Cover Crop Benefits

Catchment Sensitive Farmers Meeting

I had a really fascinating day in Dorset today, exploring the use of cover crops to capture nutrients, (mainly nitrates) and stop them being washed through the soil profile and into ground water. The day was organised by Wessex Water and the Catchment Sensitive Farming organisation. Fellow Nuffield Scholar Tim Stephens and his team have put together a matrix of different cover crop species and seeding rates to look at establishment, nutrient capture and suitability within the field. The catchment also has porous dotted around the fields and every 2 weeks from October to the end of January samples are taken and the levels of nitrate in the water assessed. These values are then turned in to a Kg/Ha of nitrogen to give farmers a vital guide as to where leaching is occurring in the farming rotation and how much is being lost and when leaching occurs.

It seems that the worst leaching occurs after Oilseed Rape and Wheat, probably due to the higher levels of fertiliser they receive but also due to the very inefficient way these plants use the applied nitrogen. Wheat for example is only about 60% efficient when looking at applied or synthetic nitrogen fertiliser. Where cover crops are used the leached level of nitrogen is about 50% that of land farmed without cover crops. This has a significant impact when looking at the catchment as a whole. It is great credit to Wessex Water that they have decided to take this partnership approach to try and reduce diffuse pollution working together with the farmers in the catchment.

Beneficial and Pest Numbers in the Cover Crops

Also on hand were a team of entomologists who have been assessing the impact of cover crops on the insect population. The picture above shows the numbers of insects trapped over a 7 day period. The 2 trays on the left show the beneficial's and on the right are the pests. The top half shows the numbers and type captured in the cover crop field (oil radish and phacelia) and the bottom half shows what was captured from a control field i.e. no cover crop. This level of detail is very encouraging. This clearly demonstrates that cover crops are hosting higher insect levels all round but significantly more beneficial insects that will predate the pests. This is so important when looking at using an Integrated Farm Management approach to try and reduce the amount of pesticides we might want or need to use. There were lots of carabid beetles, who's larvae predate slugs which is also very positive. The benefits weren't just in this cover crop species mix. the photo below shows what was captured within a buckwheat, oil radish and phacelia mix. The predator list includes, carabid beetles, springtails, harvestman spiders (not really spiders at all), ants, spiders, and parasitic wasps. The pest list includes mites, flies, aphids, snails and a slug.

Beneficial Insects and Pests trapped from Cover Crops

The session continued in the local village hall with a great presentation on 'Making the Most of Cover Crops' by Ian from Oakbank. Ian talked about different cover crop mixes, how and when to establish them, and the merits of diverse mixes and the impact of this type of farming can have to the on the bottom line and the better environment. A great day of learning.

Inter-Cropping Insight

Inter-cropping Sprayed Off

Back when we planted the oilseed rape crops in August we also planted their companion crops. This year we used 10Kg of vetch and 2kg of  Berseem clover/per hectare. These companions have grown very successfully through the autumn and into the winter. During the winter we sprayed a weed killer on most of the crop to remove the companion crops as well as weeds like sow thistle, mayweed, brome and black-grass. This herbicide does not affect the oilseed rape crop. Both of these legume crops are able to host soil bacteria to make plant available nitrogen from the atmosphere, something that the oilseed rape crop in unable to do. We hope that as much as 30-50Kg/ha of nitrogen will be available free of charge from these companion crops, easily covering the initial cost of the seed. This could be worth between £25-£41/Ha.

Canopy of Sprayed Off Inter-cropping

The picture above shows the crop where the companion crop has been sprayed off. We can measure the GAi (Green Area Index) or amount of green plant material with an app on a mobile phone. This picture shows a GAi of 1.34. There is about 50Kg/Ha of nitrogen in every GAi of an oilseed rape crop. Therefore this crop has captured 67Kg of Nitrogen. The picture below, where the companion hasn't been sprayed off, has a GAi of 3.05 indicating a potential level of 152Kg/ha of nitrogen in this whole canopy. Whether all of this canopy will relate to available nutrients for the oilseed crop to capture is something we don't know but its very interesting to see these levels of free nitrogen, even accounting for the 25Kg of fertiliser applied at planting.

Canopy Of Inter-cropping

We are planning to leave this area of the field until harvest to see what effect it has on the overall yield of the combined species. We should be able to reduce the amount of nitrogen this part of the field receives in order to keep the crop standing but also to rely on the nitrogen being created by the vetches and passed over to the oilseed rape. Time will tell what the outcome will be!