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

Abstract 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