Effects of feed intake on enteric methane emissions from sheep fed fresh white clover (Trifolium repens) and perennial ryegrass (Lolium perenne) forages

Abstract

Published analyses of enteric methane (CH4) emissions from sheep and cattle show an inverse relationship between feed intake and CH4 yield (gCH4/kg dry matter (DM) intake), which suggests opportunities for reducing CH4 emissions from feed eaten and per unit of animal production. Most relationships between feed intake and CH4 yield have been based on animals fed conserved feeds, especially silages and grains. Our research is a series of experiments with fresh white clover (Trifolium repens) and perennial ryegrass (Lolium perenne; ryegrass) forages fed to sheep at a range of feed intake levels. This study was comprised of four experiments where good quality freshly harvested white clover or ryegrass were fed to sheep over a three-fold range in DM intake, and CH4 emissions were measured in respiration chambers for two consecutive days in each experiment. Measurements were made from 16 sheep in Experiment 1 (fed at 1.6×metabolizable energy requirements for maintenance; MEm), 28 sheep in Experiment 2 (at 0.8 and 2.0×MEm), eight sheep and two measurement periods in Experiment 3 (at 1.6×MEm), and 30 sheep in Experiment 4 (fed at 0.8, 1.2, 1.6, 2.0 and 2.5×MEm). Prior to each experiment, sheep had a 10d acclimatization period to diets. Apparent digestibility was measured over 7d from sheep in Experiments 1, 3 and 4, along with collection of rumen digesta for volatile fatty acid (VFA) determination. Although CH4 yields differed when sheep were fed white clover or ryegrass at similar intakes, the differences were inconsistent and mean values similar across all experiments. This, and a similar structure of all experiments, enabled combined analysis of data from all four experiments using the restricted maximum likelihood (REML) procedure to estimate effects of feed intake level on digestibility, digestible nutrient intake, gas emissions, and VFA concentrations in the rumen. The REML analysis showed that when DM intake increased from 0.40 to 1.60kg/d, the predicted responses were an increase in CH4 production (g/d) of 187% (12.4–35.6g/d; P<0.001), and a decline in CH4 yield of 21% (25.6–20.2g/kg DM intake; P<0.001). High feed intake levels were associated with increased molar proportions (mM of total VFA) of propionate from 0.17 to 0.21 (P=0.038). Single and multiple regressions were completed on the data from all experiments, with organic matter (OM) intake predicting 0.87 of the variation in CH4 production, and molar proportion of propionate predicting 0.60 of the variation in CH4 yield. Increasing feed intakes by 1kg/d of DM reduced CH4 yield by 4.5g/kg DM intake. Plant chemical composition was weakly related to CH4 yield. High intakes of fresh forages will lower CH4 yield from fermentation, but effects of feed composition on CH4 emissions were minor. The interaction between effects of feed intake and rumen function requires further investigation to understand relationships with CH4 emissions.