Methane is a major contributor to greenhouse gases. Methane is produced by the ruminal microbiome and eructed by cattle. While there have been some investigations into the determinants of methane emissions, these have mainly been done in dairy cattle, not in grazing beef cattle. Given that the female herd spends most of their life pregnant, the effects of pregnancy on methane emissions are important to investigate.
In a cohort of 20 pregnant and 18 non-pregnant heifers, we measured methane emissions across 5 days at the end of trimester 1 of pregnancy using the SF6 tracer gas methodology. Methane yield was analysed using GC-MS and levels were averaged across the days of measurement both as g/day and in g/kg live weight and compared between the groups. Methane yield (CH4 g/DMI) was not measured in this study. Blood was obtained at the same time and glucose, triglycerides, total cholesterol, NEFA and progesterone were measured in the veterinary pathology laboratory.
Pregnant heifers had lower methane emissions than their non-pregnant counterparts both as measured in g/day (pregnant 116 ± 25 g/d vs. non-pregnant 195 ± 110 g/day, p=0.005) and as g/kg live weight (pregnant 0.27 ± 0.06 g/kg LW vs. non-pregnant 0.48 0.27 g/kg LW, p=0.003). There were no differences in circulating metabolites, but progesterone levels were higher in pregnant (43 ± 14 nmol/L) than in non-pregnant heifers (13 ± 14 nmol/L, p<0.0001), as expected. However, methane emissions were not directly correlated with progesterone or metabolite levels.
In summary, early pregnancy is associated with lower daily methane production. There was no evidence that metabolic markers or progesterone levels in heifers are major determinants of methane levels. It is possible that changes in feed intake or the ruminal microbiome in early pregnancy can explain the lowering of methane emissions.