104 N metabolism in ruminants supplemented with post-ruminally delivered urea.

摘要

Ruminants are naturally able to fertilize the rumen by transferring serum urea nitrogen through the rumen wall. To mimic the natural physiology of the ruminant, we tested the hypothesis that supplying urea post-ruminally would increase fiber fermentation and microbial protein production. Four rumen fistulated Holstein heifers (610 ± 27 Kg BW) were used in a Latin square design with abomasal infusions. Treatments consisted of infusions of either 50g or 150g of urea into the rumen as pulse dose (UR50 and UR150); 150 g of urea continuously into the abomasum (UA150) and 50 g of urea into the rumen as daily pulse dose with 100g of urea continuously infused into abomasum (URA150). Heifers were fed hay (6% CP; 65% NDF) with 1kg of compound feed (15.7%CP) supplied through the rumen cannula. Indigestible NDF (iNDF) was used as faecal marker and total urine output was measured to quantify purine derivatives for estimation of microbial protein synthesis. By design, N intake was lower (P<0.01) in UR50 (130 g/d) while all other treatments consumed on average 176g N/d. Dry matter and NDF digestibility were highest (P=0.04 and P=0.02, respectively) for UR50 and UA150 compared with UR150. Ruminal ammonia concentration was highest for UR150 (36.25mg/dL) 3h after feeding, while UA150 resulted in the lowest temporal ruminal ammonia (mean 5.2mg/dL). Nitrogen excretion was lower (P<0.01) with UR50 through lower (P<0.01) urinary N excretion. Nitrogen balance was not different among treatments (P=0.23). Serum urea N was 40% lower (P=0.04) with UR50 and 18% lower with UA150 and URA150 compared to UR150. Microbial protein N flux, relative to N intake, was higher (P=0.03) with UR50 and lower with UR150. Our study indicates that urea delivered post-ruminally improves fiber digestibility while maintaining efficient nitrogen utilization and microbial protein flux compared to a conventional urea supplementation strategy.