Nitrogen effects on species’ contributions to grazed pasture mixtures under nitrogen loss and application restrictions

Abstract

New regulations for the use of N fertiliser will impact on seed mixture decisions for new pastures. To help inform these decisions, effects of N on species and their interactions in pasture mixtures were examined. Ten mixtures of perennial ryegrass (PR), white clover (WC) and plantain (P) were sown on 31 March 2017 and grown ±N fertiliser (275 kg/ha in Year 1 reduced to 200 kg N/ha/year) under grazed and irrigated conditions. After 4 years, an equi-proportional mixture of PR and WC, based on seed count and equivalent to 12 kg PR and 7 kg WC (coated seed)/ha, produced an optimal balance of increased total yield (20.5 t DM/ha/year), weed suppression (4% of total yield), metabolisable energy (11 MJ/kg DM) and crude protein (21%), regardless of N level. Pasture yield and quality responded to change in species proportions away from the optimal mixture, including the addition of P. The magnitude of the yield response was greater with rather than without N fertiliser, because the identity effects of PR and P, and the way all three species interacted, depended on N level. The optimal mixture without N fertiliser produced as much high-quality feed as mixtures with N fertiliser.