Combining targeted grass traits with red clover improves grassland performance and reduces need for nitrogen fertilisation

To increase ruminant production efficiency, the environmental impact of growing forage must be reduced. Here we examined the role of red clover (cv. AberClaret) in minimising nitrogen (N) requirements, alongside two novel grass varieties, (1) a festulolium (cv. AberNiche), developed for drought tolerance, with potential for deep-rooting, and (2) a ryegrass hybrid (cv. AberEcho), developed for high-sugar content, which may enhance ruminant N-uptake in-vivo. Field trials were conducted at two sites growing festulolium and ryegrass ± red clover (at 29% of the seed mix weight), at a range of N fertilisation rates (0 – 600 kg N ha-1), for 2 years (six harvests). We assessed sward performance (N offtake, herbage and silage quality, grass N use efficiency), rooting depth and N transfer from clover to grasses using 15N natural abundance. Across both sites and years, dry matter and herbage-N content were overall greater from the swards that included clover. Yields from festulolium were not greater than from ryegrass under the drought conditions experienced, despite its greater root mass. Agronomic efficiency of fertiliser N was similar between grasses (19 - 22 %), however the festulolium more effectively used endogenous soil N than the ryegrass. There was no difference in soil N and C profiles between the two grasses. Inclusion of clover in the sward positively affected forage quality (crude protein, metabolisable energy), but reduced sugar and fibre (NDF) content. Among the grass types, metabolisable energy was greater and NDF content less for ryegrass than for festulolium. The effect of clover within the sward carried through to the ensiled herbage with increased N and reduced sugar and fibre in the silage from the clover mixed swards, relative to the single species grass swards. A strong reliance on biological N fixation (80 – 94%) for clover was observed, however, N transfer from clover to the neighbouring grass was not evident from the ẟ15N signatures. Inclusion of grass varieties that can deep-root or provide high-sugar content had no impact on yield, but herbage quality was relatively better for ryegrass. The capacity for festulolium to (i) deep-root and enhance grassland resilience under a prolonged drought, and (ii) promote deep soil C storage was not observed in this study. We conclude that red clover is a viable fertiliser-N replacement strategy in short-term leys, and that grass varieties with improved herbage quality may provide a better option for optimising sward performance than drought tolerant grass varieties