The objective of this study was to examine the effects substituting soybean meal with a yeast-derived microbial protein on rumen and blood metabolites, dry matter intake and milk production of high-producing dairy cows.

Sixteen Holstein cows (12multiparous and 4 primiparous), 93±37 DIM (mean ± SD) at the beginning of the experiment, were used in a 4×4 Latin square design with four 28-day periods. Cows were blocked by parity and production, with 1 square consisting of 4 animals fitted with rumen cannulas. Basal diets, formulated for 16.1 percent crude protein and 1.56Mcal/kg of net energy for lactation, contained 40 percent corn silage, 20 percent alfalfa hay, and 40% concentrate mix.

During each period, cows were fed one of four treatment diets corresponding to yeast-derived microbial protein (DEMP; Alltech Inc., Nicholasville, Ky.) concentrations of 0, 1.14, 2.28 and 3.41 percent DM. Soybean meal (44 percent CP) was replaced by yeast-derived microbial protein to attain isonitrogenous and isoenergetic diets.

Dietary treatments had no effect on pH and on most ruminal volatile fatty acid concentrations, with the exception of isovalerate, which decreased linearly with the addition of yeast-derived microbial protein. Rumen ammonia concentration decreased linearly, whereas free amino acids, total amino acid nitrogen, and soluble proteins weighing more than 10kDa showed a cubic response on rumen N fractionation.

A quadratic response was observed in oligopeptides that weighed between 3 and 10kDa and peptides under 3kDa when expressed as percentages of total amino acids and total nitrogen. Although nonesterified fatty acid concentration in blood did not differ between treatments, β-hydroxybutyrate and plasma glucose increased linearly as yeast-derived microbial protein increased.

Dry matter intake showed a cubic effect, where cows fed 1.14, and 3.41 percent yeast-derived microbial protein had the highest intake. Milk production was not affected by yeast-derived microbial protein, whereas a trend was observed for a quadratic increase for 4 percent fat-corrected milk and energy-corrected milk. Medium- and long-chain fatty acid concentrations in milk increased quadratically, which elicited similar effects on milk fat concentration and yield. Total solids percentage and yield, and milk urea nitrogen also showed quadratic effects as yeast-derived microbial protein increased in the diet. No effects were observed on feed efficiency, milk protein, and lactose percentage or yield.

A complementary in vitro study demonstrated a quadratic tendency for apparent and true dry matter digestibility as yeast-derived microbial protein was added to the diet. It was concluded that the substitution of soybean meal with yeast-derived microbial protein increased the percentage of total solids in milk and tended to improve energy-corrected and fat-corrected milk production in high-producing dairy cows consuming high-forage diets.

Source: Journal of Dairy Science/J.A. Sabbia, K.F. Kalscheur, A.D. Garcia, A.M. Gehman, J.M. Tricarico