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Row-injected cattle slurry is a promising fertiliser strategy for silage maize

According to researchers from Aarhus University, row-injected cattle slurry has the potential to replace mineral phosphorus starter fertilisers in silage maize production. This can reduce the phosphorus surplus while maintaining the crop yields.

2020.06.18 | Camilla Brodam

Photo: AU Foto

Maize is an important forage crop on dairy cattle farms, and the cultivated area with maize has been steadily increasing over the last many years. The new phosphorus regulation is particularly important for derogation farms with maize, where up to 230 kg N/ha can be applied from animal manure. At these farms, it may be difficult to comply with the phosphorus regulations if phosphorus is added as mineral starter fertilisers in addition to animal manure. According to researchers from the Department of Agroecology and the Department of Engineering at Aarhus University, new fertilisation strategies are therefore needed to adhere to the new phosphorus regulation without compromising final yields. 

“Phosphorus is an essential plant nutrient, but the young maize plants may have difficulty absorbing sufficient amounts of the nutrient from the soil. Therefore, the current recommendation is to place mineral phosphorus fertilizers near the maize row at sowing on soils with low or moderate phosphorus status or on soils with poor root development opportunities to ensure an adequate phosphorus supply to the young maize plants. However, when phosphorus also is applied with animal manure, more phosphorus is added to the field than removed with the crop at harvest, resulting in a phosphorus surplus. We have investigated how the utilisation of phosphorus in cattle slurry can be improved and hereby obviate the need for mineral phosphorus as a starter fertiliser,” explains PhD student Ingeborg F. Pedersen.

Cattle slurry as starter fertiliser

The researchers have investigated whether row-injected cattle slurry applied a few days before sowing could ensure as good a phosphorus supply as when using mineral phosphorus fertiliser. The experiment was a two-year field trial conducted on two soil types; a sandy loam (JB4) in Foulum and a coarse sandy soil (JB1) in Havris. 

“In the field trial we tested two placement methods; slurry injection in a narrow or in a broad band, placed next to the seed or below the seed, respectively, and we compared these methods to traditional slurry application with increasing amounts of mineral phosphorus fertiliser,” explains Ingeborg F. Pedersen. 

The broad slurry band is approx. 26 cm wide and placed at 10 cm depth, while the narrow slurry band is approx. six cm wide and placed next to and below the seed. The strategy requires that the maize seeds are sown accurately in relation to the slurry band, so GPS or the like is needed to ensure this.

“Placement of slurry near the maize seed ensures a targeted phosphorus supply so that we fertilise the plant and not the soil. Phosphorus is very immobile in the soil, so we have to make sure to place the slurry nutrients where the plant roots can access them. This is particularly important for maize grown on our latitudes, where cold spring temperatures can inhibit root development and thus the plant´s ability to absorb phosphorus from the soil pools. In many ways, we actually just place the slurry in the same way that one usually places the mineral phosphorus fertilizer. Our results show that row-injecting slurry in a broad band is a promising method, while we do not see as good results with narrow banded slurry,” explains Ingeborg F. Pedersen.

Acidified slurry and application of a nitrification inhibitor

The researchers have also looked at how slurry acidification and application of a nitrification inhibitor to the row-injected slurry affect maize growth. Acidified slurry increases the amount of water-soluble phosphorus, which is more plant available. Adding a nitrification inhibitor reduces the conversion of ammonium to nitrate in the slurry. When plants absorb nitrogen as ammonium, protons are released into the soil near the roots, lowering the pH in the soil. This soil acidification may increase the availability of some of the inorganic phosphorus compounds in cattle slurry.

“We saw that acidification of slurry placed in a broad band increased yield at harvest on the sandy loam, but not on coarse sandy soil. By applying a nitrification inhibitor to the slurry placed in a broad band, we also found a positive effect on the final yield on the sandy loam,” explains Ingeborg F. Pedersen. 

Reduced phosphorus surpluses

Prior to the field trial, the researchers performed pot experiments under climate-controlled conditions, where the mechanistic processes governing the phosphorus availability in cattle slurry were studied.

“We conducted pot experiments in growth chambers, where we investigated soil chemical changes, root development, etc. in the early growth stages after application of placed fertilisers. The field trial was a final testing of the promising fertiliser strategies identified in the pot experiments. In the field trial, we also had the opportunity to see how early growth affected the final yield. In some cases, we saw positive effects of both mineral phosphorus fertilizers and placed slurry on early growth, but it did not always result in higher yields at harvest. Thus, a good supply of phosphorus during early growth - whether it is supplied via mineral fertilisers or placed slurry - does not guarantee that the yield potential is fully realized, since growth conditions later in the season may affect the final yield in a different direction. The field trial also shows that the amount of phosphorus applied with cattle slurry is roughly equal to the amount removed by the crop at harvest. By omitting the use of mineral phosphorus fertilisers, a better match between P input and crop demand can be achieved, leading to reduced phosphorus accumulation in soil,” says Ingeborg F. Pedersen. 

Behind the research

Collaboration partners: Department of Agroecology and Department of Engineering at the University of Aarhus and SEGES.

Funding: The publication is part of the GUDP project "Slurry-IT", which is funded by the Ministry of the Environment and Food.

Conflicts of Interest: None

Read more: Here you can read the scientific article: “Row-injected cattle slurry can replace mineral P starter fertilizer and reduce P surpluses without compromising final yields of silage maize”, published in the European Journal of Agronomy. It was written by Ingeborg F. Pedersen, Gitte H. Rubæk, Tavs Nyord and Peter Sørensen.

Contact: PhD student Ingeborg F. Pedersen, Department of Agroecology, Aarhus University. Email: ifp@agro.au.dk.

Research, Agro, DCA