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The effect of heat damage on the digestibility and nitrogen utilization of grass silages by sheep

University of British Columbia

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<.01) negatively correlated with the apparent digestibility of DM (-.96), OM (-.98), N (-.95), ADF (-.78) and NDF (-.81). To reduce the number of variables associated with Experiment 1 and to assess the feasibility of artificially inducing heat damage in previously fermented silage, Experiment 2 was conducted using one locally produced orchardgrass silage which was either not heated (treatment C), heated at 49°C for 30 days (treatment H1), or heated at 50°C for 33 days followed by 30 days at 63°C (treatment H2). The silages were stored in 2.2 by 1m paper fibre form cylinders which contained coiled heating cables at 5 to 10cm spacings alongside the polyethylene bag within silos H1 and H2. The outside of each heated silo was insulated with a 15cm thickness of rock wool. This method of heating worked well with the exception of local perforation of the polyethylene seal caused by direct contact with the heating cables. This problem could be eliminated by coiling the cables around the outside of the silos. Artificial heating had no effect on total N content, but caused a significant (P<.01) increase in ADIN%N levels from 8.7, to 14.1 and 24.3% for treatments C, H1 and H2 respectively. Linked to the rise in ADIN%N levels was a significant (P<.01) increase in ADF content and a significant (P<.01) decrease in hemicellulose, a known substrate for the browning reaction, with increased heating. Heating expressed as ADIN%N significantly (P<.01) decreased apparent N digestibility from 62.9, to 57.2 and 39.1% for treatments C, H1 and H2 respectively. ADIN%N was significantly (P<.01) negatively correlated with the apparent digestibility of N (r = -.98). Variation in the level of ADIN%N was able to account for 96% of the variation in N digestibility, but, in contrast to Experiment 1, was a poor predictor for the other digestibility coefficients. With similar N intakes, sheep which were fed treatments HI and excreted 17 to 63% more N in their feces, but 8 to 31% less N in their urine,as compared to treatment C. ADIN%N content of the silages was significantly (P<.01) correlated with both these measures and explained 96 and 66% of the variation in fecal-N and urinary-N respectively. Reduced N digestibility was compensated for by an increase in the efficiency of N retention in treatment HI compared to C, so that there was no difference in overall N retention between these 2 treatments (.94g/day of N for both treatments). More severe heating (H2) led to a significant (P<.05) reduction in N retention (.15g/day of N). The regression equations from Experiments 1 and 2 for predicting N digestibility from ADIN%N were in close agreement and the results were pooled to obtain the following: Digestibility of N (%) = 81.76 - 1.83(ADIN%N) (r² = .92) Taken together, these results confirm that ADIN%N measurements should be a part of any routine analysis of grass silage, particularly when heat damage is suspected.

Text

2010-05-16

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http://hdl.handle.net/2429/24750

Animal Science

University of British Columbia

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