Effect Of Reducted Dietary Protein And Supplemental Rumen-protected Methionine,Leucine,Isoleucine And Threonine On The Nitrogen Efficiency And Performance Of Lactating Cows

The objective of this experiment was to evaluate the effect of supplementing a crude protein deficient diet with rumen-protected Met,Leu,Ile,Thr combined on dairy cow performance.The experiment was a 9-wk randomized complete design with 60 Holstein cows.Following a 2-wk covariate period,cows were blocked by days in milk,milk yield,and parity,and randomly assigned to 1 of the following 4 treatments:(1)Positive control(16%CP);(2)Negative control(12 CP%);(3)12%CP supplemented with RPMet,RPLeu,RPIle,RPThr(12%+AA);(4)14%CP supplemented with RPMet,RPLeu,RPIle,RPThr(14%+AA).The analyzed crude protein content of the ADMP and DMP diets was 15.7 and 14.7 to 12.6%,respectively.Crude protein intake,the apparent total tract digestibility of all measured nutrients(except ADF),were decreased by 12%CP,compared with 16%CP.Milk yield was decreased by 12%CP(31.39 kg/d),but was similar to 16%CP(34.88kg/d)for 12%+AA and14%+AA(34.72 and 34.95kg/d,respectively),paralleling the trend in DMI.Yields of milk components(protein,lactose,TS,NE_L,SCC)were decreased by 12%CP,compared with16%CP.Supplementation of the 12%CP diet with RPAA increased milk protein yield,milk lactose yield and milk TS yield.Milk fat content did not differ among treatments,but milk protein yield was increased by 12%CP+AA and 14%CP+AA compared with 16%CP and was not different from 16%CP.Relative to 16%CP,milk N efficiency increased by other diets.Milk urea N,urinary N excretion,and total excreta N were decreased by 12%CP,compared with 16%CP.Milk N secretion as a proportion of N intake was greater for the 12%CP diet compared with 16%CP.Plasma essential amino acids(EAA)(except Thr)were lower for NC compared with PC.Supplementation of the diets with RP AA increased the concentrations of plasma essential amino acids.Plasma concentrations of Gly and NEAA were increased by 12%CP+AA,compared with 16%CP,other NEAA,namely Ser,Glu,Ala,Tau,Asp were not affected by diets.Blood total protein,glucose and NO levels were not different for cows receiving different diets.Plasma urea N was decreased with all low protein diets versus 16%CP.Plasma glucagon and IGF-I were not affected by treatments,but the concentration of IGF-I was numerically higher with supplementation of the 12%CP diet with RPAA.Plasma insulin,growth Hormone,and PRL concentration was increased by 12%CP+AA and 14%+AA compared with 12%CP.Overall,The results of this study suggest that the low protein diets decreased dramatically urinary N losses and clearly increased milk N efficiency.The combination of the 4 RP AA(Met,Leu,Ile Thr)has the potential to improve milk and milk component yields in dairy cows fed low protein diet.