1a. Objectives (from AD-416):
The objectives of the proposed research are to: 1) identify limiting essential amino acids (EAA) in lactating dairy cows which are fed different levels of dietary protein; 2) quantify the effectiveness of feeding rumen-protected amino acids (RPAA) as a strategy for maintaining milk and protein yield, and improving N efficiency in lactating dairy cows fed lower-protein diets; and 3) quantify EAA flows from the rumen using omasal sampling and evaluate the reliability of the widely applied NRC (2001) model for predicting dietary EAA adequacy.
1b. Approach (from AD-416):
Over-feeding of crude protein (CP) adds expense for dairy farmers & causes environmental pollution from excessive N excretion. A preliminary in vivo study will identify the limiting EAA in lactating dairy cows fed 3 levels of dietary CP (dry matter basis): low (13.5%), medium (15.0%) and normal (16.5%) CP concentrations. This trial will be conducted as a 3x3 Latin square (with 2 replicated blocks of 3 cows each) using rumen-cannulated cows to allow infusion of the EAA solutions directly into the abomasum. It should identify the limiting EAA at all 3 CP concentrations and provide inference into whether supplementation with RPAA is a feasible strategy for lowering dietary CP without reducing milk & protein yield. Urinary N excretion would be reduced, as well as the environmental impact. Those EAA identified as limiting will be fed as RPAA in a larger, production scale trial to be conducted at the USDFRC research farm. We will initially test a 3x2 arrangement of diets: 3 CP levels (13.5, 15.0 and 16.5% CP), with or without adding the appropriate RPAA. The basal diet will consist of alfalfa & corn silages plus high-moisture corn with supplemental CP from soybean meal, the standard protein source fed to U.S. dairy cows. The 6 dietary treatments will be applied in a cyclic change-over design in four 4-week periods. Forty-eight multiparous cows averaging about 100 days-in-milk will be blocked by days-in-milk into eight 6-cow blocks. Within the 4 pairs of 6-cow blocks, cows will be randomly assigned to 1 of 2 distinct sequences of dietary treatments. Six rumen-cannulated cows also will be selected and used in a separate cyclic change-over design with 5, 3-week periods for omasal and rumen sampling. Cannulated cows will be fed the same 6 diets over 15 of the 16 weeks of the production trial. Production data from the last 2 weeks (4-week periods) or 1 week (3-week periods in the omasal cows) of each period will be analyzed using the mixed procedures of SAS. Power computations using variances from a previous cyclic change-over trial indicated that this approach will detect (P = 0.05) production differences of 1.0 kg dry matter intake/d, 1.0 kg milk/d, 80 g fat/d, and 50 g true protein/d. Cows will be fed individually for ad libitum intake. Data on feed intake and milk yield will be collected daily. Milk will be sampled midweek at 4 milkings during each sample week and analyzed for fat, true protein, lactose, solids-not fat, and urea. Blood samples and spot samples of feces and urine also will be collected at the end of each period. Blood urea will be determined in deproteinized plasma; plasmas will also be analyzed for EAA concentrations. Internal markers in urine (creatinine) and feces (indigestible ADF) will be used to estimate urinary excretion of urea N and total N, apparent nutrient digestibility, and fecal N excretion. Omasal sampling using our standard protocol will be applied to quantify ruminal nutrient flows, particularly undegraded dietary CP, microbial CP and individual EAA. Ruminal samples will be collected to assess metabolite concentrations on each diet. Samples of all feeds ingredients will be analyzed for nutrient composition.
3. Progress Report:
This project is related to the following objective: Improve accuracy, reproducibility, and ease of measuring/estimating feed digestibility. A preliminary trial has been initiated to assess the effects of abomasal infusion of essential amino acids on milk and protein yield of lactating cows that are being fed medium and low levels of dietary protein.