Evaluation of ruminally protected methionine and lysine or blood meal and fish meal as protein sources for lactating Holsteins

Forty lactating Holstein cows averaging 55 days in milk were used in a rand omized block designed experiment to evaluate the effectiveness of ruminally protected Met and Lys compared with that of ruminally undegradable protein for supporting lactation. Cows were fed total mixed diets far 15 wk. Diets were formulated to be isonitrogenous with the same base ingredients result ing in base crude protein percentage of 15.5. Supplemental crude protein su pplied by urea, soybean meal, or a 50:50 (wt/wt) mixture of fish and blood meal increased total dietary nitrogen to 18.0% of diet DM. Two additional d iets consisted of the basal diets soybean meal and urea, which were supplem ented with ruminally protected DL-Met and Lys-HCL at 10 and 25 g/d, respect ively (soybean meal + amino acids (AA), urea + AA). Mean measures of dry ma tter intake, milk yield, milk protein percentage, and milk fat percentage w ere not affected by protein supplement. Milk protein yield, milk fat yield, casein yield, and casein percentage also were not affected by source of su pplemental protein. Results indicate that at the level of crude protein int ake relative to milk production in this experiment, the source of protein d id not affect lactational performance.