Interrelationships of Urea, Protein, and Productive Factors in Lactating Cows

Efficiency of nitrogen utilization and related factors were measured in lactating cows consuming different amounts of urea and protein nitrogen. A factorial arrangement consisted of 3 ratios of concentrate crude protein (L:SI:H = 10.2:14.1:18.8 dry basis) fed with barley (Hordeum v~dgare L.) silage containing either 0 or 0.55% urea. 30 lactating cows were placed into outcome groups on 4% fat-corrected milk and randomized to treatment. Concentrates were fed according to milk production at a 1:3 ratio. Voluntm3~ dry matter consumption of both silages was equal (1.8% body weight). When corrected for pre-treatment production, milk production was affected (P < .05) by concentrate protein (L :3I :H : = 20:20:24kg per day), but a significant silage protein interaction was present (P < .05). Both urea and increases in concentrate protein improved the digestibility (P < .05) of ration dry matter, protein, and fiber. Higher concentrate protein increased nitrogen utilization for productivity (milk plus tissue nitrogen, P < .01). Urea improved nitrogen utilization when fed with L concentrate. Cows on urea silage and ~[ or H concentrates had apparent digestion of 31 and 49g more (P < .05) nitrogen daily but were 45 and 70g per day lower in utilization of nitrogen (P < .10) for milk protein seeretion plus tissue accumulation.