Influence of Level and Source of Nitrogen on Milk Production and Blood Components

In Experiment 1, following a 22-day preliminary period 24 Holstein cows were used in a 70-day continuous feeding trial employing a 4 3 factorial design with four levels of urea (0, 82, 160, and 232 g urea/cow daily) and three levels of crude protein (average of 2.5, 2.2, and 1.7 kg crude protein/cow daily). There were no significant differences in milk production, milk composition, or body weight gains. Plasma urea nitrogen was increased by urea feeding (P < .01), and there was a linear response to crude protein level (P < .001). In Experiment 2, using 12 Jersey cows in a switehback design, urea supplementation of a corn silage and ground shelled corn ration (179 g/cow daily) increased milk produetion, body weight gains, and plasma urea nitrogen levels (P < .01) as compared with no added nitrogen. Nitrogen balances and plasma levels of glutamic acid were also increased (P < .10) by urea feeding. In Experiment 3, 12 Holstein cows were used in a 2 2 Latin-square design to compare nonprotein nitrogen as the only added hitrogen in a corn silage and ground shelled corn ration (423 g urea/cow daily) with an isonitrogenous ration providing 81 g urea/ cow daily. Milk production was lower on high urea (P < .1O), as were weight gains and plasma urea nitrogen (P < .01) ; nitrogen balances were higher (P < .10). Differences in milk composition and plasma amino acids were not significant.