Effects of sheep breed and grass silage quality on voluntary feed intake and enteric methane emissions in adult dry ewes

Abstract

Forty adult dry ewes in early pregnancy participated in a 2 × 2 factorial design study to investigate the effects of two grass silage qualities (Early vs. Average; 7.12 vs 5.74 MJ of net energy lactation per kg dry matter (DM)) and two breeds (Norwegian White Sheep, NWS; Old Norwegian Spæl, ONS) on voluntary feed intake and enteric methane (CH4) emissions. Ewes of the two breeds were equally mixed in two groups and assigned to Early and Average silage quality for first part of the experiment and switched for the second part of the experiment. Each period of the experiment lasted for three weeks; one week of adaptation and two weeks of measurements. The ewes were housed in individual pens and fed silage ad libitum. Feed intake was continuously logged using the BioControl System. Enteric CH4 was measured for 10 weekdays in Portable Accumulation Chambers in each period. Average body weight (BW) for NWS and ONS were 91 and 60 kg, respectively. ONS had significantly (p < 0.0001) lower daily DM intake (DMI), DMI/kg BW, daily NDF intake, NDF/kg BW, g CH4/day and g CH4/kg BW, compared to the NWS. The Early grass silage (p < 0.0001) resulted in higher daily DMI (kg DMI/day, kg DMI/kg BW), and higher enteric CH4 emissions/kg BW when offered to the ewes, compared to the Average grass silage. Significant interactions (p < 0.05) between breed and grass silage quality were observed for both measures of feed intake and CH4 emission. The results show that breed differences in CH4 emission is strongly linked to DMI, however breed differences were still present after correcting for DMI, which indicated breed differences not explained by BW or DMI. Persistent breed differences after correcting for DMI may be related to contrasts in rumen passage rate, NDF digestibility, rumen microbiome, fermentation rate etc. We conclude that improved grass silage quality may be a potential mitigation option. However, to avoid weight gain in early pregnancy in an ad libitum feeding regime, the practical applicability may be limited to periods with higher feed requirements, such as in late pregnancy and early lactation. The breed differences in enteric CH4 emission need to be related to productivity (e.g., fecundity, lamb growth rate). Potential breed differences not explained by body size and/or DMI need to be further investigated.