‘Abolition of derogation could actually lead to more nitrate leaching’

De Boer already conducted an extensive literature study on this subject in 2017. This led the researcher in Wageningen, Gelderland, to suspect that the nitrate leaching from cattle slurry on permanent grassland may be lower than from the commonly used nitrate-containing fertilizer calcareous ammonium nitrate (KAS). Five years later he completed a field trial that confirmed his initial findings. These research results led to parliamentary questions in 2022.

Caroline van der Plas (BBB) ​​announced last month that she wants to submit the results to European Commissioner Virginijus Sinkevicius in order to regain the derogation for dairy farmers. Although De Boer does not want to get involved in political decision-making, he does think it is relevant that the results of his research are taken into account.

What is the most important finding?

‘On permanent grassland, nitrate leaching from cattle manure should be lower than from CAN. We tested that hypothesis in a two-year field trial.

‘When replacing 60 percent of the absorbable nitrogen from CAN with absorbable nitrogen from cattle manure, the leaching of nitrate into the groundwater was on average 40 percent lower. Because 40 percent of the nitrogen input in the combination still consisted of CAN, you can deduce from these results that the difference in leaching between cattle slurry and CAN was even greater.’

How do you explain that?

‘Half of the nitrogen from KAS fertilizer is already in the form of nitrate and is therefore susceptible to leaching below the root zone after fertilization during rain. After the end of the growing season, this nitrate finally leaches into the ground and surface water.

‘There is no nitrate in cattle manure; that must first be formed from the ammonium and organic nitrogen. This conversion takes time. Complete conversion of ammonium to nitrate can take two to ten weeks, depending on circumstances. And during the conversion, the grass absorbs part of the nitrate formed for growth.

‘As a result, the nitrate content in the soil when fertilized with cattle manure is on average lower than when fertilized with CAN. This reduces the risk of nitrate leaching from cattle manure.’

How did weather influences affect the results of this research?

‘In the field trial, we took measurements in both the dry year 2020 and the wet year 2021. In 2021, grass growth was very good, the grass therefore absorbed more nitrogen and nitrate leaching was a lot lower than in 2020. But still nitrate leaching was 35 percent lower when replacing 60 percent nitrogen from CAN with cattle slurry.

‘Our research also proves that nitrate from artificial or liquid fertilizer can leach out to at least 1 meter below ground level during the growing season.’

Derogation as an exceptional position has been abolished by the European legislator in order to limit nitrate emissions. How do you view that decision?

‘The amount of animal manure that could be spread on grassland under the derogation is an agronomically appropriate amount for the Dutch situation. Both in terms of nitrogen and phosphate.

‘Loss of derogation is also contrary to circular agriculture, where you try to use the manure from your own cows as optimally as possible on your own land. Dairy farmers now have to pay large sums to dispose of animal manure and at the same time purchase additional fertilizer to continue producing sufficient feed.

‘I estimate that nitrate leaching from grassland will increase rather than decrease. That seems to me to be an undesirable development.’

Your research conclusions are open to question.

‘Other researchers find that there is no difference in leaching between cattle slurry and CAN on grassland. This opinion is based, among other things, on the assumption that no significant nitrate is leached from CAN during the growing season.

‘That reasoning is not only illogical if you look at processes in the soil, but there are also sufficient research results available that contradict it. An argument that you often hear is that nitrogen from liquid manure mineralizes after the growing season, while the grass can no longer absorb it. This nitrogen then rinses out. As far as I know, there is no research that confirms this view.

‘Nitrogen mineralization and uptake by the grass both depend on largely the same factors, such as sufficient temperature and gas exchange in the soil. So if nitrogen can mineralize, that will also be absorbed. In that case, the absorption capacity of grass is higher than the mineralization.’

What about grazing?

‘During grazing, the nitrogen in the urine spots is distributed irregularly over the grassland. As a result, less nitrogen is used on average and more nitrate can leach out, compared to liquid manure spread in full fields. Leaching increases with grazing later in the year, because the grassland can absorb less and less of the nitrogen from urine spots for the rest of the season.

‘One of my first studies was into the effect on nitrate leaching if cows are housed earlier in the autumn. Reducing grazing in time and housing cows can help reduce nitrate leaching from grazed grassland.’

Herman de Boer (47) has been a researcher at Wageningen Livestock Research since 2001. He studied Plant Sciences in Wageningen and started working as a researcher at the then Praktijkonderzoek Cattle Farming in Lelystad. He conducts research into fertilization, soil quality and grassland cultivation. The field trial into nitrate leaching was co-financed by parties such as Mesdag Zuivelfonds, the public-private partnership Roughage, Soil and Cycle Agriculture and the LTO Noord Innovation Fund. ‘As follow-up research, I would like to conduct a long-term field trial to study what happens if more nitrogen is released from liquid manure and less fertilizer is applied in the long term.’