Amino acid supplementation and nitrogen balance studies for lambs consuming fresh ryegrass/white clover herbage

Abstract

The grazing of fresh herbages by ruminants has been demonstrated to result in approximately 30% of the nitrogen derived from the forage subsequently being lost across the rumen. Nitrogen balance studies have demonstrated benefits in terms of increased nitrogen retention as a result of post-ruminal supplementation of nitrogen to lambs receiving either a purified urea diet or consuming fresh herbage. Eight Dorset Down x Coopworth lambs (38±2.9 kg) fitted with rumen and duodenal cannulae were housed indoors. The lambs were fed freshly harvested ryegrass/white clover pasture at a rate of 0.47kg fresh herbage/kgW⁰·⁷⁵/d. They each received duodenal infusions of water(control),3,6,9 and 12g casein protein/kgW⁰·⁷⁵/d or three levels of a mixture of amino acids (AA, methionine, lysine, histidine and arginine) equivalent to that supplied by the first three casein infusion levels. Treatments were allocated in an 8 x 8 latin square design. Mean flow of non-ammonia-nitrogen (NAN) to the duodenum was 1.46gNAN/kgW⁰·⁷⁵/d, with 33% of the forage N intake having being lost across the rumen. The casein infusion of up to 6g casein/kgW⁰·⁷⁵/d resulted in significant (P<0.01) linear increases in NR, but with infusion levels above this having resulting in only small NR responses. The NR response to duodenal supplementation of the AA mixture was not significant (P<0.5). This indicates that the NR response from the casein above that obtained with the AA supplementation was due to the increased duodenal supply of the remaining six limiting essential AA found in the casein. The coefficient of efficiency of utilisation of absorbed AAN for NR (BVaan) using the first three casein infusion levels was 0.64±0.01, and with the AA supplementation it was 0.56±0.16. Any means whereby a reduction in nitrogen losses accross the rumen or an increase in the supply of limiting essential AA could be achieved, would have the potential to significantly increase liveweight gains of lambs consuming fresh ryegrass/white clover.