Dose response to post-ruminal urea in lactating dairy cattle

Inclusion of urea in dairy cattle diets is often limited by negative effects high levels feed on dry matter intake (DMI) and efficiency rumen N utilization. We hypothesized that supplying post-ruminally would mitigate these limitations allow greater inclusion diets. Four rumen-fistulated Holstein-Friesian cows (7 ± 2.1 lactations, 110 30.8 d milk; mean standard deviation) were randomly assigned to a 4 × Latin square design examine DMI, milk production composition, digestibility, fermentation, balance, plasma constituents response continuously infused the abomasum (0, 163, 325, 488 g/d). Urea doses targeted linearly increase crude protein (CP) content total DMI (diet plus infusion) 0, 2, 4, 6% equated 0.7, 1.4, 2.1% expected respectively. Each 28-d infusion period consisted 7-d dose step-up period, 14 adaptation, measurement period. The diet was fed ad libitum as mixed ration [10.9% CP, 42.5% corn silage, 3.5% grass hay, wheat straw, 50.5% concentrate (dry basis)] formulated meet 100, 82, 53% net energy, metabolizable protein, rumen-degradable requirements, Linear, quadratic, cubic assessed using polynomial regression assuming fixed effect treatment random cow. Dry energy-corrected yield responded quadratically dose, increased with increasing dose. Apparent total-tract digestibility CP ruminal NH3-N concentration Mean VFA not affected proportion excreted feces decreased urine Urinary excretion Microbial flow cubically but microbial synthesis affected. Plasma Regression analysis estimates when supplemented top low-CP diet, 179 g/d post-ruminal maximize at 23.4 kg/d, corresponding dietary level 0.8% which line current recommendations for Overall, results indicate delivery does depression increases.