Care needed to choose right cover crop for the job

Green manures, cover crops and catch crops have become one of the hottest topics in farming over recent years. Whether you want to fix nitrogen, reduce pests, improve soil structure or simply prevent soil erosion by avoiding bare soil, there is probably someone who claims to have the right cover crop mixture for you. However, with so many different crops, mixtures and claims, how do you decide which crop might be best for your situation and how do you manage it?

As Elsoms’ Deputy Chairman Robin Wood explained at a recent event organised by the company at its offices in Spalding, it is therefore important to know what you want to achieve and choose the appropriate crop for your needs. Head of Vegetable Sales, Justin Solly, explained that Elsoms’ partnership with Saaten Union means that they can offer a range of catch and cover crops and mixtures including oil radish, black oat, Phacelia and others.

Saaten Union itself is owned by a number of independent breeders such as Ackermann, Strube and P.H. Petersen, and Michaela Schlathölter of the family-owned German plant breeder gave an overview of its breeding activities on cover crops and the products that are available. “You have to be open to other ideas and we are always open to working together,” she said, explaining why the company’s partnerships with Saaten Union and Elsoms are so important.

With approximately 50 ha of breeding and trial fields at Lundsgaard near the Danish border, nematode control was one of the first targets of the company’s breeding programme, resulting in the Colonel radish variety which was the first crop to be awarded resistance level 1 certification – showing a 90 per cent reduction in nematode numbers, and a number of other multi-resistant varieties which reduce the impact of a range of nematode species, including Beet Cyst Nematode, in rotations.

“We look to have strong varieties with strong beneficial characteristics as this allows us to create strong [crop] blends to [improve] soils. Our Viterra blends only contain certified seed and therefore offer very high technical purity,” Michaela added. She pointed out that there are other reasons to grow cover crops, such as increasing soil fertility or reducing pest and disease pressure. However, she also pointed out that, “Growing green manure crops should improve yield and quality of the main crop, otherwise growing them makes no sense.”

While some blends of cover crop may have a strong performance against some pest nematode species, they can also make infestations of other species worse. For example the Saia oats used in Pratex provide good control of the Northern root-knot nematode Meloidogyne hapla, but can worsen populations of other root-knot nematodes such as M. fallax and Meloidogyne chitwoodi. She also stressed that it is important to consider cover crop destruction and incorporation when making your selection, as well as the crops in the rest of the rotation, particularly those immediately following the cover crop. “You cannot use every [crop] against every [pest],” she stressed. “You have to make a decision about what you want.”

Biofumigation

In the UK, the use of catch and cover crops for the control of potato cyst nematode (PCN) using biofumigation has become increasingly common and Dr Matthew Black of Harper Adams University has been working on the subject since 2006.

“The idea of biofumigation is that you are trying to harness the chemistry that happens in the soil,” he explained. This works as the enzyme Myrosinase is activated by the water in the soil. This converts the glucosinolates released by the plant cells when the crop is incorporated into the soil, through hydrolysis into compounds such as isothiocyanates which poison certain species such as some nematodes.

However, the technique can be complex as Dr Back explained: “There are 132 different types of glucosinolates and not all produce the same gases or have the same efficacy. You have to know what species of nematode you have in your soil and choose your cover crops accordingly. They are very specific. Glucosinolate concentration peaks around the point at which the plant flowers, but the chemicals present in the leaves are very different to what is in the roots.”

After reviewing the science behind the technique, he shared the results of some PhD work and explained the best way for growers to use the technique for the management of PCN. “The best time to sow is in the summer as the longer day length and higher radiation increases the concentration of glucosinolates,” said Dr Black. “The plants also need looking after and need the right levels of nitrogen and sulphur in particular to achieve sufficient yield and glucosinolate levels.”

Trials in Norfolk and Shropshire suggest that macerating the crop with a flail topper, followed by immediate incorporation with a spader gives the best results, although flailing followed by rotovating was also effective.

With so few other methods of PCN control available, particularly for Globodera pallida, more and more growers are looking at biofumigation as an option, and pot trials conducted by SRUC suggest that some varieties of Caliente mustard may also have an affect against free living nematodes which could help growers of other vegetables, such as carrots. However, before using biofumigation Matthew Black warned that growers needed to carefully consider the potential adverse affects that may arise from adding a new crop or species to their rotation.

 Wider benefits

Another reason for increased interest in using cover crops is the wider benefits that they can bring to soils, such as preventing nutrient leaching, improving soil structure and organic matter, and reducing erosion. These were all discussed by Elizabeth Stockdale, Head of Farming Systems at NIAB. “These wider benefits are often the reason that non-vegetable farmers adopt cover crops, but they are also relevant to vegetable growers,” she pointed out.

One of the first things to consider is the period that you want the crop for. Very short season crops, or ‘catch crops’, of less than three months tend to be used to trap nutrients, such as nitrogen, that may otherwise be lost ahead of the next crop, or for trap cropping or biofumigation of pests and pathogens.

Short season crops may be in the ground for 3-7 months and are typically sown in late summer after harvest and destroyed after the winter prior to the following crop. Longer seasons of cover crops tend to be used in organic systems for building fertility, and the length of time that cover crops remain in the ground is one of the key things which currently concerns policymakers.

“The main roles of cover crops are improving organic matter and soil health and in most of these areas cover cropping isn’t new or exciting,” commented Liz. “There are benefits from simply having ground cover and that is what frequently makes the policymakers most excited. However, it is not just about having cover; if we have crops that cover the soil for longer then we also have more soil biology – the soil biology needs to be fed and it is plants that feed the soil.”

She added that it is important to choose the right cover crop for each situation, adding that there are pros and cons associated with each crop. “When we are choosing a cover crop there are no right answers, but there might be wrong ones,” Liz warned. One example might be including a mustard in a rotation with oilseed rape, increasing the club root risk.

It is also important to consider how to deal with the cover crop before the following crop. Will it die off or will you need to kill it? Will it need to be incorporated or can you direct drill through the stubble?

When considering legumes it is important that they have enough time to bulk up and photosynthesise significantly if you are to get any benefit from nitrogen fixation. For this reason Liz suggests growing leguminous crops into the spring, possibly ahead of summer vegetable crops such as brassicas or salads.

Some crops, such as buckwheat, can help move nutrients like phosphorus through the soil profile. By taking up elements at the depth available and then reincorporating the plant tissue nearer the surface, following crops may have better access to certain key elements.

Different species also have different root morphology and Liz added that; “you need to choose species which use nutrients in a different way. You need a whole-rotation approach.” Many growers hope that using cover crops will boost the level of organic matter in their soils, and reduce compaction. Not only does this improve soil structure and pore size, but in sandy soils cation exchange capacity is improved.

However, on their own Liz warned that cover crops might prevent further damage and maintain levels or organic matter, but that more effort would be required to undo damage which may have resulted from years of farming operations. “Cover crops on their own will not un-compact a soil, but they are part of the solution,” she stressed.

“We are also starting to look at cover cropping as part of strategic weed management. For example, there are anecdotal suggestions that vetch may help stimulate blackgrass germination, which could then be controlled with glyphosate.”

Grower experience

Many growers considering cover crops for the first time will want to know what the practical implications are for their day-to-day farming operations and Colin Nobel of VCS Agronomy discussed his experiences with cover crops over the last few years. For example, growing a cover crop ahead of lettuce on the South Downs had allowed the salad crop to be transplanted a week earlier than on a bare field, but as well as stressing how difficult it can be to establish a cover crop – particularly one with small seed size – he also warned not to let them set seed and increase the weed population.

“Just look at how many problems we see from game cover strips,” he said. “We also need to think about other risks such as Sclerotinia carry-over from red clover to carrots. Where will you put the cover crop in your rotation? If you don’t get the timing right it will be a waste of time. What are you trying to achieve with your cover crop? Until you know that you have no idea what to grow.”