White clover incorporation at high nitrogen application levels: results from a 3-year study

Context: High nitrogen (N)-fertiliser application levels and high stocking rates have been shown to optimise herbage dry-matter (DM) production in grass-only systems. Conversely, swards including white clover (Trifolium repens L.) are commonly suited to production systems at low N-fertiliser application levels (<150 kg N/ha) and low stocking rates (fewer than two livestock units (LU) per hectare). Despite this, the use of N fertiliser on grass-clover swards has generally increased during the past few decades, particularly in more intensively managed swards under dairy farming. Increasing inorganic N decreases sward white clover content in grass-clover swards, and increasing stocking rate on grass-clover swards can result in damage to white clover plants. However, under high N-application levels, a high stocking rate can also potentially minimise the negative effects of increased grass growth on sward white clover content. Aims: The objective of the present study was to investigate the persistency of white clover in an intensive animal-grazing system, under a high N-fertiliser application level (250 kg N/ha) and at a high stocking rate (2.75 LU per hectare) over a 3-year period (2014-2016). Methods: The study was a 2 × 2 factorial design, consisting of two perennial ryegrass ploidies (diploid, tetraploid) and two white clover treatments (grass-only, grass-clover). Four sward treatments (diploid-only, tetraploid-only, diploid + clover, tetraploid + clover) were evaluated over a full grazing season at a system scale. Sward measurements were taken at each grazing occasion for 3 years. Key results: Over the 3 years, grass-clover swards produced an additional 1468 kg DM/ha when compared with grass-only swards. Sward white clover content decreased by 17% over the three study years. Conclusions: This decrease in sward white clover content led to a decrease in contribution to cumulative herbage DM production. Implications: Although the present study showed that white clover inclusion can increase herbage DM production at high N-fertiliser application levels, in association with high stocking rates, the initial extra herbage DM production on grass-clover swards decreased each year. Further studies should be undertaken to investigate whether the decline in white clover persistence observed is progressive over a longer time period.