Effects of the Ruminal Protein Degradability on Milk Production during Early Lactation of Cows fed High-energy Diets

Abstract

Three diets, Hdg : high degradability, Mdg : middle degradability, Ldg : low degradability, were tested to investigate the effects on milk production of ruminal protein degradability in early lactating cows fed on high-energy diets (roughage to concentrate ratio of 35 : 65).
Trial 1 : The ruminal degradation parameters and passage rate were investigated to evaluate the ruminal protein degradability of the three diets. The ruminal degradation parameters were evaluated by a nylon bag technique using 3 rumen cannulated sheep. The passage rates were estimated by pulse dose procedure of rare-earth element (Lanthanum) marked soybean, and by empirical equation shown in feeding standard of the Agricultural and Food Research Council, using 4 early lactating cows. The passage rates were estimated to be 2.72%/h and 10.5%/h, respectively. The former value was too low for concentrates and the latter was more suitable. The degradability of protein calculated using the latter passage rate was 72.4% for Hdg, 60.9% for Mdg and 53.5% for Ldg.
Trial 2 : The three diets were fed to 9 early lactating cows, based on one-way layout design to investigate the effects on milk production for 16 weeks after parturition. Ruminal protein degradability of total ration including roughages were 74.6% for Hdg, 65.7% for Mdg and 60.0% for Ldg during the experimental period. Dry matter intake (kg/d) and total digestible nutrients (kg/d, %) were not significantly affected. The cows fed on Ldg diet tended to reach the peak of milk yielding faster than the cows fed on Hdg or Mdg, but there were no differences in milk yield during the experiment. Milk protein (%) of the cows fed on Hdg diet was significantly low (P<0.05) compared to that of cows fed on Ldg, before the peak of milk yielding (from 3 to 6 weeks after parturition), but not different after the period. Therefore, the results obtained in this study suggest that at early lactating period, ruminal degradability of dietary protein has different effect on milk protein production before and after the peak of milk yielding.