One of the key benefits derived from planting cover crops is their ability to soak up nutrients that, otherwise, would leach out into watercourses.
But to have the desired impact, the crops must be planted within days of the previous harvest.
The current edition of the ‘Tillage Edge’ podcast reflects on the value of cover crops and their evolving use within an arable cropping scenario.
Teagasc’s Richie Hackett explained: “We have done a lot of work on cover crops. The first thing to be pointed out is that cover crops mean different things to different farmers.
“One of their main functions is that of reducing nitrate loss to watercourses. This issue is causing more and more challenges, in terms of how it is feeding into legislation.”
Hackett went on to confirm that the use of cover crops can also help to improve soil quality.
“This is all about giving benefits to the next crop,” he said.
Teagasc research has confirmed that cover crops will act to reduce leaching rates from fields. But the earlier they can be sown out in August the better.
“This approach ensures the maximum benefits to the next crop while also minimising nitrate losses from the soil,” Hackett continued.
Planting cover crops
The latest Teagasc research indicates that dry matter losses of up to 2t/ha will be accrued for every two-week delay in sowing out cover crops from early August through until early September.
“This is quite a dramatic drop-off in growth,” Hackett confirmed.
“Most farmers who have sown out cover crops in September will know that they do not come to very much in any season.”
In terms of using cover crops within a specific arable rotation, there are certain pitfalls to be avoided.
“E.g., brassicas should not be included within a cover crop mix if oilseed features within a rotation,” Hackett explained.
“Such an approach is likely to enhance problems with clubroot.
“In addition, legumes should not be included in a cover crop mix that immediately proceeds a crop of beans.
“If it’s a case of growing spring barley after spring barley, then farmers have a lot more freedom in terms of what plant mix within a cover cropping scenario.
“But restrictions will apply if an extensive cropping rotation is already in place,” he added.
Legumes
According to Hackett, the tailored inclusion of legumes within a cover crop mix can act to take nitrogen out of the air and deposit the same crop nutrient into the soil.
“Other plant species will only act to mop up whatever nitrogen is already in the soil,” he said.
“Cover crops, which feature legumes, do have a relatively high nitrogen concentration within them. This is quickly released to the subsequent crop, offering the potential of reducing the amounts of chemical nitrogen applied thereafter.
“However, it is not easy to predict the actual impact the previously locked up nitrogen will become available to the new crop,” Hackett said.
Benefits of cover crops
According to Teagasc, there are many different reasons why cover crops may be suitable for a farmer and this will depend on what the farmer is trying to achieve.
This could be improving crop productivity, improving soil health, building biomass, capturing carbon, soil protection over the winter, reducing run off, preventing nutrient loss or as part of an environment scheme.
One of the main functions of a cover crop is to mop up any remaining nutrients especially nitrogen after the previous cereal crop.
On free-draining sites, prone to nitrate leaching, research has shown that overwinter green cover can substantially reduce nitrate leaching to groundwater from fallow crop land.
The Agricultural Catchments Programme has been monitoring a tillage dominant river catchment in Castledockrell, Wexford since 2008 and has been focusing on winter green cover research.
Ireland’s 5th Nitrates Action Programme (NAP), requires in all circumstances that 75-80% of harvested crops be shallow cultivated or sown with a crop within 14 days of harvest.
The earlier the green cover is established, the more nitrogen it can capture.
The cover crop will also have beneficial effects in terms of soil structure improvements and protects the soil from potential phosphorus loss through surface run-off over the winter period.
Research at Oak Park, Carlow has shown that, in general, the effects of cover crops, compared to bare fallow or natural regeneration, on yield of succeeding cereal crops under Irish conditions are variable, often small and sometimes negative.
However, significant yield benefits in succeeding crops through the use of cover crops occurred infrequently. This concurs with findings in other European countries.
When planting cover crops, early sowing is vital to achieve good autumn growth. Growers should aim to sow in mid- to late-August or the first week of September.
Crops sown after mid-September are unlikely to produce much growth.
Mix
Mixtures of at least two species are required for an Ecological Focus Area (EFA).
Mixtures that contain a legume (e.g., vetch, peas) are more likely to give a yield benefit in succeeding crops.
Growers should avoid complicated mixes particularly where cover crops are being grown for the first time.
It is important to avoid using species that could cause problems in the rotation (e.g., brassica cover crops in a rotation containing oilseed rape).
Cover crops can be grazed from the beginning of December.
If grazing is an objective, farmers should choose species that are suitable for grazing (e.g., forage rape, leafy turnip, fodder radish, oats, and peas).
Ploughing to establish cover crops is not allowed for GLAS or EFA purposes.
Growers should use either use a drill capable of sowing into stubble, or broadcast the seed and lightly cultivate before rolling. If different sized seed are being broadcast, it may be appropriate to broadcast the large and small seed separately.
Establishment rates will normally be lower for broadcast crops.
If cover crops are not being grown for grazing purposes, applying fertiliser is not going to give an economic benefit.