Effect of dietary metabolizable protein level and live yeasts on ruminal fermentation and nitrogen utilization in lactating dairy cows on a high red clover silage diet

The objective of this study was to determine the differences in nitrogen (N) use and rumen function in lactating dairy cows fed red clover-based diets with two levels of metabolizable protein (MP) and live yeasts. Eight rumen-fistulated Holstein dairy cows were used in a duplicated 4 × 4 Latin square design, with a 2 × 2 factorial arrangement of treatments. Isoenergetic diets, highly (HD) or moderately (MD) deficient in MP [−14% or −5% less than requirements, respectively], were fed with or without live yeast supplement (10 g/d of Saccharomyces cerevisiae (2 × 1010 cfu/g). Total mixed ration (60:40 red clover silage:concentrate barley based) was fed 12 times daily. Under the conditions prevailing in this study, yeast supplementation had limited effects. The dry matter (DM) intake was 1.1 kg greater in cows fed MD vs. HD diets. Given that milk production was not different between treatments, this lead to a tendency to improved milk efficiency for cows fed HD. The concentration of isoacids in the ruminal fluid was reduced by yeast supplementation (P ≤ 0.02). Ruminal fluid lactate concentration was increased in the HD diet compared with the MD diet (0.14 vs 0.12 mM, SEM = 0.01; P = 0.02), whereas that of ammonia-N (NH3-N) was decreased in HD compared with MD diet (2.5 vs 4.8 mg/dL, SEM = 0.6; P = 0.02). Apparent digestibility of N (664 and 616 g/kg, SEM = 17; P < 0.001) and urinary N excretion (264 and 163 g N/d, SEM = 9; P < 0.001) were greater in the MD than in the HD diet. Apparent digestibility of DM, organic matter, neutral detergent fiber (NDF) and acid detergent fiber were not different among diets. Yeast supplementation tended to reduce total tract apparent digestibility of NDF (565 vs 589 g/kg, SEM = 28; P = 0.09). The diet HD reduced plasma urea concentration by 28% compared with MD. Average microbial counts, protozoa counts, total viable bacteria and cellulolytic bacteria were not affected by treatments. Yeast supplementation tended to decrease in situ effective DM degradability of corn and decreased that of barley DM, without affecting DM degradability of timothy-dominated hay. In conclusion, the present study showed that diets designed to reduce MP supply from 0.95 to 0.86 of requirements improved N efficiency and consequently contributed to the decrease in N excreted to the environment. Under these dietary conditions, there were no advantages of supplementing with yeasts.