Cover crops: How to cover all the bases in your cropping system

John Hogan from Teagasc Oakpark, Kevin Gardiner of Gardiner Grain and trials manager Patsy Kehoe from Seedtech in a field of Graham winter wheat at a recent Seedtech Open Day.
John Hogan from Teagasc Oakpark, Kevin Gardiner of Gardiner Grain and trials manager Patsy Kehoe from Seedtech in a field of Graham winter wheat at a recent Seedtech Open Day.

Mark Plunkett

Using cover crops in your cropping system offers many soil, crop and environmental benefits. Cover crops present an opportunity to improve soil structure, crop rotation diversity and water quality. It is recommended to establish crops rapidly after cereal crops have been harvested to get the full benefits from the crop in terms of soil and carbon capture.

Questions and answers address the main issues on the role and function of cover cropping for improving crop production and water quality.

1 What is the difference between a cover crop and catch/cash crops?

Cover/catch crop: this where a crop is sown between two main tillage crops, usually autumn/ winter, and used to maintain a green cover on the land and to take up nutrients left over (especially nitrogen) from the previous cropping season. These crops are usually grown for their soil and environmental benefits.

Cash/fodder drop: this is where a short-term fodder crop is grown to feed animals, usually over the winter period. These crops can be grown between two main tillage crops but usually receive N, P & K (fertilisers/ manures) to maximise dry matter production.

However, depending if they are grazed in-situ they tend to lead to increased soil fertility. However, if they are harvested and removed, nutrient off-takes should be considered in soil fertility planning.

These crops have the potential to reduce nutrient losses, soil compaction and soil erosion depending on weather conditions and grazing/harvest strategy. For example, leaving a growing buffer of catch crop between grazed/bare areas can act to reduce losses.

2 What's the difference between natural regeneration and a cover/catch crops?

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There is a requirement to maintain a continuous green cover on tillage ground from cereal harvest to December 1 each year. Natural regeneration of green cover after harvest may be sufficient if greening up of stubbles takes place. However, total plant biomass and nutrient recovery can often be low and spatially variable with natural regeneration. By establishing a cover crop immediately post-harvest, much higher plant biomass and nutrient uptake can be achieved compared to natural regeneration. This can lead to multiple additional benefits for soil quality, for long-term soil nutrient supply and for the environment.

3 What are the best cover/catch crop to sow?

Any crop that will grow vigorously during the late summer into autumn period and produce high ground cover and biomass can be suitable for growing as a catch/cover crop. Typically, cereals such as oats or rye, brassicas such as mustard or tillage radish, or phacelia meet these requirements. Crops with deeper rooting can be more beneficial from a nutrient (nitrogen) uptake and soil structure improvements.

4 How is nitrogen (N) recovered using a cover crop?

As the cover crop establishes in the soil it generates a large root system which recovers nitrogen that has been left over from the previous crop.

If there is little or no green cover, this N can be leached from the soil and lost to ground water or streams leading to poor water quality. Therefore, a cover crop with a large and deep root system is best. The cover crop stores the nitrogen it recovers from the soil in its above ground biomass over the winter until its incorporated back into the soil in spring.

This N recovered by the cover crop adds to the soil organic N pool and will become slowly available to the subsequent tillage crops over time.

5 How does a cover crop reduce P loss?

Phosphorus is mainly lost over the winter through runoff which can increase on bare soils. P is usually tightly attached to the soil particles and will be lost where erosion or sediment loss occurs. Cover crops reduce the risk of erosion following heavy rainfall as they provide a green canopy over the soil and by the action of the root mass in binding the soil together, stabilising it from loss.

6 How is soil structure improved using a cover crop?

Increased root volume and penetration through the soil from the cover crop breaks up the soil layers. This improves soil structure, leading to increased soil stability, improved drainage and is a source of organic matter. The cover crop also provides exudates which feed the soil biology. The soil biology produce compounds which help to orient the soil silt, sand and clay particles into water-stable packets which keep their structure when wet.

7 Why is organic matter so important to tillage land?

One of the major threats to tillage soil productivity worldwide is the loss of soil organic matter. Organic matter is the main store of plant nutrients in the soil, especially soil N.

Organic matter also helps to reduce soil compaction as it acts like a shock absorber between the soils particles (sand/silt/clay). Soil organic matter also provides food for earthworms, often referred to as natures plough, and soil microbiology which recycle nutrient for crop uptake.

Overall, building soil organic matter leads to multiple soil, crop and environmental benefits.

8 Do you spread fertiliser on a cover crop?

No fertiliser should not be applied to cover/catch crops as they are used to mop up surplus nutrients over the winter period. However, crops grown for over winter fodder production have nutrient requirements.

9 What issues are caused by out wintering animals on a catch crop?

Out wintering animals can lead to a number of issues such as soil compaction, poaching, erosion and nutrient loss.

Water quality can suffer due to the many benefits found with ungrazed catch crops being undone.

Once grazed during the winter, the potential of the plant to mop up remaining nutrients in the soil such as nitrates/phosphates is reduced, leading to increased risk of nutrient leaching. Careful planning of fields suitable for catch cropping can help.

For example on sloping fields erosion and runoff can occur when the catch crop is grazed off.

10 Can a cover crop improve crop productivity?

Yes, productivity will improve in a tillage system due to the additional organic matter increase over time, leading to improved soil structure and soil nutrient availability.

Additional carbon supplied by increased organic matter provides food for soil biology, leading to more earthworms, biodiversity and improved biological activity.

It is important to remember that cover crops form part of a tillage system over years with the benefits being accumulated and reaped over a farming career rather than in a single year.


Winter wheat: Outstanding in the field

John Hogan from Teagasc Oakpark, Kevin Gardiner of Gardiner Grain and trials manager Patsy Kehoe from Seedtech (pictured above) in a field of Graham winter wheat at a recent Seedtech Open Day. "Using plant breeding to solve problems is as old as farming, but it is increasingly more important as we lose old reliable sprays," was the message from Seedtech's technical director, Tim O'Donovan, at the open day in Belview Port, Kilkenny.

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