A recent episode of the Teagasc Tillage Edge podcast saw crop specialists Michael Hennessy and Ritchie Hackett discussing the benefits of catch crops.
The chat started with the two men defining what these crops actually are, and the different terminology that can be used to describe their purpose.
“Catch crops, green cover or cover crops are all about maintaining a growing plant between two main crops,” Hackett explained.
“Previously, land used to grow continuous spring barley might well have been maintained bare over the winter period.
“Natural regeneration can take place. However, if we are talking about the actual sowing of a seed into the soil, then this is a cover or catch crop.”
What are catch crops?
The differing names applied – catch crops, cover crops or green manures – reflects their end use.
Hackett explained: “A catch crop would have been used to catch or capture nutrients, whereas the term cover crop would have arisen if the planting had taken place to protect the soil from wind or other forms of erosion.”
According to Hackett, the same physical crop can be classified under any of the aforementioned names.
He continued: “In our environment the main role of any growing cover over the winter months is that of reducing nutrient leaching from the soil.
“We get quite a lot of rainfall over the winter period with very little evaporation taking place. So the potential to lose soil nutrients between crops is very high.
“If we put in a growing vegetation, it is going to soak up these nutrients. And the main one is this regard is nitrate. This is a particular challenge in the southeast of the country,” he added.
Other climate issues
According to Hackett, water erosion is another issue in our climate.
The growing roots of a green cover will help to improve soil structure. There is also the benefit of enhancing the supply of nutrients to the next crop, thereby reducing future fertiliser bills, according to the researcher.
“The potential to improve soil organic matter levels and overall soil health is also significant,” he explained.
Hackett went on to point out that an August-sown cover crop can put up to 1.3t of carbon dry matter (DM) per hectare (ha) back into the soil during the period up to Christmas.
The equivalent figure for a 25t/ha dressing of farmyard manure is 2.5t.
He further explained:
“Chopped straw from a barley crop yielding 7.5t/ha can deliver 1.8t/ha of carbon. But the reality is that cover crops can help accumulate significant amounts of carbon.”
Cover crops
Where subsequent nitrogen (N) availability is concerned, Hackett differentiated the role that can be played by leguminous and non leguminous cover crops.
He said: “If we are talking about crops like mustard and radish, there are very inconsistent results when it comes to determining how much of the nitrogen they trap is actually made available to subsequent crops.
“However, we are currently looking at the impact of legumes as a source of subsequent crop nitrogen. And they are showing a lot of potential regard in this regard.”
Where the actual leaching of nitrogen is concerned, Teagasc work has confirmed that the amount of nitrate leaving the soil is very variable.
According to Ritchie Hackett, this depends on the amount of actual nitrogen left in the soil after the crop has been removed.
He said: “Up to 90kg/ha is possible in light, free-draining soils. However, where a cover crop is put in, leaching-related reductions of up to 60% can be achieved.
“This is dependent on securing good growth, where the cover crop is concerned.”
Nitrogen in autumn
Hackett went on to admit that the relationship between actual green cover secured, and the amount of nitrogen ‘locked up’ during the autumn months is an extremely complex one.
He continued: “This is because farmers tend to grow cover crop mixtures and not monocultures.
“Retained carbon will help feed the microorganisms in the soil. It will also feed earth worms. These are the most obvious impacts of cover crops when it comes to assessing their impact on overall soil health.
“The annual impact of cover crops tends to be quite modest. So, it’s a case of building up their influence over a number of years.”
Sowing date
Recent trial work carried out at Teagasc Oak Park has highlighted the impact of sowing date on the success of cover crops.
Hackett explained: “We looked at three sowing dates: the end of July; mid-August; and early September. In each case, we sowed two cover crops. Mustard is a fast-growing non legume, while hairy vetch is a winter hardy legume.
“The decision to stay away from cover crop mixtures was taken for two reasons. First off, we wanted to analyse fully the impact of the cover crops on the growth of a subsequent cereal. So, on that basis, it was a case of keeping the project as straightforward as possible.
“But beyond that, there is no evidence to confirm that cover crop mixtures will out-perform monocultures. But in a practical farming scenario, there is a strong case to be made for including two species in a mix.
“This approach gives a degree of cover, in the event of one crop type not growing.
“And, of course, a mix of species must be specified if cover crops are grown under the GLAS [Green Low-Carbon Agri-Environment Scheme] scheme.”
The Oak Park work has shown that sowing date is a key factor in determining cover crop growth rates and yields.
Crop samples taken in early December confirmed that mustard sown out at the end of July was yielding 4.5t/ha on a dry matter basis, whereas crop established in mid-August was delivering only 2.5t/ha.
Early September-sown crops were losing a further 2t/ha.
Later-sown crops had high levels of weed cover relative to those established in late July. Significantly, the early-sown mustard crops did flower in late September and had produced non mature seed pods by the beginning of December.
Despite this vigorous vegetative growth, no problems were incurred when it came to ploughing the crop in the following spring.
“We were dropping 2t of dry matter for every two weeks that we delayed in sowing out the cover crop,” Hackett said.
“We will be repeating the trial again this season. However, the initial results were very significant,” he concluded.