Methods to Evaluate Protein Quality of Diets for Lactating Cows

Ten multiparous and five primiparous cows (x= 62 ± 33 days in milk) were utilized in a replicated 5 x 5 Latin square design with 21-d periods to investigate the differences between three systems for evaluating dietary protein quality for lactating cows: a) milk protein score (MPS); b) Cornell Net Carbohydrate and Protein System (CNCPS); and c) supplying methionine (Met) and lysine (Lys) as 5 and 15% of the predicted total essential amino acids in duodenal digesta, respectively (PEAA). Diets were isonitrogenous at 16% crude protein. Treatments were: 1) a control diet with soybean meal as the protein supplement, MPS = 0.74, CNCPS - deficient in Met (94% of requirements) and Lys; 2) canola meal (CM) as the protein supplement, MPS = 0. 77, CNCPS deficient in Met (91 % ) and Lys; 3) treatment 2 plus ruminally protected Met and Lys (RPLM) to provide 11.5 g/d Met and 15 g/d Lys; 4) blend of protein sources (BCCF), MPS = 0. 77, CNCPS - adequate amino acids (Met = 100%); 5) treatment 4 plus RPLM. Dry matter intake, energy-corrected milk yield, and fat yield were lower (P \u3c 0.05), and 3.5% fat-corrected milk yield tended to be lower (P = 0.06) for cows consuming BCCF diets (4 and 5) than when consuming CM diets (2 and 3). Cows consuming soybean meal had the lowest percentage of protein, solids-not-fat, total N, true protein N, and casein Nin their milk when compared to cows consuming all other diets (P \u3c 0.05). The addition of RPLM increased (P-\u3c 0.05) · the percentage of protein in milk (3.10, 3.15, 3.23, 3.15, and 3.20% for diets 1 through 5, respectively). Casein N tended to be greater for cows consuming CM+ RPLM than CM. The percentage of nonprotein N in milk, ruminal ammonia, and serum urea N were greater for cows consuming CM diets than BCCF diets, probably due to the greater ruminal degradability ·o( protein in CM diets. Supplementing both the CM and BCCF diets with RPLM did not increase the amount of Lys and Met in serum sampled from the coccygeal vessel. This probably indicated that the amounts of RPLM did not eliminate the deficiencies of those amino acids (AA). Both transfer efficiency and extraction efficiency of AA from blood by the mammary gland indicated that Lys and Met were the most limiting AA with histidine, isoleucine, leucine, and phenylalanine also indicated in the top five limiting AA. Although this study indicated that all the three systems (MPS, CNCPS, and PEAA) have validity, the PEAA predicted the increases in milk protein percentages most effectively. A second study was designed to help build the knowledge of ruminal degradability and intestinal digestibility of some of the protein supplements used in experiment 1. The in situ portion involved measuring duplicate 4 g samples of blood meal (BlM), canola meal (CM), com gluten meal (CGM), and menhaden fish meal (FM) into dacron bags, soaking them in water, then incubating them for 0, 3, 6, 12, 18, and 24 h on 3 different days in the rumens of two lactating, ruminally cannulated Holstein cows. Four extra samples of each feed were incubated in the rumen · for 12 h to be used in determining in vitro intestinal digestibility. Both the original feeds and the residues remaining after 12 h were analyzed for AA content. The CM was the most degradable (P \u3c 0.01) in the rumen and BlM was the least (8.1, 60.5, 18.7, and 43.7% of CP for BlM, CM, CGM, and FM, respectively). The mean coefficient of variation for protein degradation was 12 %. Intestinal digestibilities (76.5, 72. 7, 94.2, and 83.2 % of ruminally degradable protein) were all quite high with CM estimated to have the lowest intestinal digestibility and CGM the highest. The AA content of the 12 h residues differed only slightly from the AA content of the original protein supplements. Comparing the AA content of feed residues to the AA content of milk showed that isoleucine was first limiting in BlM, CM, and FM; lysine was first limiting in CGM. Although CM was extensively degraded in the rumen, its 12 h residue still provided an AA profile which was closest to the AA profile of milk protein. The BlM and CGM are good sources of ruminally undegradable protein but are more severely deficient in some AA and therefore shouldprobably only be fed in combination with other protein sources that complement their AA profiles