Submitted to: Journal of Dairy Science
Publication Type: Abstract only
Publication Acceptance Date: 7/25/2005
Publication Date: 7/25/2005
Citation: Reynal, S.M., Broderick, G.A. 2005. Effect of RDP source on production and ruminal metabolism of lactating dairy cows [abstract]. Journal of Dairy Science. 88 (suppl. 1):321. Interpretive Summary:
Technical Abstract: Twenty-eight lactating dairy cows (8 with ruminal cannulas) averaging 137 DIM were assigned to seven 4 x 4 Latin squares and fed diets differing in the proportions of RDP coming from urea and non-urea sources. Diets contained (DM basis) 15% alfalfa silage, 40% corn silage, 29 to 27% high-moisture corn, and 16 to 18% concentrate mix containing vitamins and minerals. Proportions of ingredients (DM basis) in the four concentrate mixes in diets A to D were changed in equal increments to replace RDP from soybean meal (SSBM) with RDP from urea as follows: ground shelled corn, from 0 to 6.3%; SSBM, from 13.7 to 2.7%; lignosulfonate-treated SBM, from 0 to 6.0%, and urea, from 0 to 1.0%. Diets contained on a DM basis 16.3% CP, 26% NDF, and 14% ADF. Estimated (NRC, 2001) dietary concentrations of NEl, NFC, RDP, and RUP were constant across diets and were, respectively, 1.60 Mcal/kg and 50, 10.6, and 5.7% of diet DM. As urea replaced SSBM in diets A to D, there were linear decreases in DMI (23.6 to 22.3 kg/d; P < 0.01), milk yield (39.3 to 36.0 kg/d; P < 0.01), 3.5% FCM (38.0 to 35.4; P = 0.04), and body weight gain (BWG; 0.57 to 0.34 kg/d; P = 0.04). Milk content and yield of fat, protein, and SNF were not significantly affected by RDP source and averaged, respectively, 3.24% and 1.26 kg/d, 3.08% and 1.19 kg/d, and 8.79% and 3.43 kg/d across diets. Replacing non-urea N with urea N in the RDP resulted in linear increases in the concentrations of urea-N in milk (from 6.8 to 9.1 mg/dl; P < 0.01) and blood (from 8.9 to 12.8 mg/dl; P < 0.01) and of ammonia-N in the rumen (from 8.4 to 10.8 mg/dl; P < 0.01). Total free-AA concentration and pH in the rumen were not significantly affected by treatment and averaged 18.5 mM and 6.53, respectively. Replacing SSBM with urea in the RDP negatively affected milk production and BWG (mainly through an effect on DMI) and N utilization in lactating dairy cows.