Nitrogen strategy on clover swards - what does the research say?

At the Teagasc Dairy Conference 2022, Dr. Deirdre Hennessy presented on Nitrogen strategy on clover swards - what does the research say?

Summary

• White clover is a It can fix atmospheric N and make it available for grass growth
• Fixed N can replace chemical N fertiliser, especially in the summer period (May to August)
• White clover will allow Irish farmers to continue to grow large quantities of grass at lower rates of chemical N fertiliser application.

Introduction

Grass-based milk production systems are heavily reliant on nitrogen (N) availability for pasture production. Sources of N in grazed grassland include chemical N fertiliser application, animal return (dung and urine patches, slurry, manure), atmospheric deposition (this is small; approximately 6 kg N/ha per year), background N (N stored in the soil and released through mineralisation) and, if clover is present in the sward, biological N fixation. Sown pastures in Ireland generally consist of perennial ryegrass (Lolium perenne L.) which requires chemical N fertiliser application to maximise productivity. In the last 10-15 years, the use of chemical N fertiliser has received a lot of focus, particularly in terms of the negative impact of N on water quality as well as gaseous emissions. More recently, the increased cost of chemical N fertiliser has also resulted in an increased focus on reducing its use. Incorporating clover in grassland swards must be accompanied by a reduction in chemical N fertiliser use to reduce nitrous oxide emissions and contribute to reducing agricultural greenhouse gas emissions.

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Reducing chemical N fertiliser input

In Ireland, perennial ryegrass swards receiving 250 kg N/ha can grow 13-14 t DM/ha, sustaining stocking rates of ~2.5 livestock units/ha (O’Donovan et al., 2020). Restrictions in N fertiliser use are of concern as reducing N fertiliser input can reduce grass production (Enriquez-Hidalgo et al., 2018). Analysis by Ruelle et al. (2022) concluded that pasture- based milk production systems in Ireland stocked at 2.5 cows/ha and receiving ~250 kg N/ ha were self-sufficient in terms of grass-grown for spring to autumn grazing and winter feed. Reducing chemical N fertiliser input reduced farm feed self-sufficiency. Ruelle et al. (2022) concluded that for every 50 kg N/ha reduction in chemical N fertiliser from 250 kg N/ha, stocking rate would have to be reduced by 0.18 LU/ha for systems to remain self-sufficient. To maintain herbage production in an environment with reduced chemical N fertiliser application, farmers must consider other options, including optimising soil fertility, optimising slurry use, and introducing white clover to their grassland (Dillon et al., 2020).

Role of white clover

White clover (Trifolium repens L.; hereafter referred to as clover) is the most important legume species in grazed grassland in Ireland. Clover grows well in association with grass, is tolerant of grazing, promotes high dry matter (DM) intake, has high nutritional quality and can grow over a wide range of climatic conditions (Whitehead, 1995). In addition, there are herbage production and milk production benefits associated with the inclusion of clover in grass swards. Perennial ryegrass-clover swards can make an important contribution to the future sustainability of ruminant production systems in Ireland and Western Europe (Peyraud et al., 2009). The capacity of white clover to fix atmospheric N and make it available for plant growth allows farmers to reduce chemical N fertiliser application, which is beneficial in reducing nitrous oxide (a greenhouse gas) emissions and emissions associated with the manufacture and transport of chemical N.

Sward white clover content

To achieve maximum benefits of clover inclusion in the sward, an average annual clover content of approximately 20% is required. Sward clover content varies across the year. It is low in spring, and contributes little to the DM intake of dairy cows. Clover needs a soil temperature of about 8°C for growth (compared to about 5°C for grass). Sward clover content increases from approximately 0-5% in February to a peak of 35-50% in early September and then declines.

Benefits of white clover

Nitrogen fixation

Clover is a legume; this means it has the capacity to fix atmospheric N and make it available for plant growth. This occurs through a symbiotic relationship whereby rhizobia bacteria in the soil infect clover root hairs forming nodules. The clover supplies the bacteria with energy from photosynthesis to fix N which the bacteria makes available to the clover plant for growth. In frequently grazed swards (8-10 times per year), up to 250 kg N/ha per year can be fixed when zero chemical N fertiliser is applied.. The rate of N fixation is influenced by the N fertiliser supply to the sward and the sward clover content. Generally, as N fertiliser application rate increase, N fixation declines.

Herbage production

Incorporating clover into grazed grassland can increase herbage production, particularly at lower N application rates. Research from Clonakilty Agricultural College found that incorporating clover (average clover content 23%) into intensively managed swards increased annual herbage production by 1.5 t DM/ha relative to grass-only swards (where both sward types received 250 kg N/ha). Research at Moorepark shows that grass-clover swards receiving 150 kg N/ha grew the same quantity of herbage as grass-only swards receiving 250 kg N/ha (13.4 t DM/ha) over an eight year period.

Milk production

Grass-clover swards tend to have higher quality in mid-season compared to grass-only swards as clover content increases from May onwards. Clonakilty and Moorepark research both show increases in milk production and milk solids production (P<0.05) from grass- white clover swards compared to grass-only swards.

Nitrogen use efficiency

Nitrogen use efficiency (NUE) is hugely important in grazing systems as N is a key nutrient lost from these systems. It is influenced by many factors including N fertiliser application rate, quantity and crude protein content of concentrate fed, and N removed from the system in milk and meat. The farm-gate NUE of a farm systems experiment undertaken at Teagasc, Moorepark from 2013-2016 was examined using a farm-gate N balance model. The experiment compared herbage and milk production from a grass-only sward receiving 250 kg N/ha per year (Grass250) and grass-clover swards receiving 150 kg N/ha per year (Clover150). Each treatment was stocked at 2.74 cows/ha. The N inputs were purchased concentrate, fertiliser and replacement animals, and the N outputs were milk and livestock. The N fixed by the clover was not included. The NUE of the systems increased from 37% on Grass250 to 55% on Clover150 due to the reduction in N fertiliser application and the increase in milk production on that treatment.

Long term research at Moorepark

Eight years (2013-2020) of research at Moorepark comparing the standard grass-only grazing system receiving 250 kg fertiliser N/ha with a grass-white clover system receiving 150 kg N/ha have been completed. Both systems were stocked at 2.74 cows/ha. Cows were assigned to their respective system post-calving each spring and remained on that system until housing in late November each year.

N fertiliser application strategy on grass-clover swards

In high stocking rate systems (>2.5 cows/ha), it is necessary to apply similar levels of N fertiliser on grass-clover swards as on grass-only high N systems until April. Clover makes a small contribution to the sward herbage mass in the early part of the year due to requirement for higher temperature for growth. Applying similar levels of N fertiliser is necessary to ensure adequate availability of herbage for early spring grazing. From May onwards, N fixation is actively occurring and supplying the clover component of the sward directly with N. In addition, the N stored in the organic component of the soil is higher in established grass-clover swards due to the addition of decomposing clover parts to the organic matter. This results in more N being released from the soil for grass growth through mineralisation. Chemical N fertiliser can be reduced by at least 100 kg N/ha on established grass-clover swards from May onwards. Some chemical N fertiliser (12-15 kg N/ha) is required for application in September to help build cover for the autumn.

Conclusion

Incorporating white clover into grassland swards is an effective way of maintaining herbage production when chemical N fertiliser application is reduced. Reducing chemical N application reduces nitrous oxide emissions. Grass-clover swards receiving 150 kg N/ha have similar or greater herbage production than grass-only swards receiving 250 kg N/ha. The Moorepark long-term clover research shows that milk solids production is greater on grass-clover swards compared to grass only-swards.

Read the full National Dairy Conference 2022 publication here