Cover crops and moderate N fertilisation can compete with year-round green manure

Disclaimer: This article has been machine translated from Danish to English, so please note that there may be errors in the translation.

Cover crops can have a yield effect similar to year-round green manure in crop rotation. The decisive factor for the economy is whether you can generate income from the sale of feed or biomass from the year-round green manure. This is shown by calculations of the overall economy based on results from Aarhus University's crop rotation trials.

The organic crop rotation trials at Foulumgård have since 1997 produced data to inspire organic plant breeding. A number of different crop combinations have been tested in two organic crop rotations with and without year-round green manure, with and without catch crops, and with and without the addition of degassed manure.

The effect of year-round green manure on the total yield of the crop rotation has been documented in the trials, and positive effects have been found up to eight years after a clover-grass green manure has been ploughed in. The challenge is still to document the economic effect of using up to 25 per cent of the area to produce nitrogen and carbon for the crop rotation. An area that could alternatively be used to produce a cash crop.

Additional yield with fixing catch crops

In the latest rotation, carried out under the auspices of the RDD6 project CC-Rotate in 2019-2021, the focus is on comparing and combining the effects of year-round clover green manure with catch crops in the crop rotation. The crop rotation with all-year green manure gives good additional yields when you harvest the clover grass and return approximately the same amount of nitrogen in the form of degassed fertiliser to the other crops.

Crop rotation without year-round green manure gives good additional yields by using fixing catch crops in 75 per cent of the crops. In fertilised winter rye and spring barley, the same yields are achieved as in crop rotations with all-year clover grass, while a crop of lupin/spring barley can be harvested.

Economic effects

A simple comparison of the economics of the six strategies shows that with the yields achieved and the prices used, it is a prerequisite that the clover grass dry matter has a sales value if you are to expect a positive economic effect of all-year green manure in a four-year crop rotation, i.e. 25% clover grass, as tested in the trials. If you can achieve a price of DKK 1.50/kg clover grass fertiliser and pay for the harvesting and handling of two or three plots for biogas, for example, then the strategy with year-round green manure without catch crops is the top scorer among the strategies.

If a relatively good price cannot be obtained for the biomass produced on the green manure field, the same calculation will show that crop rotation with manure and catch crops but without year-round green manure is the top scorer. With the crop prices and costs used in the example, the balance will be achieved at a price of DKK 1.00/kg clover grass fertiliser.

The unfertilised strategy with unharvested year-round green manure and the use of cover crops gives an economic result that is approximately 25 percent lower than the fertilised joints. It is therefore an economic advantage to harvest the green manure and return it as slurry or degassed fertiliser for the other crops.