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Reduced enteric methane emissions on legume vs. grass irrigated pastures
Life cycle assessment (LCA) that compared the cow-calf and feedlot phases of beef production in western Canada demonstrated that the greatest source of greenhouse gas emissions, in carbon dioxide equivalents (CO2 eq.), was enteric methane (CH4). Further, the cow-calf phase was responsible for approximately 80% of CO2 eq. Perennial legume forages contain less fiber than grasses and are therefore more digestible, and condensed tannins (CT) have been reported to reduce ruminant enteric methane emissions. Our objective was to measure enteric CH4 emissions of beef cows and heifers grazing irrigated pastures. We compared a grass with two non-bloating legumes, one that had CT and one that did not. Our treatments were meadow bromegrass (Bromus riparius Rehmann), CT-containing birdsfoot trefoil (Lotus corniculatus L.), and non CT cicer milkvetch (Astragalus cicer L.). The study was a randomized complete block design with 5 replications. The experimental unit was a 0.365-ha rotationally stocked pasture containing one forage treatment and one cow in late gestation (616 ± 8 kg; 2014) or two heifers (each 439 ± 7 kg; 2015). Following a 5- (2014) or 2-week (2015) adjustment period, enteric methane was sampled on 4 d/wk for 5 wk on 1 (2014) or 2 (2015) reps/wk using the sulfur hexafluoride method (Johnson et al., 2007). Forage disappearance from pastures was estimated from pre- minus post-grazing herbage DM, measured using a rising plate meter calibrated for each species. This value is presented as percent of body weight. The herbage of the cultivar of birdsfoot trefoil used in this study, Langille, contained 20-30 mg CT/g DM while the CT concentration of the other two pasture species was negligible. We conclude that methane emissions were reduced by approximately half for cows grazing legumes compared with grass, and by approximately one-third for heifers grazing legumes compared with grass. However, there did not appear to be an effect of CT on enteric CH4 emissions.
Keywords: Birdsfoot trefoil, Cicer milkvetch, Meadow bromegras