The current state of scientific knowledge on matters relating to catch crops was the focus of a recent workshop organised by the Irish Tillage and Land Use Society. The workshop looked at the facts and regulations that apply to catch crops. Introducing the subject, crop consultant John Metcalfe explained that changing times have resulted in the need to help our soils, as horsepower has so often been used to replace common sense. John said it should be the ambition of every farmer to hand over the land in better condition than it was received. That means it must be looked after.
The current state of scientific knowledge on matters relating to catch crops was the focus of a recent workshop organised by the Irish Tillage and Land Use Society. The workshop looked at the facts and regulations that apply to catch crops.
Introducing the subject, crop consultant John Metcalfe explained that changing times have resulted in the need to help our soils, as horsepower has so often been used to replace common sense. John said it should be the ambition of every farmer to hand over the land in better condition than it was received. That means it must be looked after.
John explained that cover crops have come very much into focus in recent years for three main reasons. GLAS has generated a lot of interest through public support. Greening has also given credence to catch crops. The third reason is those growers who have been using catch crops without any support purely to improve the health and productivity of their land. This must still be the underlying justification of every grower who plants catch crops.
It is important to state that catch crops are still only one of a number of ways in which farmers can improve soil health and land quality. But they best suit those who have difficulty getting other organic matter sources and they also provide the option to graze, either as a source of cheap winter feed or as a source of cash. Cover crops have one other advantage – having plants and roots growing all the time helps protect the soil.
Technically, we plant catch crops to prevent nutrient loss and to provide organic matter to help soil health. GLAS regulations force min-till planting, but so does timeliness of planting. The benefit of catch crops is proportionate to the volume of dry matter grown and early planting is key to unlocking the benefits.
UK study
A recent study in the UK looked at all of the credible research information that was available for catch crops and this was compiled into a single reference. This was done for two reasons. Firstly, it provided the information and guidance where it was available. Secondly, it identified the information gaps that required further study.
Charlotte White from ADAS provided a rundown on that report. She said that the likely benefits depended to some degree on the species sown and the purpose or intent. One of the key comments made by many was to identify the reasons why a cover crop is to be sown, because this is critical to help achieve that goal.
To help achieve the many benefits that catch crops can offer, it is important to have more than one species. Species have the potential to be complementary and it is up to research to provide guidance on this. Such synergies can be simple and easily explainable, but most are quite complex relationships.
Legumes
Legumes are useful because they can fix nitrogen (N) to fuel the growth of that crop and leave residual N for the following crop. But legumes also tend to have deep roots and this can be useful for catching nutrients that may be below the fibrous root systems of grasses and cereals.
Non-legumes often have more rapid growth in terms of accumulating biomass, but they can also have a range of unique root properties with potential for bio-fumigation. And combinations of grasses and legumes tend to result in increased N production.
Leguminous catch crops can capture quite a bit of N, but this will vary depending on species, planting date and location. Research indicated 30kg to 150kg N/ha. Fixation of N is correlated with biomass, so you need to be able to make bulk to get the benefit. Fixation is also a carbon-hungry process requiring 25% to 33% of the carbon trapped in photosynthesis.
The release of this N is more tricky. Charlotte said research has found evidence of 10kg to 100kg N/ha released to the following crop. The amount released is influenced by many factors, such as the species grown, the amount of biomass produced, the timing and method of desiccation of the catch crop, the carbon:nitrogen ratio of the catch crop etc. Charlotte said that a C:N ratio of less than 20 is needed to have N left over after the degradation process.
Species effects
More is being learned about the potential allelopathic effects of plants and their root systems. It is long known that either root exudates or decaying plant material can have dramatic effects on other species when tested under laboratory situations.
This can be a stimulatory or inhibitory effect (known as allelopathy). It tends to be species specific, but is not well proven under fields conditions, Charlotte said, as nature can impose its own moderating effect. This potential effect of catch crops is real, but is very complicated.
However, other plant effects, such as their biofumigation potential, are quite real and predictable in the field, Charlotte said. Trap cropping is also a useful element, where a specific species is more attractive to a specific pest than the crop itself and thus it helps to offer protection.
Soil benefits
The benefit of catch crops to the soil can be seen in a number of ways. Decreased erosion is the most obvious benefit, Charlotte said. Research has shown a reduction of up to 80% in the level of soil erosion in fields growing catch crops for some time.
She also reported that run-off levels were reduced by 50%. Improved soil structure and water infiltration are the main reasons for these improvements.
The objective of increased soil organic matter is a more vexed target. Research results have been very variable, with figures of 0% to 40% reported in research findings. Soil structure is visibly improved by catch-cropping and the continuous presence of growing roots helps aggregate stability, which makes the soil more durable against future abuse.
The fact that roots are continuously growing in the soil also helps to maintain the beneficial microorganisms around the roots, which helps them survive from crop to crop. Charlotte also reiterated the importance of earthworms in the soil system and stated that they prefer ryegrass as a food source and definitely dislike oats.
Yield benefit
Yield benefit remains a vexed question for catch crops. It is variable, but this may be more a factor in the early years of soil improvement. Charlotte said that the second year of catch crops is more likely to show a yield benefit, as the soil biology evolves to a new equilibrium. She said that the average yield benefit over sites and seasons was approximately 0.36t/ha for spring barley crops. This figure must, therefore, be borne in mind when considering seed and establishment costs.
Charlotte reported that yield seemed to benefit particularly with shallow cultivation systems and there may be good reasons for that.
One other yield issue that Charlotte reported was the presence of a 6% yield loss when growing oilseed rape following a brassica in the catch crop mix.
Starter fertiliser
Research which looked at the use of starter fertiliser with catch crops produced some interesting results. Trials used 20kg to 40kg N/ha and this generally resulted in higher green area index (GAI) levels in the autumn, but also in higher weed levels. By springtime, the GAI effect had generally levelled out with or without N, but the extra weeds were still there.
Research showed that the production of roots – the root biomass – was lower where starter N was used (about 5% lower). And as roots are an important structure-forming entity, this benefit is adversely affected by the use of starter N. Observations found that only 16% of the applied N was actually captured by the catch crop and so increased leaching is a potential consequence.
Many questions remain
The study identified many areas where further research is needed to help guide decisions on catch crops production. These include the optimum methods of establishment and destruction and there may be species interaction here.
Catch cropping has also helped identify the need for a better understanding of how the different nutrients are cycled in the soil. More accuracy will be needed on the potential for N savings, or not, in specific situations. And, of course, there is a need for a better understanding of the complexity of soil organic matter.
Catch crop pointers
Know what you aim to achieve if growing catch crops. Is it to improve the soil or to produce nitrogen or something else? Focus initially on just one. It can be useful to test a few options in your field, along with a control, so that you can see more clearly what seems to be working for you. Be aware of the different management, rotational and destruction challenges that different species may present. It is always a good idea to monitor progress and a phone camera can be very useful. If you have yield maps, you will also watch for the level of variability on your fields. Try to evaluate the costs and returns associated with what you are doing. There will inevitably be wider benefits, but you must at least be growing enough biomass to offer potential for real benefits. If you are learning, do not be afraid to modify what you are doing for your own benefit. This might even mean altering the cash crops you plant to enable timely catch crop panting.Catch for forage
Catch crops can be used to provide useful fodder sources for farms with livestock or with access to livestock. Dave Barry of Goldcrop said that 5t DM/ha must be the minimum target to help make the option work. He advised that only high-yield species be used in this regard and that early sowing is critical for success to help get these high yields.
Because many of the species have very small seed, Dave advised that the seedbed is critical to success and this is often neglected. After that, the basics are important for fertility and pH and an early sowing date are critical. He suggested that up to 80kg N/ha could be used to drive forage yield.
Dave said that a diet of forage rape plus baled silage gives performance equivalent to silage plus 2kg concentrate. Based on this valuation Dave valued 5 t/ha of fodder rape at €160/t. And with 5t DM/ha and 70% utilisation the fodder rape crops could be valued at€560/ha.
In terms of suitability, Dave rates fodder rape and leafy turnip as the most suited of the species for forage catch crop. This is based on its utilisable yield, feed quality and animal performance.
GLAS
Speaking on the GLAS options for catch crops, David Buckley from the Department of Agriculture explained that the measures in GLAS aim to target water quality, promote biodiversity and mitigate climate change. Specific measures for tillage relate to arable margins, min-till and cover crops and the payments relate to income foregone and costs incurred.
There were almost 25,000ha of catch crops last year between the different GLAS measures with nearly half that amount again in min-till. Farmers have the option to combine different measures in GLAS up to a maximum €5,000, or up to €7,000 from GLAS Plus.
Out in the crops
In the afternoon we headed out to Walter Furlong’s farm where Philip Reck gave a rundown on some of the more recent catch crop practices. This farm has been growing catch crops ahead of spring-sown crops for many years now and the soil structure is testament to the practice. Water percolation is much improved and there is much less evidence of soil run-off during heavy rain.
Philip continues to test a range of crop mixes but these generally include a legume, with peas and vetch still widely used, along with radish and phacelia. We visited one crop that was still growing in February but at that point much of the pea and vetch had died back and the space was given over to the radish. The key point is that there were still plants growing in the field with active roots to help keep the soil open and the water moving.
Given the importance of desiccation to catch cropping, Philip said that the potential loss of glyphosate would be a serious worry. Arising from this possibility Philip wondered if there is any other useful way to kill off these species. Having studied various techniques in other countries, he decided to try rolling the crop in frosty conditions.
He picked a good hardy day when the vegetation was frozen and the temperature around -4°C. The challenge was to be able to crack the vegetation rather than just bruising it.
The results were dramatic. Everything was gone, including the tillage radish that can even prove challenging for glyphosate.
The greater challenge might be to secure days at -4°C when the ground is good and trafficable.
This is a subscriber-only article
SHARING OPTIONS: