25 (SE 0.06) of total flow) and direct passage from the reticulo-rumen (0.07 (SE 0.002) of total flow). The disappearance from the small-particle pool was to VFA and CO2 production and to the omasum accounting for 0.14 (SE 0.18) and 0.86 (SE 0.24) respectively, of the total flow. It was concluded that the low-level-mordanting technique in combination with appropriate sampling yielded a realistic quantitative description of forage breakdown and movement processes in the digestive tract of cattle.

title: Changes in forage quality, ingestive mastication, and digesta kinetics resulting from switchgrass maturity.
abstract: Five maturities of switchgrass hay harvested at 14-d intervals (vegetative through 20% heading) were fed to Hereford steers (297 kg) in a 5 x 5 Latin square. Relationships with switchgrass maturity were negative and quadratic (P < .05) for DMI and cubic (P < .05) for digestible DMI. Declines in apparent digestibilities of DM, ADF, and cellulose were cubic (P < .05), whereas these were quadratic (P < .05) for NDF, hemicellulose, and CP. Whole masticates from the least, mid, and most mature hays showed linear (P < .05) declines in DM concentration and IVDMD with increasing maturity, whereas NDF concentrations increased linearly (P < .05). Mean retention time of gastrointestinal DM increased linearly (P < .01) from 64 to 94 h from the least to the most mature hay, and the associated rate of passage declined linearly (P < .01) from 3.3 to 2.1%/h. Sieving of masticate DM showed a reduced proportion of large particles (> or = 2.8 mm) and an increased proportion of small particles (< or = .5 mm) with advancing forage maturity. More than 94% of the sieved fecal DM passed a 1.0-mm sieve, but particle sizes showed the same relationship with forage maturity as noted for masticate DM. This occurred despite the comminution from the rumination and digestive processes.

title: Theoretical considerations in the estimation of rumen fractional outflow rate from various sampling sites in the digestive tract.
abstract: The digesta markers 103ruthenium phenanthroline (Ru-P) and 51Cr-EDTA were continuously infused into the rumen of young lambs offered clover hay ad lib. indoors or grazing pure species swards of either lucerne (Medicago sativa), white clover (Trifolium repens), ryegrass (Lolium perenne) or prairie grass (Bromus catharticus). Following the cessation of infusion the fractional outflow rate (FOR) of markers from the rumen was estimated from the rate of decline of marker concentration in rumen and abomasal digesta (Expt 1), abomasal digesta and faeces (Expt 2) and duodenal digesta and faeces (Expt 3). Estimates of FOR from the rumen were similar when based on rumen and abomasal sampling but estimates from abomasal and duodenal sampling were significantly higher than those from faeces. A simulation of a two-compartment model with variable FOR was performed to calculate faecal excretion curves following continuous marker infusion or the administration of a single marker dose into the rumen. Analysis of the descending portion of these theoretical faecal excretion curves yielded values of FOR from the rumen which differed from the values entered in the equations. The magnitude of the difference depended on the ratio of the values of FOR from the caecum and rumen entered in the equation. This suggested that the experimental variation observed in these young lambs was caused by the high FOR from the rumen in relation to the FOR from the caecum-large intestine.

title: Ruminal evacuation's effect on microbial activity and ruminal function.
abstract: The influence of evacuating, mixing and returning ruminal contents on microbial populations, volatile fatty acid (VFA) concentrations and liquid flow rate was investigated with four ruminal-cannulated Hereford steers (247 kg avg wt). Ruminoreticular contents were sampled, then completely removed, mixed for 5 min and returned to the rumen. Subsequent samples were taken immediately, 1 h and 4 h later. Non-evacuated steers were sampled at identical time intervals either 1 d before or after evacuation. Averaged over time, there was no significant difference between evacuated and non-evacuated steers in total anaerobic, cellulolytic and facultative bacteria, protozoa, oxidation-reduction potential, VFA concentrations, and liquid flow rates. There were no treatment X time interactions and, except for holotrich protozoa and VFA, no differences from time of sampling. Ruminal evacuation does not appear disruptive to anaerobiosis or detrimental to ruminal microorganisms and digestive processes.

title: EFFECT OF RATE OF INTAKE ON RUMINAL RATE OF PASSAGE OF SMALL FORAGE PARTICLES
abstract: Circadian rate of intake and rumen parameters were studied in three rumen-fistulated steers fed long timothy hay once daily at 13:00 h. A sinusoidal model was used to relate rate of intake (INTAKE, kg h−1) (Y) to the rate of passage of small particles from the rumen. Parameters of the model Y = χ0 + χcos (time + θ) were the fitted mean intake at zero time (χ0), the fluctuation of intake (χ) and the phase. INTAKE was recorded seven times daily for the duration of the experiment. At the end of each week, fluctuations in individual INTAKE were regressed against recording hour over 24 h. The 7-wk experiment was divided into three periods. In the first period, fecal particles were examined. Small particles (  0.05) between grab samples of feces collected in the morning (07:00 h) and evening (19:00 h). In the second period consisting of 22 d, all rumen contents (54–123 kg) were removed at eight different times of the day, sampled and retu...

title: Ability of mathematical models to predict faecal output with a pulse dose of indigestible marker.
abstract: The aim of the work was to compare the faecal output and digestibility estimated by two mathematical approaches with the actual amount of faeces excreted or feed digested by Simmental cows. Experimental data (intakes and digestibility measured over 5 d) and faecal Cr concentrations (measured at 0, 4, 8, 12, 16, 24, 32, 48, 56, 72, 96, 120 and 144 h after a pulse dose of Cr-mordanted forage) were collected from published experiments and fitted to a multicompartmental (MC) model and a gamma age-dependent (AD) model. From a statistical point of view, the MC model was very satisfactory while the AD model produced lower r2 and higher SE values and reached satisfactory statistical values only for higher DM intakes (lactating animals). The MC model produced higher correlations with the digestibility values while the AD model generated better correlations with the intake data; DM intake and digestibilities were more highly correlated with the model's parameters than neutral-detergent fibre terms. The regression between the estimated faecal outputs obtained with the two models showed an intercept close to 0 (P > 0.05) and angular coefficients near 1; there was a good correspondence of the estimates especially for the lowest values of output. The r2 values of the regressions were 0.800 and 0.829 for the MC and AD models respectively and their SE were 2.93 and 2.63. The ability of the two models to predict faecal output and digestibility was very similar, independent of the statistical accuracy of fitting the Cr-concentration data. The results indicate that variation of Cr concentration is the result of the entire digestive process, i.e. dilution and passage, which interact in a competitive or associative way.

title: Fluid and particulate retention times in sheep as influenced by intake level and forage morphological composition.
abstract: Objectives of this study were 1) to measure small-particle and liquid mean retention time (MRT) of 12 grass hays similar in NDF (61.3 +/- 1.9% NDF) but differing in morphological composition and to relate passage rates to proportions of blade, sheath and stem and 2) to evaluate the influence of MRT of small and large blade and stem fractions and large sheath fractions, in addition to morphological composition, on intake and digestibility of the 12 hays. In each of two periods, 24 sheep (wethers) were offered one of 12 hays at three consecutive levels of feeding: (L1) ad libitum, allowing 15% refusal; (L2) restricted to 100% of hay consumed ad libitum by an individual wether during L1; and (L3) 1.8% of BW on a DM basis. Hays offered included two sorghum-sudan, four barley, four oat and two pearl millet. Marked particles were pulse-dosed in L2 and L3. Large and small particles of stem and leaf were extracted with neutral detergent and marked with Cr or rare earth metals. Marked large and small stem particle MRT generally were longer (P less than .05) than those of corresponding large and small blade particles. Large sheath particles generally had an intermediate (P less than .05) MRT between those of large stem and blade particles. Mean retention times of marked fractions were lower (P less than .05) in L2 than in L3. Small particle MRT in L2 was longer than MRT for liquid, though correlations were high (r = .74 to .86, P less than .01). Large particles were retained longer than small particles. Particle size, morphology, and percentage of stem in the forage influenced the mean retention time. Use of representative samples of all fractions fed to measure retention times may lead to a better understanding of ruminal function and to improvements in explanations of intake regulation, because marked fractions do not behave identically for all forages.

title: INFLUENCE OF AMMONIATION OF, HIGH-MOISTURE BARLEY ON DIGESTIBILITY, KINETICS OF RUMEN INGESTA TURNOVER, AND MILK PRODUCTION IN DAIRY COWS
abstract: Three dairy cows with large rumen cannulae were fed totally mixed diets twice daily. Diets contained 49% whole crop oat silage, 38.5% high-moisture barley (HMB), and 12.5% protein-mineral-vitamin supplement on a dry matter (DM) basis. HMB had been ammoniated at target levels of 0, 0.65, 1.30, and 1.95 g per 100 g HMB DM. Actual levels achieved were 0, 0.63, 1.30, and 1.95 g ammonia per 100 g. Apparent digestibility of organic matter (OM), neutral detergent (ND) fiber, starch, and N were not significantly influenced by level of ammoniation of HMB, although there was a trend for increased N digestion. Total rumen pool sizes of wet ingesta and DM were not influenced by ammoniation levels of HMB, although proportion of ND fiber in rumen OM declined as level of ammoniation of HMB increased. Milk yield and production of milk protein and lactose increased as level of ammoniation of HMB increased. Data are interpreted to suggest that rumen digestion of dietary nonfiber components declined as level of ammoniation ...

title: Markers for quantifying microbial protein synthesis in the rumen.
abstract: Measurement of ruminal microbial protein is necessary to quantify ruminal escape of dietary protein and microbial yields. Microbial markers used most widely have been the internal markers, diaminopimelic acid and nucleic acids (RNA, DNA, individual purines and pyrimidines, or total purines), and the external isotopic markers (e.g., 15N and 35S). Combined with digesta flow markers in ruminally and abomasally or intestinally cannulated ruminants, microbial yields can be estimated. An ideal marker system must account for both the bacterial and protozoal pools associated with both the fluid and particulate phases of digesta. No marker has proven completely satisfactory; hence, yield estimates are relative rather than absolute. Total purines represent robust microbial markers that should be adaptable by most investigators. Principal concerns about total purines relate to unequal purine: N ratios in protozoal and bacterial pools and to the need to assume that dietary purines are completely degraded in the rumen. A theoretically sounder, but more costly, method is continuous intraruminal infusion of 15N ammonium salts. However, 15N enrichments of bacterial and protozoal pools are not equal, so the basis for calculating microbial yield in faunated ruminants is uncertain. Urinary purine excretion may prove to be a noninvasive method for estimating microbial protein yields in intact dairy cows.

title: A comparison of markers for estimating magnitude of rumen digestion.
abstract: 1. Two experiments were carried out with sheep and cattle receiving high-concentrate rations to estimate the amount of digestion taking place in the rumino-reticulum and omasum. Two techniques were used for the collection of samples of abomasal contents. In the experiment with sheep, animals were slaughtered at different times after feeding and samples of digesta were collected. In the experiment with cattle, samples were collected through an abomasal fistula at 2 h intervals throughout the entire 24 h. Chromic oxide powder was incorporated into the ration. 2. Digestion of DM in the rumen estimated by the chromic oxide ratio ranged from 36 to –7%. Estimates based on the lignin ratio ranged from 57 to 68%. Estimates based on lignin as the marker were more consistent within experiments. 3. Starch digestion in the rumen based on the chromic oxide ratio ranged from 56 to 92% while estimates based on the lignin ratio ranged from 89 to 96%. The amount of starch digested, according to chromic oxide ratios, was up to 500 g more than estimated organic matter digested in sheep, and up to 2 kg more than organic matter digested in cattle, both impossible results. Estimates based on lignin were always less than estimated dry-matter digestion. 4. In view of the untenable results obtained by using chromic oxide powder mixed in the ration as an indigestible marker, while lignin in the same samples always yielded credible results, it appears that chromic oxide is not always a suitable marker for estimating rumen digestion from abomasal samples.

title: An assessment of chromic oxide as an indigestible marker for digestion studies in sheep
abstract: Fistulated cross-bred sheep were given, by means of continuous feeders, concentrate diets containing chromic oxide (Cr 2 O 3 ). Dry-matter digestibility coefficients predicted from the faecal concentration of Cr 2 O 3 were compared with those obtained by total collection of faeces, and the concentration of Cr 2 O 3 in spot samples of faeces taken at different times of day was compared with that in faeces bulked over an 8-day collection period. The rate of flow of digesta from the abomasum and the partition of digestion between the stomach and intestines, calculated from the concentration of Cr 2 O 3 in samples of abomasal digesta, were compared with values obtained using two markers, one of which remained in solution while the other was adsorbed by particulate matter. Recovery of Cr 2 O 3 in the faeces ranged from 91 to 101% and predicted dry-matter digestibilities were similar to those obtained by total collection. There was no diurnal variation but there was marked random variation in the concentration of Cr 2 O 3 in spot samples of faeces. Cr 2 O 3 was shown to behave independently of the particulate matter in digesta and, as a single marker, it significantly underestimated the flow of digesta from the abomasum relative to the double marker method. Cr 2 O 3 thus overestimated digestion in the stomach and underestimated digestion in the intestines. It was concluded that Cr 2 O 3 was not a satisfactory marker for studies of the flow of digesta and hence the partition of digestion within the gastro-intestinal tract when digesta samples are taken from a simple cannula.

title: Influence of niacin supplementation on in vivo digestibility and ruminal digestion in dairy cows.
abstract: The effect of niacin supplementation on digestion was investigated using four cows in a crossover design. After peak lactation, cows received a diet of corn silage (55%) and concentrates (45%), including a formaldehyde-treated mixture of soybean meal, rapeseed meal, and urea. This diet was either supplemented with niacin (6 g/d) or not supplemented. Total tract apparent digestibility of OM and fiber and ruminal digestibility of OM were not modified by treatment. Niacin supplementation enhanced the theoretical degradability of ruminal DM in situ (44.7% vs. 40.6%) and concentration of protozoa in ruminal fluid (459 vs. 311 x 10(3)/ml), especially Ophryoscolecidae. The percentage of butyrate in the VFA mixture of ruminal fluid was increased by niacin supply, but acetate and propionate percentages and total VFA concentration did not vary. Ruminal digesta, ruminal pools, and ruminal kinetics were not affected by treatment. Duodenal flow of nonmicrobial N tended to increase with niacin supplementation. Duodenal flow of microbial N did not vary, as measured using a microbial sample, for fluid-associated bacteria or for a mixture of fluid-associated and particle-associated bacteria. These results are discussed in relation to the characteristics of the diet.

title: Effect of protein source on ruminal fermentation and passage of amino acids to the small intestine of lactating cows.
abstract: Four lactating Holstein cows, fitted with T-type cannulas in the proximal duodenum, were used in a 4 x 4 Latin square design to determine the effect of protein supplement on production, ruminal digestion, and profile and the quantity of AA available for absorption. Supplemental protein sources were blood meal, corn gluten meal, blood meal plus corn gluten meal, and sunflower meal, which constituted 8, 10.5, 9.3, and 13% of dietary DM, respectively. The DMI and milk production were not influenced by treatment. Ruminal NH3 concentration increased with the sunflower meal diet, and molar percentage of propionate decreased with the blood meal diet. Duodenal flow of NAN was unaffected by protein source, but dietary N flow decreased, and bacterial N flow increased, when cows were fed the sunflower meal diet. Bacterial N flows were 46.0, 45.3, 46.4, and 65.8% of NAN for the respective diets. The AA profiles of isolated ruminal bacteria differed among dietary treatments but were not correlated with the respective supplementary protein sources. The essential AA profiles of duodenal digesta and duodenal flow of individual AA closely reflected AA differences in protein sources, suggesting that the composition of RUP profoundly affected the composition of protein entering the intestine when supplemental protein provided 35% of total CP intake.

title: Ruminal, intestinal, and total digestibilities of nutrients in cows fed diets high in fat and undegradable protein.
abstract: To study relationships of high undegradable intake protein and dietary fat on intestinal AA supply, the ruminal, intestinal, and total digestibilities of diets with or without added fat (5% of DM) and animal protein (blood meal: hydrolyzed feather meal, 1:1; 8% of DM) were examined with four cows in a 2 x 2 factorial design in a 4 x 4 Latin square experiment. Ruminal degradabilities were 14.9 and 18.6%, and intestinal digestibilities were 98.9 and 68.3%, respectively, for CP in blood meal and feather meal. Treatment effects on ruminal digestibilities were small. Protein supplementation increased total N intake by 29%, duodenal AA N flow by 39%, and AA N absorbed by 37%; absorption of Leu and Lys increased 60 and 33%, and absorption of Ile and Met increased 11 and 7%, respectively. Measured duodenal AA N flow (Cr2O3 marker) was 33% higher in cows cannulated adjacent to the pylorus compared with cows cannulated 100-cm distal to the pylorus, but only when cows were fed protein-supplemented diets; the estimates from those diets caused calculated microbial protein efficiency to exceed theoretical values. We postulated that blood meal and feather meal segregated near the pylorus, yielding high estimates of duodenal AA N flow. Removal of data for protein-supplemented diets obtained from cows cannulated at the pylorus yielded estimates of microbial protein synthetic efficiency consistent with literature values. Microbial synthesis of AA N was related linearly to ruminal digestion of carbohydrate. Location of intestinal cannulas may influence accuracy of nutrient flow estimates.

title: Effects of fat saturation and source of fiber on site of nutrient digestion and milk production by lactating dairy cows.
abstract: Six primiparous cannulated cows were assigned to six treatments in a 6 x 6 Latin square design to evaluate the effects of degree of fat saturation and amount and source of effective fiber on site of nutrient digestion and milk production. Cows were fed for ad libitum intake a control diet with no added fat or diets with 5% added fat from saturated tallow, tallow, or animal-vegetable fat; the diets with animal-vegetable fat had 40% forage, 40% forage plus 20% soyhulls, or 60% forage. Ruminal acetate:propionate was higher when soyhulls replaced forage NDF because of the higher digestibility of soyhulls in the rumen and total tract. Ruminal digestion of NDF was decreased as unsaturation of fat increased. True and apparent efficiencies of bacterial protein synthesis were increased as fat unsaturation increased, probably because of reduced recycling of microbial N in the rumen. The digestibility of fatty acids in the small intestine was higher in cows fed no fat than in those fed fat and was reduced as fat unsaturation decreased, primarily because of the saturated tallow. Dry matter intake was decreased 14% by increased fat unsaturation. All fat supplements depressed milk protein percentage. Production of 4% FCM tended to decrease linearly as unsaturation of fat increased. In diets with 5% added fat, a degree of saturation between saturated tallow and tallow (iodine values of 18 and 62) appears to be optimum for fatty acid digestibility and DMI.

title: Influence of amount and degradability of dietary protein on nitrogen utilization by dairy cows.
abstract: Four Holstein cows fitted with ruminal and T-type duodenal cannulas were utilized in a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. The TMR contained 25% alfalfa haylage, 25% corn silage, and 50% concentrate and provided either 16.4 or 19.6% CP, with ruminal degradability calculated to be 30 or 45%. Intakes of DM, OM, ADF, NDF, and N were not altered by either amount or degradability of CP. Intake and ruminal and postruminal digestibility of starch were greater when cows were fed diets high in undegradable CP but was not altered by amount of CP. Apparent total tract digestibilities for DM, OM, starch, ADF, and NDF were similar among treatments. Apparent total tract digestibility of N was 4.7 percentage units greater for diets low in ruminally degradable CP. Apparent digestibility of OM, ADF, and NDF and true digestibility of OM in the rumen were not altered by amount of CP or undegradable CP. Increasing the CP content of the diet and the proportion of undegradable CP in the diet increased NAN flow to the duodenum. Except for Met, flows of all AA to the duodenum were increased when CP was increased. Flow of Met to the duodenum was not altered by undegradable CP content of the diet. Production of milk, 4% FCM, and milk CP was not altered by amount of CP or undegradable CP. Milk fat content and yield were increased when diets high in undegradable CP were fed. Results suggest that all diets supplied adequate amounts of AA for these cows or that Met was deficient for all cows.

title: Omasal sampling technique for assessing fermentative digestion in the forestomach of dairy cows.
abstract: A procedure allowing digesta sampling from the omasum via a ruminal cannula without repeated entry into the omasum was developed. The sampling system consisted of a device inserted into the omasum via the ruminal cannula, a tube connecting the device to the ruminal cannula, and a single compressor/vacuum pump. Eight cows given ad libitum access to a total mixed diet were used in a crossover design to evaluate the effects of the sampling system on digestive activity, animal performance, and animal behavior. Results indicated that the omasal sampling system has minimal effect on normal digestive and productive functions of high-producing dairy cows. Dry matter intake was reduced (24.0 vs 21.8 kg/d; P < .02) and seemed related more to the sampling procedures than to the device in the omasum. Observations of animal behavior indicated that cows with the sampling device were similar to control cows, although rumination and total chewing times were reduced slightly. The composition of digesta samples was biased toward an over-abundance of the liquid phase, but using a double-marker system to calculate digesta flow resulted in fairly small coefficients of variation for measurements of ruminal digestion variables. This technique may prove useful for partitioning digestion between the fermentative portion of the forestomach and the lower gastrointestinal tract. The omasal sampling procedure requires less surgical intervention than the traditional methods using abomasal or duodenal cannulas as sampling sites to study forestomach digestion and avoids potentially confounding endogenous secretions of the abomasum.

title: Comparison of nitrogen-15 and purines as microbial markers in continuous culture.
abstract: Eight dual-flow continuous-culture fermenters were used in four replicated periods to compare the effects of diet and microbial marker on estimates of N metabolism in continuous culture of ruminal microorganisms. A basal diet was supplemented with urea and tryptone, soybean meal (SBM), lignosulfonate-treated SBM, corn gluten meal, blood meal (BM), hydrolyzed feather meal, fish meal (FM), or meat and bone meal (MBM). Microbial protein flow and protein degradation in fermenters were estimated using purines, purine N, and 15N in bacteria obtained from fermenter flasks or from the effluent. The ratio of purine N to total N in bacteria averaged .083 and was not affected (P > .05) by treatment. Dietary purine content (percentage of DM) ranged from .033 in BM to .084 in FM. Escape of feed purine N (percentage of total purine N flow) averaged 1.7% (SE = 2.9) and was not different (P > .05) among treatments. Bacterial N flows obtained using purines were more variable than estimates obtained using 15N. Bacterial N flows calculated using 15N in bacteria isolated from fermenters were more variable than those obtained using bacteria isolated from the effluent. The use of purines as a microbial marker resulted in lower estimates of protein degradation and smaller differences among treatments compared with use of 15N. Data suggest that escape of feed purine N seems to be a minor factor affecting calculation of bacterial N flow and that the use of 15N in effluent bacteria may be a more accurate procedure when using continuous-culture fermenters.

title: Modeling ruminal pH fluctuations: interactions between meal frequency and digestion rate.
abstract: A steady periodic analysis of ruminal carbohydrate digestion was developed to predict the effects of diet and frequency of eating on ruminal pH fluctuation. Tests of the model against previous data showed that pH fluctuations were too large when previously published rates of carbohydrate digestion were used but were improved using rates from an in vitro gas production system, which were lower. With the original digestion rates, the minimum meal frequency to maintain steady-state conditions in the rumen increased from 4 to 12 meals/d as dietary effective neutral detergent fiber (NDF) decreased from 34 to 6% of dry matter (DM); with the revised rates, the minimum frequency was 3 to 6 meals/d, respectively. The minimum effective NDF to maintain a pH value above 6.0 increased from 14 to 23% of DM as meal frequency decreased from steady state to 2 meals/d using the original rates; with the revised rates, the minimum effective NDF was slightly smaller, increasing from 13 to 21% of DM, respectively. Effects of DM intake and body weight on pH fluctuation were minor, and dietary buffers, when used at rates less than 1%, did not reduce fluctuation. Different methods of calculating mean ruminal pH yielded different results for the effect of meal frequency on mean pH.

title: Physical constraints on voluntary intake of forages by ruminants.
abstract: Voluntary dry matter intake (VDMI) of forages by ruminants may be limited by distention resulting from restricted flow of digesta through the gastrointestinal tract. An animal's capacity for fill depends on the weight and volume of digesta that causes distention and the flow rate of digesta from the organ in which distention occurs. The reticulorumen is generally regarded as the site in the gastrointestinal tract for which distention limits VDMI with high-fill diets, although evidence suggests that distention of the abomasum may also limit VDMI. Linear decreases in VDMI have been noted with increasing amounts of inert fill inserted into the reticulorumen, but results have not been consistent across several experiments. Reduction in VDMI depends on the extent to which intake is limited by fill before insertion of inert fill; hence animals with high energy requirements consuming relatively low-energy, high-fill diets are affected to the greatest extent. Because NDF generally ferments and passes from the reticulorumen more slowly than other dietary constituents, it has a greater filling effect over time than non-fibrous feed components and has been found to be the best single chemical predictor of VDMI. However, many other factors affect fill, including particle size, chewing frequency and effectiveness, particle fragility, indigestible NDF fraction, rate of fermentation of the potentially digestible NDF, and characteristics of reticular contractions. These factors are only partially accounted for in models that have been developed to predict VDMI. Increased accuracy of prediction of VDMI is expected as models continue to evolve.