Lactation performance of dairy cows fed yeast-derived microbial protein in low- and high-forage diets.

The objective of this study was to investigate the effect of substituting soybean meal products with yeast-derived microbial protein (YMP) on lactation performance in diets containing 2 forage-to-concentrate ratios. Sixteen Holstein cows (4 primiparous and 12 multiparous) were randomly assigned to multiple 4 × 4 Latin squares with a 2 × 2 factorial arrangement of treatments. Diets contained low (LF; 45% of diet DM) or high forage (HF; 65% of diet DM) and YMP at 0 (NYMP) or 2.25% (WYMP) of the diet. The forage mix consisted of 67% corn silage and 33% alfalfa hay on a DM basis. No interactions of forage and YMP were noted for any of the production parameters measured. Feed efficiency (energy-corrected milk/dry matter intake) was greater for cows fed NYMP compared with WYMP. Regardless of the addition of YMP, cows fed LF had greater dry matter intake and produced more milk than cows fed HF. In addition, cows fed LF produced more energy-corrected milk than those fed HF. Milk fat percentage was lower in cows fed LF compared with HF, whereas fat yield was similar between forage concentrations. Fat yield tended to decrease with feeding YMP. Interactions of forage and YMP were observed for propionate concentration, acetate and propionate proportion, and acetate-to-propionate ratio. A tendency for an interaction of forage and YMP was also noted for ruminal pH. Cows fed HF diets had greater ruminal ammonia and butyrate concentrations, as well as proportion of butyrate. Arterial concentrations of Ile, Leu, Met, Thr, and Val were greater in cows fed LF. Cows fed NYMP had greater arterial concentrations of Ile, Lys, Trp, and Val than cows fed WYMP. Substitution of soybean proteins with YMP did not improve performance or feed efficiency of high-producing dairy cows regardless of the forage-to-concentrate ratio of the diet.