Effects of the amount and frequency of nitrogen supplementation on intake, digestion, and metabolism in cattle fed low-quality tropical grass

Abstract

This study evaluated the effects of the amount and frequency of N supplementation on intake, digestion, and urinary and blood characteristics in cattle fed low-quality tropical grass hay. Five ruminally and abomasally fistulated crossbred bulls averaging 262 ± 20 kg of body weight were used in a 5 × 5 Latin square design. The basal diet was composed of a low-quality signal grass hay (47.2 g CP/kg DM). Treatments were designed according to a 2 × 2 + 1 factorial arrangement, as follows: supplemented with 30 % of rumen degradable protein (RDP) requirements daily or infrequently every three days; supplemented with 60 % of RDP requirements daily or infrequently every three days; plus a control without supplementation. The N supplement (1,140 g CP/kg DM) was a mixture of casein, urea, and ammonium sulfate. Overall, N supplementation did not affect forage intake when compared to the control. There was an interaction between the amount and frequency of N supplementation on all intake measurements. Infrequent supplementation decreased voluntary intake only when 60 % RDP was infrequently supplied, and this effect was attributed to a decreased intake on the day when supplementation was provided. Supplements generally improved digested organic matter intake but not digested fiber intake, revealing a lack of effects on forage intake and digestion. The N balance and efficiency of N utilization were increased by N supplementation but were not affected by supplementation frequency. Supplementation improved blood IGF-1 but did not affect 3-methylhistidine excretion, indicating a supplemental N effect on muscle anabolism, although it lacked effects on muscle catabolism. Both rumen ammonia-N and serum urea-N were affected by an interaction between treatments and day of supplementation cycle, where variation among days were only observed for infrequent supplementation. The rumen ammonia-N concentration peaked on supplementation day. However, a delay was observed for the peak of serum urea-N, which took place one day after supplementation. This pattern indicates that metabolic mechanisms may contribute to N salvage in animals that are infrequently supplemented. We concluded that infrequent N supplementation does not compromise digestion, N utilization, and metabolism in cattle fed low-quality tropical grass. However, when supplementing with large amounts of N, the infrequent supplementation may negatively affect forage intake.