Prediction of methane energy output in dairy and beef cattle offered grass silage-based diets

Since 1992 a number of lactating dairy cows (n = 247) and beef steers (n = 75) were offered grass silage-based diets in a range of feeding experiments and subjected to gaseous exchange measurements in calorimetric chambers at the Agricultural Research Institute of Northern Ireland. The objective of the present study was therefore to use the energy metabolism data from these studies to evaluate the relationship between methane energy output (CH4-E) and a number of animal and dietary factors. There were no significant differences between dairy and beef cattle in terms of silage dry matter (DM) intake as a proportion of total DM intake (S-DM1/T-DM1), total acid detergent fibre (ADF) intake as a proportion of T-DM1 (T-ADFI/T-DM1) or silage ADF intake as a proportion of T-ADFI (S-ADFI/T-ADFI). Animal type also had no significant effect on CH4-E as a proportion of gross energy (GE) intake (CH4-E/GEI) or digestible energy (DE) intake (CH4-E/DEI). The data from both dairy and beef cattle were thus pooled to predict CH4-E. CH4-E/GEI and CH4-E/DEI were each significantly related to feeding level or dietary factors (P < 0.001). These two ratios (CH4-E/GEI and CH4-E/DEI) were reduced by proportionately 0.0078 and 0.0123, respectively, as feed intake increased one level above maintenance. However an increase of 0.10 in S-DM1/T-DM1, T-ADFI/T-DM1 or S-ADFI/T-ADFI would increase CH4-E/GEI by 0.0025, 0.0069 or 0.0048; or CH4-E/DEI by 0.0035, 0.0107 or 0.0067. CH4-E (MJ/day) was significantly related to GE or DE intake (P < 0.001) with a coefficient of 0.055 or 0.071 and a constant of 3.23 or 3.32. The prediction of CH4-E was thus examined using various combinations of intake (GE or DE) with feeding level above maintenance or/and dietary factor (S-DM1/T-DM1,T- T-ADFI/T-DM1 or S-ADFI/T-ADFI). The relationships were all highly significant (P < 0.001) and the R-2 values ranged from 0.851 to 0.888. The equations relating CH4-E to DE intake, feeding level above maintenance (FL-1) and S-ADFI/T-ADFI or S-DM1/T-DM1 had the highest R-2 value, and these two equations also gave the most accurate prediction when using published results. These two equations are CH4-E (MJ/day) = DEI (MJ/day) (0.094 + 0.028 S-ADFI/T-ADFI) - 2.453 (FL-1) CH4-E (MJ/day) = DEI (MJ/day) (0.096 + 0.035 S-DM1/T-DM1) - 2.298 (FL-1) (C) 2000 Elsevier Science B.V. All rights reserved.