The inclusion of a companion legume or herb, in combination with perennial ryegrass increased growth performance and reduced enteric methane emissions in lambs postweaning.

Increasing lamb growth rates directly from pasture and reducing enteric methane (CH4) emissions are key drivers to enhancing production efficiency and achieving more sustainable lamb finishing systems. The objective of this study was to assess the influence of binary sward mixtures of perennial ryegrass (Lolium perenne L.) plus a companion forage on growth performance, enteric CH4 output, reticulo-rumen content weight (RRcw), rumen fermentation parameters and the composition of the rumen microbiome in growing lambs. A randomized block design was employed to investigate 5 treatments, namely, perennial ryegrass (PRG), PRG plus white clover (Trifolium repens L.; PRG + WC), PRG plus red clover (Trifolium pratense L.; PRG + RC), PRG plus chicory (Chicorium intybus L.; PRG + Chic) and PRG plus plantain (Plantago lanceolate L.; PRG + Plan). At weaning, 120 lambs (n = 24 per treatment) were selected in both 2021 and 2022 for CH4 measurement using portable accumulation chambers. A further subset of 60 lambs (n = 12 per treatment) were selected from the initial 120 lambs each year for rumen measurements and sampling. Lambs were weighed fortnightly and drafted for slaughter upon reaching their target live weight (LW). Rumen fluid was harvested using a transoesophageal sampling device postweaning and manually immediately postmortem. The average sward companion forage content was 28% on a dry matter basis. Lambs grazing PRG + WC, PRG + RC, or PRG + Chic had a higher postweaning average daily gain (ADG) (P < 0.001) and an increased lifetime ADG (P < 0.001), compared with lambs grazing PRG. The addition of any companion forage resulted in a reduction in age at slaughter of 16 to 50 d (P < 0.001). Methane production (g/d) and CH4 intensity (g/kg LW and g/kg ADG) were reduced for lambs grazing PRG + WC, PRG + RC, and PRG + Plan (P < 0.001), compared with those grazing PRG. There was a lower relative abundance of Methanobrevibacter (P < 0.001) in lambs grazing PRG + RC over those grazing PRG, PRG + Chic, and PRG + Plan. The presence of Prevotella was more abundant in lambs grazing the legume treatments, PRG + WC and PRG + RC, than those grazing PRG (P < 0.001). The RRcw of the PRG lambs was heavier than lambs grazing PRG + WC, PRG + RC, and PRG + Plan (P < 0.001). These results suggest that the manipulation of pasture species composition is a promising, practical, and feasible strategy to enhance production efficiency and mitigate CH4 from pasture-based lamb production systems.