Urea for Lactating Dairy Cattle. II. Effect of Various Levels of Concentrate Urea on Nutritive Value of the Ration

The objective of the experiment was to determine the effect of several levels of concentrate urea on ration utilization in lactating cattle.A high-quality concentrate mixture containing 0, 1.25, 2.0, or 2.5% urea (42% N) in place of an equivalent amount of plant protein nitrogen was fed with good-quality timothy hay to Holstein cows during early lactation.Concentrate was fed according to milk production, and hay was limited to 2% of body weight daily.Sixteen digestibility trials, including energy and nitrogen balances and rumen metabolite analyses, were conducted.Ration intake was high on all treatments.There were no significant treatment differences in milk production or composition, or in ration digestibility.Nutritive value, expressed as digestible or metabolizable energy per unit dry matter, was significantly depressed at the two highest urea levels.Molar proportions of acetic, propionic, and butyric acids in rumen fluid at intervals before and after feeding were unaffected by treatments.Rumen fluid urea levels were very low, even 0.5 hr postfeeding.Rumen fluid ammonia concentration rose markedly after feeding in response to the treatments, but the differences were not statistically significant.In a previous report from this laboratory (2) it was shown that an increase in concentrate fiber level from 5 to 8% caused a serious depressing effect on ration digestibility and nutritive value in dairy cows of low (17 kg/day) milk-producing ability.When 18 kg urea (42% N) per ton was used to replace an equivalent amount of plant protein nitrogen in the same concentrate mixture, ration digesti-