Grass-Clover Mixtures: High Yields, Low Nitrate Leaching

The advantages of grass-clover mixtures in ley farming are manifold. An Agroscope study now shows that their highly efficient use of available nitrogen also keeps the risk of nitrate leaching low.

Grass-clover mixtures offer major advantages in ley farming: they supply high-quality feed, promote soil structure and fertility, use resources efficiently and provide the rotation with nitrogen from symbiosis which the clover fixes from the air.

In collaboration with the ETH Zurich, an Agroscope team investigated the effects of clover-nitrogen fertiliser combinations on yields and the risk of nitrate leaching. Researchers tested the influence of different quantities of nitrogen fertilisers on pure grass swards, grass-clover mixtures and pure clover swards. Among others, they analysed soil nitrate levels and nitrogen balance, recording the risk of nitrate leaching both during the growth of the temporary leys and after tilling of the leys for the follow-on crop in the rotation.

Pure clover and pure grass swards both have their drawbacks

Pure grass swards with either very low or moderate nitrogen fertilisation (50 and 150 kg N, respectively) are not at risk of nitrate leaching, but were low-yielding. They require very high nitrogen fertilisation to achieve similar productivity to mixtures. This very high fertilisation rate can lead to a time-lagged risk of nitrate leaching.

Even at the very low fertilisation rate, pure clover swards exhibited an increased risk of nitrate leaching. The similarly high nitrate-leaching risk of pure grass swards at high fertilisation rates and pure clover swards at low fertilisation rates suggests that leaching risk is influenced more by the nitrogen balance (total input minus nitrogen in harvested plant mass) than by the source of the nitrogen, i.e. fertiliser or symbiosis.

Grass-clover mixtures use nitrogen efficiently…

By contrast, grass-clover mixtures maximised biomass production at a low risk of nitrate leaching. This was accomplished by the efficient use of nitrogen from symbiosis, fertilisers and soil. Even after the tilling of the mixtures, nitrate leaching remained comparable to or even lower than after the tilling of pure grass swards.

… and yield more forage with lower nitrate leaching

The study reaches the conclusion that moderately fertilised grass-clover mixtures are markedly higher-yielding than pure grass swards. At the same time, the mixtures regulate symbiotic nitrogen fixation according to the needs of the plants and the availability of nitrogen in the soil, leading to a nitrogen balance of close to zero. As a result, they do not increase the risk of nitrate leaching compared to pure grass stands.