Methane emissions changed nonlinearly with graded substitution of alfalfa silage with corn silage and corn grain in the diet of sheep and relation with rumen fermentation characteristics in vivo and in vitro

Feeding grain and maize silage have been proposed as practises to reduce methane (CH4) emissions from ruminants, but inclusion level required is not known. The objectives of this study were to determine the response in CH4 emissions and rumen fermentation characteristics in sheep fed lucerne silage substituted with increasing levels of maize silage or maize grain and its relationship with in vitro gas production parameters when fed the same diets. Romney ewe hoggets (~14 months old; n = 64) were randomly divided over eight dietary treatments, which included: chaffed lucerne silage alone or, substituted by either 25, 50, 75 and 100 % maize silage or 25, 50 and 65 % rolled maize grain, on a dry matter (DM) basis. After acclimatization to the diet, dry matter intake (DMI) and CH4 emissions were measured from individual sheep for two consecutive days in open circuit respiration chambers and a rumen sample collected at 3 h after feeding. The same diets were also incubated in an automated in vitro gas production system for 48 h. Increasing the substitution of lucerne silage by maize silage or maize grain in the diet of sheep resulted in a quadratic response (P <;0.01) in CH4 emissions per unit of DMI (CH4/DMI), with either supplement. For both supplements, CH4/DMI increased up to 50:50 lucerne:supplement mixtures and then decreased with higher supplement inclusion in the diet, especially with maize grain inclusion. The ratio of (acetate+butyrate)/(propionate+valerate) and propionate alone in rumen liquid were the strongest single predictors for CH4/DMI in the overall data set and explained 37.1 and 32.5 % of the variation in CH4/DMI, respectively. These relationships were stronger in the maize silage inclusion data set than in the maize grain data set. In vitro CH4 production (mL/g DM; CH4/DM) increased linearly (P <;0.001) and also showed a quadratic relationship (P <0.05) with increasing substitution of lucerne silage with both maize grain and silage. There was a weak quadratic relationship (r2 <0.35) between in vivo CH4/DMI and the in vitro parameters of gas and CH4 production, total volatile fatty acids and CH4%. In conclusion, CH4/DMI changed in a non-linear fashion with increasing supplement inclusion in the lucerne forage diet of sheep, which has implications for predicting the influence of feeding supplements on animal greenhouse gas emissions.