Variable Dietary Protein and Urea for Intake and Production in Holstein Cows

Abstract

A central composite design with protein 9.4-16.2% and urea 0-40% of N, all expressed as dry matter, was the basis for 9 rations to which 36 cows were allotted equally. Complete rations (18% crude fiber) were corn silage in constant ratio with a mixture of variable corn, soybean meal, or urea. After 30 .+-. 5 days in lactation, cows were fed the midpoint ration (12.8% protein, 20% urea) for 4 wk, then fed assigned rations for 20 wk. Milk production and feed intake were recorded. After 5 wk ruminal ammonia was determined on fluid aspirated 2-4 h postfeeding. Initial means of milk and for treatments were 28.1 and 23.8 kg. Linear components of protein and urea for milk yield accounted for 74.4% of the sum of squares between treatments. Linear slopes were 2.58 kg milk for each percentage increase in protein and -.22 kg milk for each unit of urea N substituted for crude protein N. Feed efficiency for milk was enhanced by higher protein and lower urea. Mean ruminal ammonia was 27.5 mg/100 ml; and 94.6% of sum of squares of treatments were accounted for by linear components of protein and urea. Families of response curves were presented to portray the influence of the significant components described above.