Corn distillers grains versus a blend of protein supplements with or without ruminally protected amino acids for lactating cows

In a replicated 4 x 4 Latin square design with 4-wk periods, we used 12 mul tiparous Holstein cows averaging 83 d postpartum to compare corn distillers grains (CDG) versus a blend (BLEND) of other protein sources with CDG (fis h meal and soybean meal), and to determine the effectiveness of ruminally p rotected lysine and methionine (RPLM) in improving the utilization of CDG a s a protein supplement for lactating cows. The 2 x 2 factorial arrangement of treatments was as follows: CDG diet, CDG diet plus RPLM, BLEND diet, and BLEND diet plus RPLM. All diets contained 30% corn silage, 20% alfalfa hay , and 50% the respective corn-based concentrate mixture. The array of amino acids available for absorption when cows were fed the BLEND diet was more desirable than for the CDG diet according to Milk Protein Score and Cornell Net Carbohydrate and Protein System. Dry matter intakes were similar among all diets. Milk yields (32.6, 31.7, 32.8, and 32.8 kg/d, respectively) wer e similar for cows fed all diets. Milk fat yields and percentages (3.72, 3. 76, 3.67, and 3.63%) were unaffected by diet, but milk protein percentages (3.23, 3.26, 3.25, and 3.26%) tended to be higher when fed RPLM. Concentrat ions of most protein fractions in milk were similar for all diets, although beta-lactoglobulin was increased slightly when cows were fed BLEND diets. Lysine, Met, and Phe were indicated as the most limiting amino acids for al l diets according to extraction efficiency and transfer efficiency of amino acid from blood by the mammary gland. Methionine status was apparently imp roved by RPLM supplementation; Lys status was improved by the BLEND diets. Milk yield and composition when cows were fed CDG were not further improved by feeding blends of protein sources or RPLM; however, such dietary change s improved Lys and Met status of the cows.