Care needed when applying slurry to post-silage grassland

Dairy farmers planning slurry applications immediately after grassland cuts should consider whether it provides all the grass nutrients required.

More farmers are making better use of slurry through accurate application and detailed analysis to measure its nutrient content. Refining this approach using detailed analysis, can increase slurry value still further, maximising homegrown forage.

Slurry analysis isn’t as high in potash (K) as some producers might think, says Tom Wells, agronomist at Origin Soil Nutrition, who advises number of dairy farms using slurry as a base for grass nutrition.

“In many cases, it doesn’t provide enough potash to support and maintain current production. This can leave offtake shortfalls between the nutrient value of the slurry applied and the recommended application rates to grassland following silage cuts.”

Potash applications

The Potash Development Association (PDA) say that 40% of grassland soils have below target potassium levels, which should be a concern for growers given potash’s role in the nutrient synergy.

Potash is essential for water regulation in plants and will support continued growth and resistance in drought conditions. It will also help crops to make better use of nitrogen (N), the main nutrient required for increasing yield.

Large quantities of potassium are removed with each cut and, without replacing these offtakes, subsequent deficiencies will impact the efficacy of other nutrients. Farmers taking silage cuts should be aware of the crop requirements.

PDA guidelines for a silage crop at 25% DM suggest that 6kg of potash per tonne of fresh weight will be removed so a 47t/ha crop will remove 300kg K20/ha.

Fertiliser handbook

Furthermore, AHDB’s nutrient management guide RB209 advises that a soil with a potassium index of 2+ requires a total potassium input of 60kg/ha each, following first and second cut.

This is reduced to 40kg/ha each for third and fourth cuts – a total of 200kg/ha over the season. Ganular nutrition will be required to support offtake requirements as the slurry nutrient analysis he’s seen this year won’t be sufficient.

“The potassium content in slurry alone is unlikely to meet the offtakes for index 2+, and the crop demand for K is much higher at lower indexes, for example, an index 1 soil requires 360 kg/ha.

Depending on slurry nutrient values, applying 2,000 gal/ha could provide 20kg/ha of potassium to the soil, which is lower than the offtake requirement above and will leave the crop deficient,” says Tom.

He emphasised that we can only manage what we measure, so getting slurry analysed will enable a FACTS advisor to prescribe a nutrient management programme in accordance with the nutrient value of slurry.

Don’t rely on nitrogen alone

The importance of linking nutrition requirements to soil analysis following cuts will help grow good quality forage. Without this data, farmers and agronomists across the Midlands don’t have the right information for nutrient recommendations.

“As we know, grass is the cheapest form of feed, and we are advising farmers that have a budget in mind for grassland nutrition to let us tailor requirements based on slurry and soil analysis to maximise fertiliser inputs.

Vital nutrients

“We are formulating after-cut grades that match offtakes for potassium and account for slurry inputs, while providing the soil with other vital nutrients based on deficiencies.”

Farmers only applying bagged or liquid nitrogen, and relying on slurry for other nutrients, will be limiting crop performance.

“Nitrogen is required for yield, but it won’t grow grass on its own. It creates a synergy with several other nutrients, including potash, where the relationship supports nitrogen conversion into protein and assists uptake via roots.”

“A potash deficiency will weaken nitrogen’s response, which leads to an increased risk of nitrogen loss to the environment. We can look at optimising and adjusting nitrogen rates all we want, but if a potash shortfall is apparent, then we are limiting its potential at the start.

“We aren’t just growing grass for yield; it’s the quality of the forage that we want to improve as well. Refining this provides the animal with a more rounded and nutritionally balanced feed source.”