Use of Urea by Early Postpartum Holstein Cows

Open Access
Publisher Website
Google Scholar
Abstract
Lactating Holstein cows (39) were fed high-energy complete rations ad lib with crude protein: 1) 11.7% (negative control); 2) 13.9% (1% urea); 3) 16.6% (1% urea); or 4) 16.6% (positive control) in a continuous 12 wk study beginning at wk 5 postpartum. Milk production of 27.7, 31.8, 34.0 and 30.4 kg/day showed the use of urea N by groups 2 and probably 3. Two digestion-N balance trials with each cow also provided evidence that urea N was used for milk secretion. Energy digestibility averaged 59.4, 64.2, 65.4 and 65.8; and lower for the negative control diet. N solubility in the diets was 28, 36, 32 and 21%, which reflects the objective of selecting ingredients with low N solubility for use in urea diets. Concentrations of ammonia N before and after feeding were 1.1, 3.3, 3.5, 4.2 and 2.2, 11.2, 11.9 and 9.3 mg/100 ml of rumen fluid. The prefeeding amounts were probably too low for maximum microbial growth. Urea-N concentrations in plasma were 8.65, 10.32, 18.00 and 17.03 mg/100 ml. Lactating cows in early lactation apparently can use urea N when high-energy complete rations with ingredients of low N solubility are fed ad lib.