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

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Medicago satL. 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 ammo- nia 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 promi- nent effect of lignin partitioning in the initial cell wall on inhibition of digestibility. Ammonia treat- ment 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 flo- wering 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 grea- ter and composed of hemicelluloses and phenolic acids. The in situ digestibility of the treated CW frac- tion was enhanced slightly in lucerne and more strongly in cocksfoot. For both forages, the chemi- cal extractability by ammonia reflected the biological susceptibility of the CW to the rumen microbes. (© Elsevier / Inra)