Abstract
Slow and rapid ruminal ammonia release rates were simulated by intermittent urea feeding with steers fed winter range grass. Four 360-kg, ruminally-cannulated steers were fed 208 g prairie hay (2.56% crude protein) hourly with the following daily dietary supplements: (1) Urea provided continuously (C) at a rate of 3.5 g/hr for 24 hr; (2) urea provided moderately (M) at a rate of 14.2 g/hr for 6 hr (3) urea offered rapidly (R) as 85 g in 1 hr, or (4) no supplemental urea (O). Ruminal ammonia remained stable with treatments C and O at 12.8 and 1.6 mg/dl, respectively. Treatments R and M peaked 1.5 and 6.5 hr after the start of urea feeding at 41.8 and 37.1 mg/dl, respectively. Nonammonia nitrogen concentrations in the rumen were increased by an average of 23% with urea supplementation. Dry matter digestibility was increased by 5% with addition of urea, regardless of the rate of urea administration. Daily nitrogen retentions were 12.4, 12.2, 11.4 and -7.1 g (P < .01) for C, M, R and O, respectively. Simulated slow ammonia release rates enhanced neither dry matter digestibility nor nitrogen retention. Metabolic use of ammonia in the rumen of steers fed forage was not improved by an attenuated release rate.
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