Nitrogen utilization, ruminal fermentation and abomasal nitrogen flow in sheep fed caucasian bluestem hay supplemented with lucerne or urea

Abstract

Four crossbred wether lambs with ruminal and abomasal cannulae were used in a Latin square design to measure nitrogen (N) utilization, ruminal fermentation parameters and abomasal N flows for diets of caucasian bluestem hay alone (BLUESTEM) and supplemented with 50% lucerne (BLUE + LCRN), 9.6 g urea once daily (DOSED UREA) or 9.6 g day−1 urea continuously infused (INFUSED UREA). Lambs consuming BLUE + LCRN had lower (P < 0.05) intakes of neutral detergent fiber (NDF), hemicellulose and cellulose, and lower (P < 0.05) digestibility of acid detergent fiber (ADF) compared with those fed BLUESTEM, DOSED UREA or INFUSED UREA. Supplementation increased (P < 0.05) N intake and apparent digestibility regardless of the source. At 4 h post-feeding, ruminal ammonia (NH3-N) was increased (P < 0.05) by N supplementation over the BLUESTEM diet. The BLUE + LCRN diet resulted in lower (P < 0.05) ruminal pH from 2 to 8 h post-feeding compared with BLUESTEM, DOSED UREA or INFUSED UREA. Total concentration of ruminal volatile fatty acids (VFA) at 2 h post-feeding was greater (P < 0.05) for BLUE + LCRN compared with BLUESTEM, DOSED UREA or INFUSED UREA. Proportions of acetate, butyrate, iso-butyrate and iso-valerate were unaffected (P > 0.10) by supplementation. Propionate was increased (P < 0.05) on the BLUE + LCRN diet. No differences (P > 0.10) in abomasal flow of particulate dry matter (DM), non-NH3-N (NAN) and microbial N were evident. More (P < 0.05) NH3-N reached the abomasum in lambs receiving supplemental N. Ruminal escape of feed N was greater (P < 0.05) for the BLUESTEM diet compared with BLUE + LCRN and INFUSED UREA. Microbial N, as a percentage of total abomasal N, did not differ (P > 0.05) between diets. Results of this study indicate that low levels of ruminal NH3-N found on the BLUESTEM diet were not limiting microbial protein synthesis. Supplementation with lucerne appeared to stimulate the rate of early ruminal fermentation as evidenced by lower ruminal pH and greater concentration of total ruminal VFA 2 h post-feeding. This may help to explain the improved performance observed with the addition of a legume to a warm-season grass diet.