INTERACTIONS OF RUMINALLY PROTECTED METHIONINE AND LYSINE WITH PROTEIN-SOURCE OR ENERGY-LEVEL IN THE DIETS OF COWS

Interactions between supplementation with ruminally protected Met and Lys and the nature of protein or energy concentration of the diet were studied using 16 and 12 multiparous lactating dairy cows in two trial s of 8 and 12 wk, respectively, commencing on approximately d 40 of la ctation. In trial 1, cows received a semicomplete diet plus concentrat es. The diet consisted of 62 to 63% corn silage, 2.2% corn gluten meal , .4% urea, 11% soybean meal (untreated or treated with formaldehyde), and 23 to 24% barley. In trial 2, cows received a complete diet with corn silage, untreated and formaldehyde-treated soybean meal, and barl ey in the ratio 78:12:9:0 or 49: 13:4:33. All treatments were replicat ed in a 4 x 4 Latin square design. In both trials, Met plus Lys (10 g/ d of intestinally available Met and 30 g/d of Lys) had no significant effect on DMI, milk yield, fat content, casein as a percentage of true protein, or urea content of the milk. Mean increase of milk protein y ield was 46 g/d with Met plus Lys, and mean increase of true protein c ontent was 1.1 g/kg of milk. The increase in content of milk true prot ein was greater for cows receiving the low energy diet. Protein source had no effect on milk yield or composition. Glucose, urea, NEFA, BHBA , and total free AA in plasma were unaffected by supplementation of ru minally protected Met plus Lys. However, concentrations of Met and Lys in blood were slightly, but not significantly, higher in supplemented cows.