Optimizing nutrient utilization of a precision feeding system for dairy heifers using low and high forage diets

Four experiments were conducted to optimize nutrient composition of diets used for a precision feeding system for dairy heifers. The main focus of this work is to understand the relationships between forage to concentrate ratios and dietary components when nutrients are provided precisely to meet the requirements of growing dairy heifers. The objective of the first experiment was to assess the effects of manipulating dietary fiber by replacing corn silage (CS) by lower quality forage as corn stover (CST) when used in high concentrate (HC) and low concentrate (LC) diets for precision-fed dairy heifers. Eight Holstein heifers (335.6 ± 7.41 kg body weight) were randomly assigned to 2 levels of concentrate HC (20% forage) and LC (80% forage) and to a forage type sequence (0% of forage as corn stover (CST), 100% corn silage (CS); 20% CST, 80% CS; 40% CST, 60% CS; 60% CST, 40% CS) within concentrate level administered according to a split-plot, 4 × 4 Latin square design (21d periods). Heifers fed HC had higher apparent total tract dry matter digestibility (DMD). Increasing fiber level by increasing amount of CST in the diet resulted in a linear decrease of DMD and organic matter digestibility. Heifers fed LC diets had higher neutral detergent fiber (NDF) digestibility and tended to have lower acid detergent fiber (ADF) digestibility than those fed HC diets. Substituting CS with 20% CST resulted in the highest NDF and ADF digestibilities. Digestibility of N was not different, but N retention increased for HC and decreased quadratically for LC diets. Heifers fed HC diets decreased fecal output, and CST linearly increased these parameters. Urine volume tended to be higher for HC-fed heifers, and increasing dietary fiber through CST inclusion tended to decrease urine output. This shift in water excretion resulted in similar total manure output. Total purine derivative excretion did not differ between treatments, but interacted with CST addition, resulting in a linear increase in microbial protein flow to the duodenum in HC-fed heifers and in a linear decrease in LC diets as CST increased. In conclusion iv increasing dietary fiber through CST decreased DM and organic matter digestibility linearly, while NDF and ADF digestibility were maximized when 20% CST was added to HC and LC diets. Microbial protein synthesis increased and decreased linearly with CST addition in HC and LC diets respectively. Retention of N increased and decreased quadratically with CST addition in HC and LC diets respectively. Total manure excretion was not different between HC or LC diets. The objective of the second experiment was to determine the effects of manipulating the degradability of the protein fraction in limit-fed heifers diets containing three dietary fiber level within two F:C maintaining a constant level of RDP. Eight cannulated Holstein heifers (486.98 ± 15.07 kg body weight) were randomly assigned to 2 levels of concentrate HC (45% forage) and LC (90% forage) and to a forage type sequence [33% of forage as an proportionally equal combination of grass hay and wheat straw HS, 67% corn silage CS (Low fiber; LF); 50% HS, 50% CS (Medium fiber; MF); and 67% HS, 33% CS (High fiber; MF)] within forage level administered according to a split-plot, 4 × 4 Latin square design (21-d periods). Increasing dietary fiber through HS while maintaining a consistent RDP decreased DM, OM, NDF, ADF and cellulose AD linearly. Microbial protein synthesis predicted from urinary purine derivatives decreased linearly with HS addition resulting in a linear decrease in N retention with HS addition, which was opposite to ruminal NH3N and BUN, reflecting the inefficiency in N utilization as more HS was added to the diets. Rumen fermentation parameters and DM and fractional passages (solid and liquid) rates support the reduction in digestibility, N retention and microbial protein synthesis observed as more dietary fiber is added to the rations of dairy heifers precision-fed a constant level of RDP. A third experiment was conducted to determine the effect of replacing soybean protein (SBM) with a slow-release non-protein N source (SR-N) in precision-fed dairy heifers in the tropics. Eight Holstein heifers (237.6 ± 5.45 kg body weight) were allocated to 2 treatments in a cross over design. Treatments were control (SBM) and SR-N (Optigen II; Alltech Inc.). The v forage to concentrate ratio was 50:50 (dry matter basis), and fresh chopped sugar cane was the sole source of forage. We conclude that when SR-N replaced SBM, it tended to decrease dry matter and decreased organic matter, crude protein, hemicellulose, and starch total tract apparent digestibility, but did not affect neutral detergent fiber and increased acid detergent fiber digestibility. Even though control diets were more digestible across the total tract, differences between treatments were small, suggesting that replacing SBM with SR-N is feasible in sugar cane-based diets in the tropics. The objective of the fourth experiment was to determine the effects of replacing dietary starch with soluble fiber and the dose at which the effects of Saccharomyces cerevisiae (YeaSacc ®1026 , Alltech Inc.; YC) are maximized based on nutrient total tract digestibility (AD), N utilization and blood metabolites of precision-fed dairy heifers. A split plot design with starch level as the whole plot and YC dose as sub-plot was administered in a 4 period (21 d) 4 × 4 Latin square. Eight Holstein heifers (432.49 ± 6.81 kg BW) were allocated to 2 starch treatments (28% starch: HS; 17% starch: LS) and to a sequence of YC doses (0, 10, 30, and 50 g/d). Results from this experiment demonstrated that starch level did not affect DM AD, but influenced ADF and hemicellulose AD. YC dose had a greater effect on DM, NDF, ADF, and hemicellulose AD when added at 30 g/d. YC influenced glucose and triglycerides concentrations differently according to the dietary starch concentration o the diet. The second objective of the fourth experiment was to determine the effects was to determine the effects of replacing dietary starch with soluble fiber and the dose at which the effects of Saccharomyces cerevisiae (Yea-Sacc ®1026 , Alltech Inc.; YC) are optimized based on rumen fermentation, microbial protein synthesis, and bacteria numbers and viability in precision-fed dairy heifers. A split plot design with starch level as the whole plot and YC dose as sub-plot was administered in a 4 period (21 d) 4 × 4 Latin square. Eight Holstein heifers (432.49 ± 6.81 kg BW) were allocated to 2 starch treatments (28% starch: HS; 17% starch: LS) and to a sequence of YC vi doses (0, 10, 30, and 50 g/d). The highest pH for LS and HS diet were at 0 and 50 g/d respectively. Total volatile fatty acid concentration at a point in time was not different among YC doses or starch level. Molar proportions of propionate, iso-butyrate and iso-valerate were higher for HS than LS. Acetate was reduced to its lowest value at 30 and 10 g/d YC in LS and HS diets respectively. Lactate concentration was greater for LS treatment and there was a time x starch interaction. Increasing YC dose resulted in a quadratic tendency with 10 g/d having the lowest lactate concentration. Mean ruminal ammonia concentration was increased in HS-fed heifers and a YC effect was observed. HS treatment decreased mean ruminal pH in comparison to LS diets, but there was a steeper prospandrial decrease in LS fed heifers. YC daily doses had a linear interaction effect with starch concentration. It decreased linearly rumen pH in the LS diet and the reverse was observed for the HS. Dietary starch concentration increased viable, non-viable, and total FAB, PAB, and total bacteria numbers. YC daily dose linearly decreased viable, and total FAB numbers with 10 g/d having the highest numbers of FAB. No effects of YC were observed for FAB or total bacteria numbers. Protozoa numbers were similar among dietary treatments and YC doses, but the ratio of bacteria to protozoa numbers was linearly reduced by YC doses. Estimated microbial N outflow was not different among starch treatments or YC doses. We conclude that fermentation profiles respond differently in HS and LS when YC is added to precision-fed dairy heifers.