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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. White clover can improve herbage dry matter production in grass-based milk production systems. This study investigated the persistency of white clover under a high N-fertiliser application level and at a high stocking rate. White clover inclusion initially increased herbage DM production under these conditions; however, the initial extra herbage DM production on grass–clover swards decreased each year. [ABSTRACT FROM AUTHOR]

Grazed grass is considered the cheapest feed available for dairy cows in temperate regions, and to maximise profits, dairy farmers must utilise this high-quality feed where possible. Recent research has reported that including white clover (Trifolium repens L.) in grass swards can have a positive effect on milk production. The aim of the present study was to quantify the effect of tetraploid and diploid perennial ryegrass (Lolium perenne L.; PRG) swards sown with and without white clover on the milk production of grazing dairy cows. Four grazing treatments were used for the study; tetraploid-only PRG swards, diploid-only PRG swards, tetraploid PRG with white clover swards and diploid PRG with white clover swards. Thirty cows were assigned to each treatment and swards were rotationally grazed at a stocking rate of 2.75 cows/ha and a nitrogen-fertiliser application rate of 250 kg/ha annually. There was no significant effect of ploidy on milk production. Over the present 4-year study, cows grazing the PRG–white clover treatments had greater milk yields (+597 kg/cow.year) and milk-solid yield (+48 kg/cow.year) than cows grazing the PRG-only treatments. This significant increase in milk production suggests that the inclusion of white clover in grazing systems can be effectively used to increase milk production of grazing dairy cows. Recent research has reported that perennial ryegrass (PRG) ploidy and white clover inclusion in grazing swards can have a positive effect on milk production. In the present study, cows grazing PRG–white clover swards had greater milk yields than did cows grazing PRG-only swards, while grass ploidy had no effect. This significant increase in milk production from PRG–white clover swards suggests that the inclusion of white clover in grazing systems can be effectively used to increase milk production. [ABSTRACT FROM AUTHOR]