Drilling a winter cover crop mix can increase soil nitrogen supply by up to 35kg N/ha and reduce nitrate leaching by 90% compared with a weedy stubble.

This is according to the latest Nitrogen Release from Cover Crops study carried out by independent crop consultant Adas, together with water companies Affinity Water and Portsmouth Water.

The aim of this study was to quantify the effect of two cover crop species mixes on overwinter nitrate leaching across two sites, in Hertfordshire and West Sussex, from 2021 to 2023.

Chemical versus mechanical cover crop destruction methods were also assessed, and the impact on soil nitrogen supply and the performance of the following spring cereal crop recorded.

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Yield boost

In all scenarios cover crops had a positive effect on the yield of the following spring cereal.

On average, spring oat yields were 0.2t/ha higher where cover crops were grown compared with a weedy stubble control. Spring barley had a 0.5-1t/ha yield increase as a result of the cover crops.

Cover crops released more spring N following destruction and decomposition in the soil than from the weedy stubble control, supplying up to 35kg N/ha, with potential to cut back artificial fertiliser requirements.

This was dependent on how well the covers had established, the size of the above-ground cover crop biomass, and the species mix’s ability to scavenge N.

Mechanical destruction of overwinter cover by chopping had limited effectiveness in destroying oil radish and its below-ground tap root.

The resultant oil radish regrowth required a more robust post-emergence herbicide for control in the subsequent spring cash crop.

Chemical versus mechanical

However, where the covers had been destroyed mechanically by either rolling on a frost or chopping, spring cereal yield, grain N offtake and total crop N uptake were reduced compared with using glyphosate.

In fact, rolling on a frost to destroy overwinter cover had implications for the quality of the following spring oat crop. The grain specific weight was reduced to below that typically accepted by millers (50kg/hl).

On the other hand, where glyphosate had been used to destroy the overwinter cover, the spring oat specific weight exceeded milling requirements.

The higher specific weight measured following glyphosate destruction possibly reflects the earlier mineralisation of nitrogen in the covers and higher nitrogen availability to the following spring oat crop than from rolling on a frost.

It is likely glyphosate breaks down vegetation more rapidly, allowing mineral nitrogen to become available for the following spring crop earlier than mechanical destruction, which is likely to have benefited spring cereal establishment and subsequent crop performance.

Legacy effect

A legacy effect of cover crop type was noted on the autumn soil mineral nitrogen content after harvest of the spring cash crops.

Across both sites, autumn soil mineral nitrogen content was 20-30kg N/ha higher on the weedy stubble control, reflecting the poorer performance and nitrogen utilisation of the spring cereals where no cover crops were grown.

There was no legacy effect of cover type or destruction technique on either the nitrate leaching losses measured over winter following spring cereal harvest, or on the subsequent spring soil N supply.

The crops grown in the legacy year (a cover crop and oilseed rape) had an autumn N requirement.

If an autumn-sown crop which did not have an autumn N requirement, such as a winter cereal, was grown, there would have been an increased risk of losing autumn soil mineral N via overwinter nitrate leaching.

Read the full Nitrogen Release from Cover Crops project (PDF).