Effect of calcium salts of fatty acids and level of rumen degradable protein on nitrogen metabolism and performance of dairy cows fed corn silage-based diets

This experiment was designed to evaluate the effects of combining calcium salts of soybean oil fatty acids (CSFA) and two rumen degradable protein (RDP) levels on feed intake, ruminal fermentation, N metabolism, and performance of lactating cows fed corn silage-based diets containing 164 g/kg of crude protein and 6.72 MJ/kg of net energy. Eight rumen cannulated Holstein cows (207 ± 23.6 days in milk and 33.4 ± 2.45 kg/d of milk yield) were used in a 4 × 4 Latin square design with a 2 × 2 factorial treatment structure. Treatments consisted of diets with or without CSFA at 33.2 g/kg dry matter (DM) combined with either low (98 g/kg DM) or high level (110 g/kg DM) of RDP. Microbial protein synthesis was evaluated using the omasal sampling technique along with the triple marker system of digesta flow and ¹⁵N as a microbial marker. There was no interaction effect (P ≥ 0.15) between CSFA and RDP for all variables analyzed. Fat supplementation reduced (P ≤ 0.010) intake of nutrients (except ether extract), tended to increase (P = 0.076) rumination time, and increased (P = 0.015) ruminal pH. Calcium salts of fatty acids decreased (P ≤ 0.014) DM and organic matter ruminal digestibility and tended to reduce (P = 0.096) neutral detergent fiber ruminal digestibility. Dietary CSFA did not affect (P ≥ 0.32) ruminal flows of total non-ammonia N and microbial non-ammonia N, but increased (P = 0.022) microbial protein synthesis efficiency. Cows fed CSFA had greater (P ≤ 0.043) milk yield, feed efficiency, and N secreted in milk (g/kg N intake), and lower (P < 0.001) milk fat content in comparison with counterparts. Cows fed the diets with low RDP tended to have greater (P = 0.096) crude protein (CP) intake in comparison with cows fed diets with high RDP. Cows fed low RDP tended to exhibit greater (P = 0.057) ruminal acetate to propionate ratio than those fed high RDP. Low RDP decreased (P ≤ 0.048) starch total tract digestibility and ruminal protein degradation, tended to increase (P = 0.055) milk yield, and reduced (P = 0.002) milk protein content, but did not affect (P = 0.76) microbial protein synthesis efficiency compared to high RDP. Thus, lower RDP increased non-microbial non-ammonia nitrogen and reduced urinary losses of N, whereas CSFA decreased feed intake and improved microbial protein synthesis efficiency in mid-lactating cows. However, RDP level and CSFA association shows no interaction effect.