UREA AS A SUPPLEMENTAL NITROGEN SOURCE FOR LACTATING COWS

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Abstract
Fifteen Holstein cows in early lactation were allocated to three groups for a 17-wk period. During a 4-wk preliminary period, all cows received a soybean meal concentrate to give a total average ration crude protein of 15%. In the next 4 wk, cows were gradually adapted to zero (ZU), low (LU), and high (HU) urea-containing rations. Based on total intake, cows consumed rations with 9, 11 and 12% crude protein. Depressed concentrate intake in the HU group due to decreased palatability limited total crude protein intake of that group to 12%. Milk yield was not stimulated by increasing dietary crude protein from 9 to 11 or 12% with urea. The percent milk persistency was not maintained by urea feeding when compared to persistency in the previous lactation at which time all cows received a 13% crude protein diet. Nitrogen balance for cows in ZU, LU and HU groups was −11.9, 13.3 and −3.6 g/day, respectively. The inclusion of urea in the ration increased ruminal NH3-N (mg/100 ml) from 1.32 for the ZU group to 3.94 and 5.78 for the LU and HU groups. Rumen microbial numbers (measured by DNA concentration) were decreased in all three groups when intact protein was removed from the ration. The sum of the plasma concentrations of histidine, isoleucine, leucine, lysine, threonine and valine were significantly decreased in cows fed the high urea-containing ration.