Cattle grazing blue grama rangeland. I. Seasonal diets and rumen fermentation.

Four field trials were conducted from early August to late October, 1982, on blue grama (Bouteloua gracilis) rangeland in south-central New Mexico to examine relationships among grazing season, diet botanical and chemical composition, and rumen fermentation in beef steers (Bos taurus). Diets contained an average of 83% grasses and 17% forbs from early August through late September and 77% forbs in late October. Cell wall content of the diet decreased from the early growing season (74.9%o) through the onset of dormancy (64.9%) while acid detergent fiber and lignin increased (41.9 to 52.9%o and 5.2 to 12.7%, respectively) and crude protein content declined from 18.4 to 11.7%. Soluble and insoluble nitrogen (N) fractions of the diet reflected crude protein; from 13 to 36% of N was in unavailable forms. The extent of in vitro organic matter digestion declined from the early growing season (66.5%) through onset of dormancy (47.9%0). Ruminal ammonia concentrations declined as season progressed: 6.0 mg/100ml was the lowest concentration observed. Declining diet quality was accompanied by an upward shift in digesta pH and altered proportions of volatile fatty acids (VFA) in rumen contents. Total VFA concentration was highest in late August (106.3 mmoles/liter). Ruminal measures generally reflected changes in dietary protein and digestibility but concentrations could also reflect changes in digesta flow rates. Finally, data suggest that crude protein may not be a good measure of protein supply to livestock grazing on ranges with diverse forage types. Several authors have discussed the influence of advancing season of growth and nutritive value of range plants and relative values of forage classes (Cook 1983, Pieper et al. 1978, Kothmann 1980). Such changes are usually discussed in relation to the daily requirements of animals grazing in the plant communities. However, few studies have related diet quality to ruminal environment in the grazer. The rumen is the primary site of forage degradation. Products of ruminal fermentation (ammonia, volatile fatty acids) and the pH of ruminal contents can influence fiber digestion (Mertens 1979), microbial protein synthesis (Satter and Slyter 1974), and, finally, energy and protein supplied to the grazing host. Therefore, knowledge of seasonal fermentation patterns in grazing range animals might suggest means of enhancing diet utilization and animal performance. The objectives of this study and another reported subsequently (McCollum and Galyean 1985) were to relate seasonal changes in diet composition with changes in rumen function and forage intake in beef steers (Bos taurus) grazing native blue grama (Bouteloua gracilis) rangeland during 4 periods of a growing season. Materials and Methods Study Area The Fort Stanton Experimental Ranch is in the foothills between the Sierra Blanca and Capitan Mountains in southern Lincoln County, New Mexico. Pieper et al. (1971) described the vegetation, topography, and climate of the Ft. Stanton Ranch. Briefly, annual precipitation averages 40 cm and mean annual At the time of this research, authors were graduate assistant, associate professor, graduate assistant, and professor, respectively, Department of Animal and Range Sciences, New Mexico State University, Las Cruces 88003. McCollum is currently assistant professor, Animal Science Department, Oklahoma State University, Stillwater 74078. This article is a contribution from New Mexico Agr. Exp. Sta. Journal Article No. 1087. The authors wish to acknowledge the assistance of Drs. A.L. Goetsch, R.E. Estell, and M.B. Judkins, E.E. Parker, and Harley Segotta. Manuscript accepted 19 April 1985. temperature is 11.10 C. Most vegetational growth usually occurs during June through September. The study pasture, an open grassland area, was situated at approximately 1,900 m elevation and received about 35 cm precipitation the year of the study (65% occurred between mid-July and the end of September). Vegetation of the study pasture was dominated by blue grama and cholla cactus (Opuntia imbricata) (Pieper et al. 1971). Field trials Four trials were conducted during 1982 (9 Aug.-17 Aug., EAug; 29 Aug.-Aug.-4 Sept., LAug; 23 Sept.-l Oct., LSept; 23 Oct.-31 Oct., LOct). Trials were conducted at these times to reflect phenological changes in the plant community. During each trial, 3 esophageal-cannulated dry, mature beef cows (Hereford X Angus) and 6 rumen-cannulated (10.2 cm ID cannulas) steers (Hereford X Angus; x weight = 303 kg) were allowed to freely graze the pasture. No other animals were grazed on the study pasture during adjustment and sampling periods. It was assumed that the diets of the cows and steers were similar. Cows were placed on pasture a minimum of 1 week before each trial, while steers were placed on pasture 2 weeks before the EAug trial and grazed continuously through the LOct sampling period. Diet samples were collected the first 2 days of each 8-day trial. The cows were penned each day at 1200 hour and samples were collected during 30-minute periods between 1700 and 1800 hours. The cows were hazed to the area being grazed by the steers during collection periods. Following the collections, an aliquot of each extrusa sample was frozen in a plastic bag and stored until analyzed. Rumen samples were collected on the third day of each trial. At 0800 hours, 250-ml sample of whole rumen contents was withdrawn after thorough hand-mixing of the rumen. Rumen pH was evaluated immediately with a combination electrode. Samples were strained through 4 layers of cheesecloth, and strained fluid was acidifed (1 ml 20% v/v H2S04 per 100 ml strained fluid) and stored frozen. After sampling, steers were allowed to return to normal grazing activities. The sampling process was then repeated at 1200, 1600, and 2000 hours. To facilitate rumen sampling, temporary pens were transported to and erected on an area immediately adjacent to the site being grazed by the steers at each sampling time. Laboratory Analyses Esophageal samples were dried at 50 0 C for 54 hours and ground through a 2-mm screen in a Wiley mill. Dry matter and ash contents were determined by standard procedures (AOAC 1980). Dietary fiber constituents (acid detergent fiber, ADF; acid detergent lignin, ADL; neutral detergent fiber or cell walls, NDF) were determined according to Goering and Van Soest (1970). Nitrogen (N) was determined by Kjeldahl procedures (AOAC 1980) and fractioned into soluble N (SN) and insoluble N (IN) by the sodium chloride method of Waldo and Goering (1979). Insoluble unavailable N (ADIN) was the N present in ADF residue. Crude protein (CP) was calculated as 6.25 X N. In vitro digestion analyses were conducted on samples composited across cows and sampling days. Rumen fluid inoculum was collected from 3 steers consuming a 50:50 mixture of alfalfa (Medicago sativa) and prairie hay. Extent of organic matter disappearance (IVOMD) was estimated using techniques of Tilley and Terry (1963). JOURNAL OF RANGE MANAGEMENT 38(6), November 1985 539 This content downloaded from 157.55. 39.78 on Sat, 25 Jun 2016 07:06:53 UTC All use subject to http://about.jstor. org/terms Microhistological examination of dried and ground esophageal masticate samples followed procedures outlined by Sparks and Malechek (1968). Ten systematically located fields per slide and 5 slides per sample (individual cow samples) were examined at lOOX