Article taken from Agronomist & Arable Farmer Magazine
Sowing a companion crop with oilseed rape can boost crop performance and lead to better yields, especially on heavy, wet soils where establishment is a challenge. This could be the year to try the technique, says Agrovista's Stewart Woodhead
OSR is a lazy rooter. Wherever there is soil damage or compaction, it will struggle to put down sufficient root to support optimum early growth. Many heavier soils are likely to provide a less-than-ideal environment for young OSR plants to thrive this autumn. After months of above-normal rainfall, such soils will be relatively cold, wet and tight, restricting establishment, root development and vigour. Ongoing research by Agrovista shows that planting the right species as a companion crop alongside OSR can overcome these adverse conditions. Choosing one that puts down powerful roots to help condition and drain the soil can have a dramatic effect on crop growth and eventual yield. ""The tougher the conditions the bigger the benefit," says Agrovista technical manager Stewart Woodhead.
""At our heavy land trials site at Stoughton in Leicestershire, we have achieved up to It/ha improvement in yield by using a companion crop. On the lighter soils at Morley in Norfolk we've seen a 0.25t/ha improvement:' At current prices, even the smaller increase would bring in an additional £70/ha. Given that Agrovista's recommended companion crop costs around £13/ha, that represents an excel-lent return on investment, says Mr Woodhead.
""After several years of trials our current species of choice is berseem clover. We do also offer a mix with common vetch, which can be important where nutrient capture is a priority. This will take up about 40kg N during late autumn and winter, which will be released back into the soil in the spring. ""However, this blend does cost more. We find berseem clover on its own offers all the structural and conditioning benefits and still captures about 10kg/ha of N over the winter."" The berseem clover is a single-cut variety chosen for its rapid establishment and early growth and, importantly, because it is susceptible to frost and will not regrow once it has been subjected to freezing temperatures.
""This ensures it is killed off even in a mild winter like the one we have just had. It has done its job by the time winter sets in and this saves the cost of applying a selective herbicide to remove it."" It also has a low carbon to nitrogen ratio, which means it can be broken down more quickly by microbes without the need for significant amounts of soil N. That also means nitrogen mopped up by the companion crop in the autumn will be available for the OSR crop to use in the spring. Berseem clover has immensely powerful roots that will break through pans and open out compacted soils, says Mr Woodhead.
""They are like jackhammers, opening up channels, draining excess moisture and allowing the oilseed rape roots to follow down into the soil. ""This means the OSR plants can scavenge much more effectively for nutrients, boosting early crop vigour, improving overwintering and giving the crop a head start when it grows away in the spring."" A trial at Agrovista's heavy land trial site at Stoughton, Leicestershire this season has clearly illustrated this benefit. Straight oilseed rape and a mix of OSR/berseem clover were drilled on 27th August using a Great Plains 00 Planter with and without its deep soil-loosening tines.
""The pictures speak for themselves," says Mr Woodhead. ""In the legs-out plots establishment was significantly better where the berseem clover was included."" Berseem clover also conditions the soil for the following crop. As well as mechanically improving the soil at depth, its large root net-work and top growth provides valuable organic matter when it breaks down in the spring. Healthy plant growth at depth also provides sugars for bacteria and other beneficial organisms in the soil biosphere, further improving soil health, says Mr Woodhead. Berseem clover also encourages the devel-opment of soil mycorrhiza that form a close relationship with roots of many plant species — but not brassicas like OSR — increasing uptake of nutrients, particularly phosphorous and nitrogen. ""Mycorrhiza are seriously depleted in some soils.
While we are unsure of how long this mycorrhizal boost will last, as they can be depleted by cultivations and intensive fungicide regimes, it seems very likely that encouraging their development early in a crop's life must be helpful to that crop's development."" Further trial work will attempt to evaluate this benefit, he adds. Slug damage is often significantly lower where a companion crop has been planted. ""There is no scientific data on this but we have seen notable benefits," says Mr Woodhead. ""It may be simply a case of there is more green material to feed on, or that the companion crop provides a varied diet."" Work in France has shown a reduction in flea beetle incidence. Although this has not been proven in Agrovista's UK trials, work in Suffolk has shown a dilution effect due to better OSR establishment. Although there were the same number of larvae/sq m, each plant containing fewer larvae.
AGRONOMY
Berseem clover should be drilled with the oilseed rape so it establishes in the row, says Mr Woodhead. ""We have not seen any problems due to competition — just the opposite in fact, as this is where the strong rooting effect will do most good."" Berseem clover seed is a similar size to OSR but both can vary, so as a rule of thumb no more than 5ha of seed should be mixed at a time to avoid separation issues in the drill, he adds. The clover should be sown at the same rate as the OSR, typically 2.5-3kg/ha. Most herbicides can be used, including those recommended for Clearfield varieties. ""The only restriction is that you cannot use clomazone, otherwise the usual chemical tool-box is fine.""
COMPANION PLANTING WITH MAIZE
Companion plants are showing promise as post-harvest soil stabilisers, reducing erosion over the winter and the associated nutrient pollution of watercourses. Agrovista is trialling a range of species including fescues and ryegrasses, says technical manager Chris Martin. ""The companion crop is foraged with the crop. The stubble then regrows, producing a canopy that reduces the impact of rain on the soil while its roots continue to bind the soil together. ""This could be incredibly useful in reducing phosphate pollution associated with soil run off over the winter months," says Mr Martin.