Ammonia treatment of lucerne and cocksfoot harvested at two growth stages: Effect on cell wall composition and digestibility

Abstract

Medicago sativa L. var. Europe and Dactylis glomerata L. var. Amply were harvested at the middle and the end of first growth. Lucerne and cocksfoot samples were treated with 30 g ammonia per kg DM for 4 days at 80°C to study the effects on nitrogen content, cell wall composition, and 48 h in situ and pepsin-cellulase digestibilities of dry matter (DM) and cell walls (CW). Lucerne had a very different CW composition than cocksfoot and changed less over maturity. Lucerne had a greater digestibility, but on a cell wall matrix (CWM) basis, it had a higher lignin content, reflecting essentially differences in tissue organization. In spite of a great increase of lignin concentration in the undigested residues, the ratio of CWM lignin concentration in lucerne to that in cocksfoot was very close, on average, in initial cell walls and in in situ undigested residues (1.72), suggesting a prominent effect of lignin partitioning in the initial cell wall on inhibition of digestibility. Ammonia treatment significantly increased the nitrogen content of all four samples both in DM and in CW. It improved DM digestibility slightly for lucerne and more strongly for cocksfoot (about 3.4 and 11.7 points in situ respectively for young stage). Increased digestibility tended to be greater at full flowering only for cocksfoot. In lucerne, the fraction solubilized by ammonia was low, composed mainly of polyuronic acids, which are highly digestible. In cocksfoot, the solubilized fraction was greater and composed of hemicelluloses and phenolic acids. The in situ digestibility of the treated CW fraction was enhanced slightly in lucerne and more strongly in cocksfoot. For both forages, the chemical extractability by ammonia reflected the biological susceptibility of the CW to the rumen microbes. (© Elsevier/Inra).