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Abstract

The effects of heat damage, expressed as the percentage of total nitrogen located in the acid-detergent fibre fraction (ADIN%N), on the digestibility and nitrogen utilization of grass silages by sheep was investigated using both naturally heated (Experiment 1) and artificially heated (Experiment 2) samples. For Experiment 1, 6 locally produced grass silages were selected for their varying levels of heat damage expressed as ADIN%N: levels ranged from a low of 4.8% to a high of 22.9%. The apparent digestibility of each silage was measured in digestion trials using 4 to 6 sheep per silage and a 14 day adaptation period followed by a 7 day total collection trial. Measures of total nitrogen (N), acid-detergent fibre (ADF) and neutral-detergent fibre (NDF) as a percentage of silage dry matter (DM) were poor indicators of apparent digestibility. For example, silages 3 and 6 had N, ADF, and NDF contents of 1.9, 44.4, 66.3 and 2.5, 43.4, 59.6% respectively, whereas the apparent digestibility coefficients for dry matter (DM), organic matter (OM), N, ADF and NDF were 65.5, 68.7, 59.8, 68.6, 70.0 and 56.1, 58.6, 41.1, 54.6, 59.0% respectively. Variation in the level of ADIN%N was able to account for 91, 96, 91, 61 and 66% of the variation in the apparent digestibility of DM, OM, N, ADF and NDF respectively. ADIN%N was significantly (P