Drilling a companion crop or seed mixture alongside oilseed rape could help control pests and add valuable nutrition to soils, new thinking is suggesting.
Strategic use of companion crops in oilseed rape could not only provide an answer to growing pest problems in the UK crop but also reduce fertiliser and herbicide costs, believes Sarah Hawthorne of DSV UK.
Trials across Europe are highlighting the benefits of the approach with increasing numbers of growers looking at companion crops in their pursuit of more integrated management systems, she says.
“In reality, use of companion crops is nothing new in arable production but they’re being looked at with fresh eyes as the agrochemical options open to growers continues to diminish.”
Whilst interest is growing, in reality very little commercial cropping has used the approach in these countries, but this could all be about to change, she believes.
“As long as we have conventional solutions that achieve the same crop management outcomes and that are cheaper and easier to use, companion cropping will always be in second place.
“But all the indications are that our traditional options are in decline so we have to look at new ways of achieving a similar effect. One things is for sure, use of companion cropping is not just a replacement for the sprayer – its impact is much more far reaching.”
Whilst most growers in the UK will focus on pest control and improved crop nutrition, there are other benefits, she points out.
“Companion planting in OSR has two main objectives. The primary one is to reduce nitrogen fertiliser inputs by using legumes to fix atmospheric nitrogen and the secondary one is repelling or distracting insect pests.
“It can be argued that in the UK, the most important of these is better pest control but there are interesting debates around herbicide use emerging too.”
For pest control, non-leguminous companion plants have been shown to prevent insect damage in autumn with species which reliably die off in winter or can easily be controlled with herbicides proving to be the most suitable, Sarah Hawthorne says.
“Yellow mustard has proved successful in trials grown alongside Clearfield oilseed rape although the precise mechanism why pests are reduced is not clearly understood.
“One possible explanation could be that the smell of mustard masks the oilseed rape, making it harder for insects to find the commercial crop and attack it. Another explanation could be that the pests, mainly flea beetle, prefer the mustard and therefore avoid the oilseed rape.”
Whatever the companion crop of choice, its ability to fix atmospheric Nitrogen must also be considered, she adds.
“One of they aims of companion planting in oilseed rape is to fix atmospheric nitrogen by using legumes, and so reduce nitrogen fertiliser inputs.
“Various clover species such as fenugreek and Egyptian clover have proved effective, while field beans have been very successful in some areas, particularly on heavy soils, and lentils have been shown to be effective in some cases too.”
Under good conditions, companion plantings can fix up to 30 kg N/ha before winter, she says.
“However, the supply of nitrogen does not become available to the oilseed rape until the following spring when the legumes have died off and this will only happen if the companion plants have enough time for their nodules to fully develop before winter sets in.
“For this reason, the oilseed rape and legumes must be sown around ten days before the actual local sowing time and in some conditions this can increases the risk of subsequent overgrowing and winterkill.
Companion plants are usually sown with the oilseed rape, either simultaneously in the same row or separately in a second pass between the rows of rapeseed, she advises.
“This planting technique has been developed by farmers in France but non-leguminous companion plants such as mustard can also be broadcast after drilling the oilseed rape.
“In terms of variety choice, it is important to choose an oilseed rape variety which is sufficiently vigorous in autumn, but at the same time less prone to extending its stem.”
As such, proven high performing hybrids such as DSV Incentive and Sparrow are good choices with the new DSV Clearfield varieties Plurax CL and Phoenix CL being ideally suited to companion cropping systems.
“Plurax CL is a very well balanced variety with a unique combination of yield-stabilising factors including a good package of disease resistance including excellent resistances and tolerances against stem canker, Light Leaf Spot and Verticillium wilt.
“It has also shown, a very high level of winter hardiness in a range of trials delivering high yields and oil contents from a simple management system without the need for extensive agronomic intervention.
“One of the few varieties to be tested by NIAB across 7 sites over two years in the UK, Plurax CL has delivered the highest gross output and highest oil content of all Clearfield varieties in the data set.”
Phoenix CL is a benchmark for Clearfield performance across Europe being one of the first such varieties to reach the yield levels of normal hybrids, she explains.
“It is now one of the best adapted varieties available, consistently delivering high yield performance in a range of very difficult conditions with its yield stability underpinned by early vigour and a deep taproot system.
“This allows the variety to access water and nutrients in deeper soil layers, so as well as good establishment it has the ability to recover quickly in the spring after a challenging winter.
Better Weed Control
Previous trials have shown that a pre-emergence treatment with a herbicide containing metazachlor has no adverse effect on the companion crop, Sarah Hawthorne points out.
“Better weed control, more cost-effective use of herbicides and a positive contribution to soil health have also been cited as further benefits of companion cropping.
“Weed control is relatively straightforward with companion cropping. Graminicides can be applied in the usual way. In France the weed control effect provided by companion plants enabled herbicide application rates to be reduced by as much as 50% of the permitted dose.”
But the cost-effectiveness of companion planting in oilseed rape resulting from reducing fertiliser and agrochemical inputs has to be assessed against the additional cost of the seed, she warns.
“Companion seed can cost up to £60/ha, depending on the species. Furthermore, the cost-effectiveness of the companion plant seed may be reduced by the possible need to apply herbicide in the spring.
“There are still many unanswered questions regarding companion planting in oilseed rape and little experience of this new technique has been gained on the ground and there are well documented downsides.
“Regardless of the specific species used as companion plant, for example, the approach bears the risk that competition from the companion plant will cause the oilseed rape to bolt and so lose its winter hardiness.
“Furthermore, all non-leguminous companion plants will invariably compete with rapeseed for light and nutrients but there can also be symbiotic effects of mixed cropping systems such as better nutrient uptake by roots.”
“The bottom line is that whilst there are significant benefits and it is likely these will become more relevant to the way we farm in the future, considerations of cost and management fit also need to be factored into any decisions.”