Adaptation of Lactating Cows to Rations Containing Urea

Abstract

Effects of a progressive change in 6 wk from a urea-free concentrate to one providing .34 (medium) or .66 g (high) urea/kg body weight on jugular blood composition and animal performance were studied with 27 cows. Consumption of dry matter was not affected even at the high rate of urea intake. Milk production was maximum when after 6 wk daily intake of urea reached .34 g/kg body weight, but higher rates of urea intake decreased milk yield and milk fat content. Butyrate through caproate were lower in milk lipids of urea-fed cows than for controls. While medium urea increased stearate, the high rate elevated linoleate. Ammonia and urea in serum were increased by transition to the high urea intake. Nonessential amino acids of serum were increased during adaptation to the medium rate of urea consumption but decreased with the high rate. Transitions to medium or high urea intakes did not affect the essential amino acids of serum. Adaptation of 6 wk was preferable to 3 wk to reach a daily intake of urea of .34 g/kg body weight in cows producing up to 18 kg of milk per day. Urea is used widely in rations for ruminants (5, 6), but urea intakes exceeding .45 g/kg body weight per day decreased feed intake and milk production in the dairy cow (4). Urea rations were less efficient than rations containing natural protein (19), because rapid hydrolysis of urea in rumen released ammonia in excess of the fixative abilities of rumen organisms (24). This resulted in more nitrogen excreted in urine than secreted in milk (9). Animals fed urea rations had higher blood urea (23), lower valine, isoleucine, leucine, and phenylalanine in blood serum (20), and they retained less nitrogen (10). However, continuous feeding of urea (15) decreased blood urea (24) and improved nitrogen balance (26). Urea utilization in lactating cows might be improved through a gradual transition over a period greater than 4 wk from a urea-free to a urea-containing ration (8, 22). It is not known, however, whether the amount of urea intake could exceed .45 g/kg body weight without limiting productivity in cows if the period of adaptation to urea were extended beyond 4 wk. This study was to observe effects on feed intake, milk yield, milk, and blood compositions of cows of a 6-wk transition from a urea-free concentrate to a concentrate providing urea consumption of .34 or .66 g/kg body weight. © 1980, American Dairy Science Association. All rights reserved.