The effects of fresh forages and feed intake level on digesta kinetics and enteric methane emissions from sheep

Published data have shown that in ruminants, methane (CH4) yields (g/kg dry matter [DM] intake) decline as feed intakes increase and, although the reduction has been attributed to a shorter digesta mean retention time (MRT), there are few supporting data. This study was undertaken to determine the association between digesta kinetics and CH4 emissions measured from sheep in respiration chambers fed either fresh white clover (Trifolium repens;WC) or fresh perennial ryegrass (Lolium perenne; RG) (Experiment 1), or RG at several feed intakes (Experiment 2). Measurements included CH4, whole tract apparent DM digestibility(DDM), total tract and rumen MRT (TMRT and RMRT, respectively) of solid and liquid fractions, as well as passage rates. In Experiment 1, eight sheep each with a rumen fistula were fed hourly either WC or RG forages, repeated over two periods (four sheep/diet/period) at about 1.6 times maintenance requirements for metabolisable energy (MEm; 1.12 kg DM/d).Diet did not affect apparent DDM (726 g/kg), CH4yield (22.3 g/kg DM intake), or TMRT of solid fractions (29.4 h). However, TMRT for the liquid fraction was shorter (P=0.037) for sheep fed RG (17.4 h) compared with WC (23.0 h), and rumen digesta analyses suggested a larger rumen liquid pool size when RG was fed (6.05 L) compared with WC (3.96 L) (P=0.041). Experiment 2 involved 30 sheep offered fresh RG twice daily at about 0.8, 1.2,1.6, 2.0 and 2.5 × MEm. The DDM did not differ greatly across RG intakes (625–648 g/kg) but, as RG intake increased (0.49–1.34 kg DM/d), there were corresponding reductions(P<0.001) in CH4 yield (27.0–23.9 g/kg DM intake), liquid TMRT (31.4–14.2 h), solid TMRT(46.4–24.8 h), liquid RMRT (18.4–7.5 h), and solid RMRT (28.4–15.8 h). When CH4 yield was plotted against rumen liquid and solid passage rates, the extent of the relationship was best explained (R2) when RG was fed at different intakes in Experiment 2 (0.71 and 0.66 for liquid and solids, respectively). The 2.7-fold increase in feed intake halved RMRT, but intakes affected passage of the rumen liquid fraction to a greater extent than solids. It can be concluded that reductions in CH4 yield from fresh forages fed to sheep are associated with shorter TMRT and RMRT. Understanding the effects of diet, digestion, feed intake,and feeding frequency on methanogenesis requires more knowledge about rumen digesta kinetics, especially relationships between outflow rates of solid and liquid fractions.