New model predicts harvest date and cover crops efficiency

Farmers in Denmark have to grow cover crops on a certain proportion of their soil in order to meet the requirements of the EU Nitrate Directive and the Water Framework Directive. Cover crops needs be sown no later than 20 August.

As a consequence, 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,” Johannes W. M. Pullens from the Department of Agroecology at Aarhus University said in an article on the university’s website.

The Danish Agency for Agriculture asked the university 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.

Phenological development of crops

Pullens said their model is based on the phenological development of crops. 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,” Pullens said.

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.

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.

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,” Pullens said.

_{Text continues underneath image}

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. – Photo: Lex Salverda

Shortening the period from maturity to harvest

According to the researchers, 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,” Pullens said.

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.