LACTATIONAL PERFORMANCE OF COWS FED LOW OR HIGH RUMINALLY UNDEGRADABLE PROTEIN PREPARTUM AND SUPPLEMENTAL METHIONINE AND LYSINE POSTPARTUM

Multiparous Holstein cows (n = 24) were fed diets containing 34 or 41% ruminally undegradable protein (RUP) for 30 d before parturition; the n each group was fed a basal diet supplemented with or without ruminal ly stable Met (10.6 g/d) and Lys (15.2 g/d) for 75 d in the subsequent lactation. Supplementation of Met and Lys increased the milk yield of cows previously fed the low RUP diet, but milk yields before and afte r amino acid (AA) supplementation were similar for cows previously fed the high RUP diet. Milk protein content (percentage) increased from 2 .83 to 2.96 for cows previously fed the high RUP diet. Milk protein yi eld increased from 1.13 to 1.21 kg/d when Met and Lys were fed. Data o n AA concentration in plasma and AA extraction by the mammary gland su ggest that the supplementation of Met and Lys corrected a Met limitati on. According to the Cornell Net Carbohydrate and Protein System, the lactation diet was limiting for Met for maximum milk yield and was cor rected by AA supplementation. Based on limiting AA, allowable milk yie ld was 42.5 kg/d, and the observed yield was 40.9 kg/d averaged across treatments. The group with the greatest allowable milk yield (45.2 kg /d) had the greatest actual milk yield (43.0 kg/d). The regression equ ation of observed milk yield on allowable milk yield was Y = 3.4 + 0.8 805X.