Productive and reproductive parameters, rumen environment and methane emission by beef cows grazing native pastures with or without inclusion of a tannin-rich legume

Beef production in pasture-based systems is often associated with relatively high methane emissions. Improving forage quality by including Lotus species may contribute to methane mitigation in rangeland systems. We aimed to evaluate the productive and reproductive performance, ruminal environment, and methane emission in beef suckling cows grazing native pastures (NPs) improved with a tannin-enriched legume (Lotus subbiflorus). Fifty-six Aberdeen Angus × Hereford multiparous cows were assigned to two treatments on day 22 ± 0.82 postpartum: (i) NP (n = 28), and (ii) NPs improved with L. subbiflorus (improved pastures, IPs, n = 28). The initial forage allowance was 4.7 kg DM/kg BW for both treatments. IP contained 27% L. subbiflorus, with 3.6 g/100 g DM of condensed tannins. Milk yield was assessed biweekly from the start of the experiment until day 133 postpartum. Body condition score and BW were assessed at calving and every 28 days. Ovarian activity diagnosis was performed on days 57 and 87 postpartum. Methane emission was measured for 49 days using the GreenFeed® system. The animals were rumen sampled via oesophageal intubation to determine pH, volatile fatty acids, and NH₃ levels. BW was higher (P < 0.05) in IP than in NP cows on day 78, but not on day 133 postpartum. Average milk production (kg/d) was greater in IP than in NP cows from days 30 to 84 postpartum (P < 0.05) and from days 84 to 140 postpartum (P < 0.05). Daily BW gain between days 22 and 78 postpartum tended to be higher (P = 0.057) in IP than in NP calves, but no differences were observed between days 78 and 133 postpartum. On day 57 postpartum, more IP cows were in superficial anoestrus than NP cows (P < 0.05); however, no differences were observed on day 87 postpartum (P = 0.979), as most of the cows in both treatments were cycling. Cows in IP presented lower ruminal acetate concentrations (P < 0.05) and higher (P < 0.01) NH₃ content than cows in NP. Methane emission was higher in cows in IP than in NP (P < 0.001). However, methane emission intensity was lower in IP than in NP (P = 0.05). Under grazing conditions, incorporating L. subbiflorus on NPs could be a strategy for nutritional management to enhance early postpartum beef herd performance and reduce methane emission intensity in rangeland systems.