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Tillage Farms are low GHG emitters but targeting nitrogen can reduce emissions further


The biggest contributor to Greenhouse Gas (GHG) emissions on tillage farms is nitrogen use. Michael Hennessy, Head of Crop Knowledge Transfer Department, Teagasc looks at some of the figures involved in reducing nitrogen rates on farms.

Based on the data from the Teagasc National Survey Sustainability Reports from the past few years tillage farmers emitted the equivalent of just over 2.3 t CO2-e/ha, however when the livestock proportion are stripped out of these farms, the tillage proportion is just over 1.2 t CO2e/ha. When we compare this to other systems, cattle farms emit close to 4.5 t CO2-e/ha and dairy farms close to 9.2 t CO2-e/ha. So, the starting point is that tillage farms are low emitters of CO2 equivalent per hectare.

How can this be improved?

The biggest contributor to GHG emissions on tillage farms is nitrogen use. This single input alone can account for 80-85% of all emissions per year. These emissions come in two forms:

  • from the manufacture of NPK, and
  • release of nitrogen in the form of N2O (nitrous oxide) and indirect emissions from leached nitrate.

The losses are associated with nitrous oxide (N2O) lost to the air and nitrate leaching to ground water. The N2O lost in a tillage system is relatively low even when using CAN and a little lower if using Urea but its still relatively small. However, the losses of nitrate through the late summer and autumn can be significant in free draining soils, which are mainly tillage soils. We also know from reports published from the EPA that rivers in the main tillage catchments, particularly in the south east, have high nitrate values and a proportion of this nitrate comes from tillage soils.

So with all that said targeting nitrogen use on tillage farms is a sensible place to start to reduce GHG emissions, although the overall reductions will be small. Considering the cost of nitrogen in 2022, it’s a good year for farmers to look at ways to reduce nitrogen use on farm. There are a number of ways to reduce nitrogen use including; growing your own nitrogen (using legume crops), reduce nitrogen inputs (by using less and increasing the precision of application), use an alternative source of nitrogen (by using organic manures), and changing the source of your nitrogen. We will look at these one by one.

1. Grow your own nitrogen from legume crops

Beans are a crop many people may have tried over the past 10 year. The crop can suffer from yield inconsistency and when the crop is poor it can be very poor (think of 2018) however growers who plant the crop on time (on the correct land) and manage it carefully are achieving quite consistent yields, year on year. Increasing the area of beans can significantly reduce fertiliser requirements. Take an example of an 80 hectare farm, growing winter barley (20 ha), spring barley (56 ha) and a small amount of beans (4 ha). If the cropping is adjusted, where winter barley would stay the same (as its already planted) and the farm maximises the area of beans ( e.g. beans at one in five years) and reduce the amount of spring barley sown, this reduces the farm’s nitrogen requirement by 14% or close to 7 tonnes of CAN (or €4,000 @ €600/t for CAN). These are significant savings and the overall profitability of the farm will not be compromised.

2. Reducing nitrogen below the optimum rate for yield is always tricky

Nitrogen recommendations for crops are based on the economic optimum. This rate generally builds in a little extra nitrogen to ensure high yield, but not necessarily the highest yield. By increasing nitrogen above this level does not guarantee a higher yield, rather it’s a sum of diminishing returns. Based on this, and knowing the cost of fertiliser and the sales price for grain, we can calculate the break-even return or BER for crops.

Teagasc calculations, where CAN is €620/t and grain can be sold for €200/t, then growers should apply 33kg/ha less nitrogen to crops. Where grain rises to €220/t, then growers should only use €27/ha less nitrogen. Given the projected cost of nitrogen and the sales value of grain, reducing nitrogen will be profitable for most growers.

3. Organic Manures

Where growers can get organic manures and maximise the embedded nitrogen then significant savings can be made. For example, a grower applying good quality cattle slurry 33m3/ha (3,000 gal/ac) to 20 ha (50 acres) can save €6,000 on chemical fertiliser (not including the costs of slurry spreading).

4. Changing from CAN to protected urea

Changing from CAN to protected urea can help to marginally reduce emissions on a plough based system. Research shows some reductions are possible but other considerations such as achieving the correct spread width (out to 24m or beyond) may be more problematic.