Clover-grass mixtures are popular because they do not need to be fertilised as much as pure grass crops. Indeed, legumes such as white and red clover are self-sufficient in nitrogen. They can simply fix nitrogen from the atmosphere and fix part of the nitrogen in the soil. In this way, they can actually also provide nutrients for the grasses that also form part of the field's plant community.

"Because legumes are nitrogen-fixing, fertilising your clover grass optimally can be complex. This is true for both organic and conventional farmers, because the more fertiliser you apply the less clover there will be, the grass simply out-grow the clover. In popular terms, you're fertilizing the clover away," explains postdoc Henrik Thers from the Department of Agroecology at Aarhus University.

This is problematic because the less clover, the less nitrogen is fixed.

Organic experiments with cattle manure

Researchers from Aarhus University have conducted organic fertilisation trials of clover grass fields to find the optimal fertilisation in relation to field yield, proportion of clover, protein concentration and, not least, nitrogen fixation. Therefore, the chosen fertiliser has been cattle slurry.

"In total, we had four fertilisation levels in this trial. We have fertilised with 0, 100, 200 and 300 kg total N per ha in cattle slurry. If you look at the dry matter yield, there is a significant increase in yield up to 200 kg N. There is also a small increase at 300 kg N, but it is not significant. Therefore, we conclude that 200 kg N is the most optimal in terms of yield," says Henrik Thers.

So, unless you're a cattle farmer with unlimited manure, you should give the 200 kg N to clover grass and save the rest for other fields.

"We have also studied the quality of the yield and we can see that you don't actually get a better quality when you fertilise, on the other hand you don't get a significantly worse one either, so you shouldn't be afraid to fertilise your clover grass," says Henrik Thers.

Autumn tattle about spring

Clover grass is fertilised several times a year, and often the fertiliser will be adjusted to the proportion of clover. So, it is problematic that the first fertilisation of the year is applied so early that the farmer can't assess the proportion of clover in his field. Researchers now have a solution to this problem.

"Of course, it would be most optimal if you could measure the proportion using specially made cameras, but this is not possible at the beginning of the season because the clover plants are simply too small. We've found that you can use the images from the autumn to tell you something about how much clover there will be next spring," says Henrik Thers.

(Read also: "A breakthrough in image recognition of clover allows optimised nitrogen supply to clover pastures")

The research shows that if you have a clover percentage in the autumn that is between 33-75%, then there is a reliable correlation to the clover percentage the following spring. However, the proportion will be some percentage points lower. In this way, you know what the clover percentage will be even before the clover percentage can be measured, and this allows you to adjust fertilisation.

Enough data for a fertilisation model

Like previous studies, the results of the new trials also establish that at the high fertilisation levels, a lower percentage of clover will be found than at the lower fertilisation levels.

"So, it's true that you can fertilise the clover away. That's why it would be a good idea to create a model for optimal fertilisation of organic clover grass. If you are going to do such a model, you need to know what happens when you fertilise clover grass with different amounts of slurry. We now have this data," says Henrik Thers, who hopes that the study will contribute to more optimal fertilisation of clover grass.