Straw incorporation has been climate success but leaves straw in short supply

The Head of Crops Environment and Land Use in Teagasc, Mr Spink, also revealed that the net emissions for winter oats are very close to zero if the straw is incorporated into the soil.

Teagasc has calculated the impact on soil organic carbon sequestration and greenhouse gas emissions of incorporating straw.

Carbon sequestration is the capture of and storing of atmospheric carbon dioxide.

Assuming a straw yield of four tonnes per hectare from a cereal crop, a little under one tonne of carbon dioxide per hectare is sequestered on light soils by incorporating the straw.

“On a heavy clay soil, it will be almost twice that, just over two tonnes”, said Mr Spink. Crop growers can earn €250 per hectare for incorporating the straw of oats, rye, wheat, or barley, and €150 for incorporating oil seed rape straw.

However, while commendable, the improved soil carbon is of little consolation to livestock farmers.

Due to incorporation and bad weather, which reduced straw yields, the straw that livestock farmers traditionally purchased from tillage farmers is in very short supply in many areas, particularly in the west and northwest. Some have turned to straw imported from the UK for livestock bedding.

With tens of thousands of livestock farmers needing straw ahead of calving and lambing, prices have reached record-high levels.

Buying straw for bedding can be particularly challenging for organic farmers, who are required to have at least 50% of their livestock housing area solid floored and bedded (rather than slats). They must use straw, rushes, sawdust, or woodchips, as bedding. Or cattle may be out-wintered if conditions permit, at less than one livestock unit per acre, without poaching, and with adequate shelter.

Bedding problems are likely to increase for livestock farmers, with a decline predicted in the cereal area for harvesting next autumn, resulting in less straw. However, the straw supply could increase if high prices attract cereal growers away from incorporation.

But they must weigh the economics of that against any financial benefits from producing crops with net emissions at very close to zero, which is the case for winter oats after straw incorporation, for example.

“The carbon footprint of Irish grain is low by international standards, given that we have some of the highest yields in the world”, said John Spink.

Even lower carbon footprints are likely if local Irish (Tier 2) emissions values are calculated, compared to the Intergovernmental Panel on Climate Change (IPCC) Tier 1 figures, which are used for the national inventory for calculating Ireland’s emissions.

Tier 1 international figure calculations are the simplest, using default values to calculate GHG emissions, requiring fewer resources to collect the necessary data and to produce emission estimates. But Tier 2 (nationally developed emissions figures) or Tier 3 (values dependent on circumstances, for example, different figures for different soil types) calculations generate more precise carbon footprints.

An example of how Tier 1 and Tier 2 figures differ stems from the Teagasc research over the years to calculate how much of the nitrogen applied on farms in Ireland is emitted as greenhouse gas. 

“In the case of CAN, it’s 0.35% of the N,” John Spink said. “If we were to use the international default [Tier 1], that would be 1% of the N”. 

This means that emissions from CAN in Ireland are about a third of what they would be if measured using Tier 1 international default values.

Tier 1 factors are necessarily employed in the instances where local Irish [Tier2] emissions values have not yet been calculated.

“That’s obviously something that we need to do more research on, because I suspect, and I think most people suspect, those figures are significantly higher than they would be in reality in Ireland”, John Spink said.

He said the net emissions for Irish winter wheat are a relatively low figure of less than 50kg of CO2 equivalent per tonne.

He also explained why continuously grown winter wheat has a slightly higher carbon footprint than winter wheat in rotation. It's because slightly higher rates of N are applied, and also because it’s slightly lower yielding.

The carbon footprint of wheat derives mainly from the fertiliser that’s used. “That’s accounting for nearly half of the total CO2 emissions for our crops”, Mr Spink said.

What also significantly influences the carbon footprint is what is emitted into the atmosphere when nitrogen fertiliser is applied to the soil. That contributes about an additional 24% to the carbon footprint.

“So, three-quarters of the emissions associated with wheat production have to do with nitrogen fertiliser”.

Fuel production and its use in cultivation, seeding, crop husbandry, and harvest contribute an additional 14% of the wheat carbon footprint. However, there is little difference in the carbon footprint from growing crops using shallow minimum tillage, versus ploughing. 

“That is because, although using slightly less diesel in the minimum tillage compared to the plough, when you look across the five years that we looked at, there’s a tendency for the minimum tillage to be slightly lower yielding, so the two things offset each other, and overall, the carbon footprint is very similar”, John explained.

“Nitrogen fertiliser use and crop yield are the main factors affecting carbon footprint. We need to do further research to refine some of the factors where we’re currently having to use Tier 1 figures”, John said.

The carbon footprint of various crops in the rotation at the Teagasc Oak Park Crops research centre in Co Carlow was calculated from five years of data, from 2016 to 2020.