Lowering the carbon footprint of pasture-based milk production at Solohead

At Solohead Research Farm, we have been investigating the potential for lowering the carbon footprint of pasture-based milk production by implementing best practices. A key focus has been using clover to replace artificial fertiliser nitrogen (N).

Typical rates of fertiliser N cause a large accumulation of N in the soil; ranging between 30 and 120 kg per ha of N. This N is taken up by the sward at a rate of 1 to 2 kg per ha per day. Hence, there can be a lot of reactive N (ammonium or nitrate) sitting around near the soil surface for long periods at risk of being lost.

A small proportion of this N is lost as nitrous oxide gas through denitrification. Nitrous oxide is a greenhouse gas with 265 times the global warming potential of carbon dioxide. Very small losses of nitrous oxide can have a disproportionally large impact on the carbon footprint of a farm.

On the other hand, clover makes N available deeper within the soil on an on-going basis; at rates that are more-or-less in line with the uptake of N by the sward. Hence, with clover there are no large accumulations of reactive N in the soil and negligible losses of nitrous oxide from clover-rich grassland.

Productivity, carbon footprint and profitability

We have investigated different systems of clover-based dairy production at Solohead compared with a system reliant on fertiliser N input of 275 kg per ha (FN275). This FN275 system was based on perennial ryegrass swards containing an average of 9% clover (Table 1) and reseeded at an annual average rate of 6% per year.

Our standard clover-based system received between 40 and 110 kg per ha of fertiliser N applied as protected urea in spring depending on growing conditions and averaging around 95 kg per ha per year (FN95).

This system received no fertiliser N input from April onwards and had clover contents of around 22% per year (Table 1). For the last four years (2019 to 2022), we have been running a system receiving no fertiliser N input (FN0). Lowering fertiliser N input has resulted in higher clover contents; approximately 30% averaged over the year on FN0, and a 1:1 increase in biological N fixation (BNF). The swards in FN95 and FN0 were reseeded at an annual average rate of 12% per year. Cattle slurry was recycled back onto each of these three systems at rates proportional to the stocking rates on each of the systems (2.6 cows per ha).

Averaged over five years, the FN95 system produced 97% of the pasture of the FN275 system (Table 1). This includes 2018 when pasture production was curtailed by soil moisture deficit during the summer. Averaged over the last three years, there has been no difference in pasture production between the FN95 and FN0 clover-based systems. The FN95 had a 16% lower carbon footprint than the FN275 system, whereas the FN0 had a 25% lower footprint, which is in line with the target outlined in the Climate Action Plan.

These differences are mainly due to lower fertiliser N use. There was no difference or improved profitability with the clover-based systems compared with FN275 mainly because there was similar milk and other sales with lower input costs. We discontinued the FN275 system at the end of 2021 because it was obsolete.

Table 1: Pasture and clover production at Solohead Research Farm between 2017 and 2022

Measured using cut data. PBI data for same paddocks is 1.5t DM/ha less.

Maintaining clover and BNF and avoiding bloat

The management of clover-based swards is different to that of N-fertilised ryegrass swards. There are a number of management practices necessary for maintaining perennial high clover DM yields and high rates of BNF on the grazing platform:

• High soil lime status (pH 6.5 to 7.2), high soil K (index 3/4) and soil P Index 3.
• Reseed 10% per year to get productive clover swards established on the platform.
• The seed mixture should contain per acre pack: 2 kg white clover varieties that are on the Irish recommended list and 2 kg of red clover. Recommended red clover varieties are Milvus, Aberclaret, Aberchianti, Fearga and Lemmon.
• Apply a clover-safe post-emergence herbicide to the establishing crop.
• Minimise fertiliser N input and maximise BNF. BNF typically commences around 6 weeks after sowing and can exceed 150 kg/ha following an April reseed.
• Minimal (in February and March) or no fertiliser N in subsequent years. Apply the saved fertiliser N on non-clover paddocks if concerned about overall pasture production on the farm. This will give best economic response to fertiliser N.
• High input of fertiliser N favours grass production to the detriment of the clover component of the sward, e.g. FN275 (Table 1). Likewise, very high rates of BNF over a number of years can also be detrimental to clover content. This can be rectified by taking off a cut of silage; a 5 t DM crop of herbage harvested for silage will remove between 150 and 200 kg/ha of N from the soil. It is good practice to integrate a cut of silage (baled surpluses) into the grazing rotation.
• Where there are very high clover contents this problem will normally solve itself; high yields of clover results in high rates of BNF, which favours the grass component of the sward. Closing up a paddock early in the last rotation and leaving a heavy pasture cover over the winter will rapidly deplete the clover content.
• Clover is very vulnerable to competition from grass over the winter. The lower the winter cover the better in terms of higher clover production and BNF in the following year.
• Bloat has not been much of a problem over 20 years at Solohead; partly because cows get conditioned to clover during the grazing season. There is always clover in the cow’s diet; clover contents increase from 15% at turnout to 50% in August. Putting more fibre into the diet is the key to avoiding bloat. We typically build pasture covers during August and early September as a means of extending the grazing season. This means we are grazing higher covers (up to 3,000 kg DM per ha above 4 cm) when clover contents are at their highest. The biggest risk of bloat that we have encountered is when overall farm cover declines during October and cows are grazing lower covers (1,200 kg DM/ha) of lush low DM pasture. Feeding 2 kg DM per cow of wilted silage/haylage (35-40% DM) per day prevents bloat under these circumstances as well as helping to lift yield of milk solids and extend the grazing season.

Conclusions

A clover-based system can substantially lower the carbon footprint of a dairy farm with no loss of profitability or with improved profitability primarily depending on the cost of fertiliser N and the price received for milk.

This paper by James Humphreys, Dan Barrett, Owen Cashman, Marion Sorley and Emma Buckley first appeared in the Moorepark 2023 open day proceedings. For more information on the research work being conducted through Moorepark, click here.