Cover crops and straw incorporation – the benefits to tillage farms

John Mahon, Teagasc Signpost Tillage Advisor, tells us more on cover crops and straw incorporation - two post-harvest sustainability actions for tillage farmers that contribute positively to reducing their farms’ greenhouse gas emissions, while also providing additional benefits.

The Signpost Programme, established in 2021, is a multi-annual campaign to lead climate action by all Irish farmers towards achieving our greenhouse gas (GHG) emissions target reduction of 25% by 2030, in addition to improving water quality and biodiversity on farm. Based on research, climate and sustainability actions may be selected and implemented on farm to achieve individual targets.

Two such actions on tillage farms which may be adopted are the growing of cover crops, and straw chopping and incorporation post-harvest. Both of these actions can contribute positively to reducing the farm’s greenhouse gas (GHG) emissions (up to 1.33t CO₂ eq/ha/year and 1.08t CO₂ eq/ha/year, respectively, MACC 2023), along with improvements to soil health, reduction in nitrogen (N) leaching and improvements to water quality.

While both are subject to additional payments on farm through the agro-environmental scheme ACRES and the Straw Incorporation Measure (SIM), providing additional income on farm, the longer-term consequences of their use are recognised by many farmers in reduced fuel use during cultivations, better seed beds and crop establishment, reduced soil erosion over winter, and the ability to reduce base fertiliser requirements in subsequent crops. 

Why carry out these measures post-harvest?

The Nitrates Directive was designed to protect water quality from pollution by agricultural sources. New rules introduced in 2022 as part of the Nitrates Directive requires tillage farmers to take measures to establish green cover (on land which will not be winter sown by 31^st October) as soon as possible after harvest to prevent the pollution of water caused by nitrates.   Shallow cultivation must take place within 10 days of the baling of straw, or of harvest if the straw was chopped, and within 14 days of harvest in all other circumstances.

As cultivations are compulsory on 75-80% of the land destined for spring cropping, this is a good opportunity to sow cover crops or incorporate chopped straw while carrying out this measure. This can also prove very useful as a cultural weed control method, allowing the germination of weed seed post-harvest before the establishment of the following crop. 

Cover crops

The method of establishment and machine choice vary widely, but the most important thing is to ensure that cultivations are shallow (50-100mm) and that consolidation by rolling is completed immediately after sowing to ensure adequate soil to seed contact and moisture retention in the soil.

Choice of species mix used will depend on the end goal required.

1. If growing as a means of grazing livestock in early spring (bearing in mind the new lie-back rules introduced in 2023) – a large biomass is required in order to maximise the benefit. Brassica species are best suited to this goal.
2. If trying to improve soil structure and increase soil organic matter levels, deep rooting species help greatly, such as tillage radish.
3. To provide an N source for the following spring crop, legume species in the mix will help greatly as they fix their own N from the atmosphere.
4. Finally, if using as part of an integrated pest management (IPM) cultural grass weed control programme to lower herbicide use on farm, open species in the mix such as vetch, phacelia or buckwheat will allow germination and growth if trying to flush out problematic weeds, while allelopathic suppression of the weed will occur if oats, rye or buckwheat are part of the mix.

Cover crops species can be grouped as per Table 1 below.

Table 1: Cover crops by species

Rotation and other considerations

It is really important to bear in mind rotational break crops grown on the farm. For example if oilseed rape is grown on the farm, brassicas should be avoided for fields in oilseed rape rotation to avoid the build-up of clubroot infection.

Likewise, if peas or beans are the main break crop on the farm, legume species must be avoided. Similarly, if oats are a major crop in the rotation on the farm, you are increasing the likelihood of oat mosaic virus problems building by using oats in the cover crop mix.

Other considerations are winter hardiness. For livestock grazing in early spring this and a bulky crop are most important, so if possible brassicas like forage rape and leafy turnip should be in the mix.

Mixes

There is very little research work done in Irish conditions on the effectiveness of very complex mixes versus simple two/three way mixes. However, if you are mixing your own straights, bear in mind the complexities and seek help from your advisor to tailor the best mix to suit your needs.  

Sowing date

Above all, sowing date will have the biggest influence on the success or otherwise of the cover crop sown. Early sown is essential to achieve good autumn growth for all end goals. Experiments at Teagasc Oak Park have shown a linear reduction in biomass production as sowing date is delayed (Figure 1). The mustard in the experiment lost 2t DM/ha for each three week delay in sowing, clearly showing that cover crop sowing should be prioritised as soon as harvest is complete and straw has been removed.

Figure 1: Variation in cover crop biomass production by sowing date

Straw chopping and incorporation

Straw chopping and incorporation increases soil organic carbon, as organic matter is directly imputed back into the soil. (For every 4t straw incorporated over 15-20 years, a 7-17% increase in SOC (top 15cm only) has been observed (depending on whether reduced tillage was also applied), MACC 2023).

The addition of organic matter will help improve the soil’s ability to retain and recycle nutrients and will improve the soil structure. Organic matter will also provide a food source for soil biology and improve soil health and function. Building soil organic matter is a slow process and takes time.

Straw contains mainly potassium (K) and a small proportion of phosphorus (P). Table 2 below shows the levels of P and K returned depending on the crop type. The majority (90%) of P is removed in the grain at harvest time and ~10% of total crop P remains in the straw. Approximately 50% of the K remains in the straw for barley and wheat, while for oats ~ 65% of the K remains in the straw at harvest time.

Table 2: Chopped straw P and K values for a range of crops based on grain yields (t/ha)

The K in straw is relatively available, as it is water-soluble and will be available to the soils reserve of potassium. The amount of P that is returned is relatively low but will add to the soil P reserves and is not as available as the K in the short term.

Key actions over the coming weeks:

• Think about where a cover crop may be of benefit on the farm (2025 spring cropping fields)
• Decide the end goal and purpose for the cover crop
• Decide what species mix would best achieve this end goal, bearing in mind rotational restrictions on the farm
• Place an order in time for the cover crop seed mix with your local merchant
• Service your cultivation equipment/seeder OR book your contractor