Last week’s budget announced a €10m spend for the tillage sector with little in the way of explanation as to what it was for. Since then, the Minister for Agriculture has indicated that straw incorporation was a targeted objective. This is not too surprising, in that this is the only measure currently included for tillage in the carbon MACC curve.

Incorporating straw is a carbon-related measure with many side benefits. But the last thing any grain grower should do is plough in straw directly. It needs to be chopped first and then it needs to be mixed with the soil immediately after harvest, to enable early and faster breakdown.

A straw is like a pipe with sealed blockages (the nodes). Rapid breakdown requires that microbes have access to the stem walls from all sides, but this cannot happen if there is no cut between two stem nodes. That is why it needs to be chopped. Smaller bits are also more manageable for earthworms.

Why mix soil with straw

A layer of straw that sits on top of a turning sod can form a series of straw sandwiches between the sods.

This concentrated straw layer can be toxic to root growth and limit where the plant roots can grow.

This is frequently seen where a lump of straw is ploughed down. We also see problems caused by chaff, where combines with big headers are not fitted with chaff spreaders.

As well as getting the breakdown to start earlier, the cultivation also helps to promote weed and volunteer seed germination

The only place where one might plough down cropped straw directly would be on very loose soils which break up and mix with the straw as they turn or on land where organic matter had been applied regularly in previous years. Having the proper bugs in the soil speeds up straw breakdown and thus reduces the risk of it becoming toxic.

Aside from these situations, initial mixing and incorporation are important in the early years of returning straw. As well as getting the breakdown to start earlier, the cultivation also helps to promote weed and volunteer seed germination. Also, the loose mixed soil and straw will bury much more cleanly if ploughing afterwards. What might have been a straw sandwich falls to the bottom of the furrow as a mix of soil and straw.

How might a scheme work?

Good straw incorporation requires an initial mixing of chopped straw with soil. These costs are not insignificant, so compensation is welcome. Decent stubble cultivators are expensive, but they have been grant-aided (40%) by the tillage capital investment scheme within TAMS.

So, how might a straw incorporation scheme work? If it is voluntary, it will only be used in the areas where straw prices are lowest. If it is obligatory, it will be met with resentment. If the €10m is entirely for straw incorporation, say on 20% of the area or 50,000ha,that would imply a payment of €200/ha or €80/ac.

At farm level, one might choose to incorporate straw from a rotating crop such as oats

Staying with one-fifth of the area to be incorporated, might this be done on the same land for five consecutive years or on one-fifth of the area over a five-year period?

I think I would rotate it. While this would slow down the build-up of soil biology, it may give you a greater overall benefit. Straw incorporation helps to build soil microbe numbers but the more they build, the faster they break down the straw.

At farm level, one might choose to incorporate straw from a rotating crop such as oats. This is likely to have more straw than other cereals, it is coarser in terms of its longevity in the soil and it is generally more troublesome to bale. Barley straw tends to be easier, is generally more valuable and is preferred by customers.

Costs and benefits

Incorporation or baling – both involve additional costs. Baling has a cost, plus the time involved, as does loading and possibly transport. If baling is delayed by weather, you risk missing a planting date, plus additional time and cost for tossing and rowing up the straw, as well as having fewer bales. If you chop, you have the cost of additional fuel for chopping and the time, fuel and metal for cultivation.

Every grower will have to analyse these costs and benefits. For most, it would mean a minimum value of €10/bale to make selling worthwhile. However, this decision may need to be made in May when completing the CAP application, rather than being able to judge the straw market at harvest.

Much has been stated about the reduced nutrient offtake where straw is not removed, leaving a lower fertiliser requirement. Teagasc indicates that the nutrient offtake from an 11t/ha winter wheat crop with straw removed is 42kg P/ha and 108kg K/ha. If straw is left in situ, this requirement falls to 37kg P and 52kg K/ha. So, removing the straw is taking off an estimated additional 5kg P/ha plus 57kg K/ha. This can be quantified, and, in my book, greener straw takes off even more than these values.

Whenever straw or any high-carbon material is incorporated, it tends to pull nitrogen away from the crop for the first few years

However, I believe the nutrients held in straw are very slow to cycle. It could be at least seven years before you see soil P and K levels rise, even if you continue to apply the maintenance amounts recommended for straw removal, ie 42kg P and 108kg K/ha. By doing this, you are putting money in the bank, but it is on a time lock. Going back to straw not removed, the rates of 37kg P and 52kg K/ha could result in a shortage after five years, especially for a spring crop.

Whenever straw or any high-carbon material is incorporated, it tends to pull nitrogen away from the crop for the first few years to fuel the organisms that break down the material. For this reason, straw incorporation sits well with the simultaneous application of high-N organic manures such as pig slurry, raw digestate or chicken litter for winter planting. Straw degradation sucks up the nitrogen, making it less likely to cause excess autumn growth. A single run of a cultivator at the time of slurry application would incorporate both the straw and the slurry.

Other benefits

The benefits of straw incorporation are not all quantifiable. It will bring a small annual increase in soil carbon. It will also benefit soil biology and help increase earthworm numbers which do so much good in tillage soils. Organic materials also help increase the number of other soil microorganisms and overall biodiversity.

For the grower, a visible improvement in soil structure is evident after about three to five years. Soil becomes easier to work. This means reduced diesel usage and cost. The window for planting is widened as soil structure improves. This makes planting more feasible in late autumn and early spring.

Better and more open soil structure helps water percolation, which should keep crops safer post-planting. This has been an issue in recent years where winter wet caused serious patchiness in crops.

In short

  • It seems that the incorporation of straw may be supported following a recent budget announcement.
  • Successful straw incorporation requires considerable thought and it needs to be mixed into the soil as quickly as possible post-harvest.
  • High-N organic manures could be incorporated at the same time as the straw, thus helping to spread the cost and the straw will help soak up much of the surplus N.