The Danish climate does not provide a basis for growing crops all year round. In other words, there is a growing season and a season where there is no cultivation. This applies to virtually all areas with the same climate as Denmark. It gives a whole season where the soil is not cultivated and it can be problematic as the large amounts of rainfall in autumn and winter combined with the lack of plant cover is one of the biggest reasons for excessive nitrate leaching. One mean of avoiding this is cover crops, which grow after the main crop is harvested.

“Cover crops are considered to be one of the most effective ways to reduce nitrate leaching and the nitrogen load on the aquatic environment. But we can see that the efficiency of cover crops varies considerably from year to year depending on the growth conditions during the fall. And especially the temperature plays a big role. A late harvest reduces the chances of a strong and successful cover crop, as they start growth correspondingly late,” says post doc Johannes W. M. Pullens from the Department of Agroecology at Aarhus University

Late harvest results in poorer conditions for catch crops

In order to meet the requirements of the EU Nitrate Directive and the Water Framework Directive, it has been decided in Denmark that farmers have to grow cover crops on a certain proportion of their soil. According to the rules, the cover crops needs be sown no later than 20 August.

“This means that the main crop - primarily grain - must be harvested before 20 August. But farmers in the northern part of Denmark have had a hard time living up to these deadlines, either because the grain is simply not ripe enough or there have been unfavourable harvesting conditions,” says Johannes W. M. Pullens.

Late harvests due to, for example, unfavourable weather have made it necessary to grant dispensation for the date of sowing the cover crops. Bad weather can, among other things, play a role in relation to the farmer's ability to get into the field if it is too wet and soft to drive on.

One step further then the policy support

In an order from February 2019, the Danish Agency for Agriculture asked DCA to develop a model for predicting harvest dates, e.g. to be better prepared for postponing the deadline for the establishment of cover crops in Danish fields.

"The model we have made for the Danish Agency for Agriculture is based on the phenological development of crops," says Johannes W. M. Pullens.

In other words, the model takes into account growth phases from sowing over emergence to maturity. In addition, a model has been developed for the period from maturity to harvest.

“The different growth phases are in different ways dependent on temperature and day length, and based on historical data for sowing time, germination, flowering, maturity, and harvest. We have been able to develop a single model that can be calibrated for spring barley and winter wheat, respectively. And with an uncertainty of 6.2 days for spring barley and 9 days for winter wheat, it can predict harvest time,” says Johannes W. M. Pullens.

Large geographical variation in harvest time

When the researchers used the model to estimate harvest time over a 20-year period, they found a significant geographical variation in harvest time of up to 18 days across Denmark.

"We could see that the earliest harvest occurs in the southeastern part of the country, while the latest harvest time is in the northwest, and there can be up to 18 days difference in when the grain is ripe and ready for harvest," explains Johannes W. M. Pullens. 

Harvest date affects the effect of cover crops

The difference in harvest time plays a big role in relation to when the cover crops can be sown and for when submerged cover crops start to grow. Thus, there are also differences in whether the cover crops can grow and become strong enough to reduce leaching of nitrogen sufficiently. 

Therefore, in an article in the scientific journal "Field Crops Research", the researchers investigated how the model can be used to assess the effect of the variation in harvest date on growth and the effect of cover crops on nitrate leaching.

“If harvested late, it can have consequences for how well the cover crop is established. And if the cover crop is not well established in the field, it will not be able to grow large enough to reduce nitrate leaching sufficiently,” says Johannes W. M. Pullens 

New possibilities

The model results show that there are great opportunities in shortening the period from maturity to harvest. This requires in some cases that the farmer chooses to harvest the grain with a slightly higher water content than desirable. But it can also mean that new harvesting methods are needed.

“If the harvest is closer to the maturity date, you will also be able to sow the cover crops a little earlier. This improves the establishment and growth of the cover crops, and it will provide a more biomass and thus reduce leaching of nitrogen,” explains Johannes W. M. Pullens.

The new model calculations thus provide a basis for developing cultivation systems based on an earlier harvest of the cereal crops, which will be able to provide better established and more vigorously growing cover crops with a greater contribution to solving the agricultural problem of nitrogen leaching to the aquatic environment.