Start Date: Mar 01, 2013
End Date: Nov 30, 2014
Overfeeding of crude protein (CP) adds expense for dairy farmers and causes environmental pollution from excessive N excretion. A preliminary in vivo study will be conducted to 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 direclty into the abomasum. This experiment 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 and protein yield. Those EAA identified as limiting will be fed as RPAA in a larger, production-scale trial to be conducted at the U.S. Dairy Forage Research Center research farm. The initial plan is to test a 3x2 arrangement of diets: 3 CP levels (13.5, 15.0 and 16.5% CP), with or without addition of the appropriate RPAA. The basal diet will be composed of alfalfa and 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 five 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. Cows will be fed individually for ad libitum intake. Data on feed intake and milk yield will be collected daily. Milk will be sampled mid-week 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.