Do establishment of multispecies swards affect nitrous oxide and methane emissions and promote soil health?

Multispecies swards containing leguminous and herb species can potentially reduce greenhouse gas emissions, especially nitrous oxide (N₂O) from grassland forage. However, there is a need to maintain or enhance the dry matter (DM) yields for livestock milk production and body weight. Multispecies swards should contribute to improved soil health and obtain nutrients and minerals from deeper in the soil profile than monocultures of grasses. A one-year plot study was conducted using increasingly complex multispecies swards; perennial ryegrass and red clover (PR), perennial ryegrass, red clover and tonic plantain (PRP), perennial ryegrass, red clover, tonic plantain and birdsfoot trefoil (PRPB) and finally perennial ryegrass, red clover, tonic plantain, birdsfoot trefoil and burnet (PRPBB) compared with a fertilised monoculture of perennial ryegrass (Pfert). The plots were cut twice for DM and quality. Additionally, the emissions of the greenhouse gases N₂O and methane were monitored. DM yields were highest for PRP, PRPB and PRPBB. The PRP DM yield was greater compared to Pfert and PR (p > 0.03 and p > 0.02, respectively). Mean emissions of N₂O were greatest for the Pfert (27.5 μg N₂O ha^-1 day^-1), compared to PR (p > 0.05), PRP (p > 0.04), PRPB (p > 0.01) and PRPBB (p > 0.01). PRP gave greater metabolizable energy than the Pfert (p > 0.03) and PR (p > 0.02). Aspects of soil health, mainly the physical structure and earthworms, were sustained by greater species mixtures. The study suggested a PRP sward gave greater yield than perennial ryegrass with less fertiliser and lower N₂O emissions and demonstrated opportunities for climate change mitigation and adaptation in dairy systems.