The effects of functional trait diversity on productivity of grass-legume swards across multiple sites and two levels of nitrogen fertiliser

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Productive and stable forage yields are essential for the sustainability of ruminal livestock production. Grassland seed mixtures composed of species of diverse functional groups have previously been demonstrated to increase yield performance and stability compared to monocultures. In this study we conducted field trials with five grass and two legume species either grown in monocultures or a range of mixtures from two-species to seven-species mixtures sown in a simplex design. The species represented different functional groups regarding ability to fixate atmospheric nitrogen (N), rate of establishment and temporal persistence.The experiments were established with the same cultivars of species at five locations in Norway with climatically contrasting environments – from mild humid, mountainous continental to sub-arctic. The experimental plots were harvested for three years at four of the sites and two years at one of the sites, and they were fertilised according to normal practise in intensive silage grass production in the respective regions (regular N). At three of the sites, a treatment with low mineral N supply rate was also included.We found that crops sown as mixtures returned higher yields and contained less weeds than the average of monoculture crops, and these effects were consistent over all sites and study years. The grass-legume mixtures managed at low N supply rate performed equally well or better than monocultures or grass-only mixtures managed at regular N supply. We found no effects of the functional groups categorised as temporal persistence or rate of establishment on the yield performance, and there were no apparent benefits of increasing the number of species beyond the species P. pratense, F. pratensis and T. pratense over the three production years the experiments lasted.The results suggest that by using grass-clover mixtures, farmers can reduce N fertiliser rates, without compromising productivity of temporary grassland under northern conditions over the first three years of production.