When should you apply early nitrogen?

A lack of nitrogen (N) supply in the soil can limit spring grass growth. The timing and rate of fertiliser N and slurry application are key decisions for every livestock farmer in early spring.

Research shows a large range in grass growth response to early spring N (between 5 to 18 kg DM/kg N applied). While the appropriate application of early N is beneficial, the incorrect application of early N is wasteful, costly, pollutes water and increases greenhouse gas emissions.

Take the following into consideration when considering applying slurry or fertiliser N:

• Check the weather forecast (met.ie) – do not spread slurry or fertiliser if rain or very cold weather is forecast.
• Only apply fertiliser N when soil temperature is greater than 5°C and rising - grass is growing at these temperatures and will take up the N in the slurry or fertiliser
• Check the soil trafficability before spreading to avoid damage to soil
• Measure farm cover and use the grass growth predictions (available in this newsletter, on the PastureBase Ireland website, on the Farming Forecast on Sunday on RTÉ 1) to inform decision making around slurry and N fertiliser application
• Target areas most likely to respond to early application of N - fields with a high perennial ryegrass content or recently reseeded, drier areas, fields with a grass cover > 400 kg DM/ha (6 cm), fields with optimum soil fertility (index 3 for P and K, pH > 6.2).

Where possible use slurry to replace fertiliser N in spring. Apply slurry to approx. 40% of the farm as soon as permitted, targeting high demand areas where possible, e.g. silage ground and low P and K grazing ground. In February, apply slurry to grazed areas (up to 30% of the farm). Apply all slurry using LESS (low emissions slurry spreading) methods. 2,000 gals/ac applied using LESS will supply ~20 kg/ha (16 units/ac) of available N and 2,500 gals/ac will supply ~25 kg/ha (20 units/ac) of available N. Manage slurry application to ensure that no more than 2,500 gal/ac are applied in each application.

Don’t apply more than 29 kg N/ha (23 units N/ac) chemical N fertiliser in February. The whole farm should receive fertiliser N in March (20-50 kg N/ha; 20-40 units N/ac). No more than 75 kg N/ha (slurry N + chemical N) in total up to early April (Table 1 and 2). All chemical N should be applied as protected urea.

Table 1. Nitrogen fertiliser and slurry application plan for the early spring period on well-drained soil

¹Application of N for February/March grazing

²Assumes slurry at 6% DM, adjust application rates based on slurry DM%

³Some of this area will be silage ground

⁴Combination of Protected Urea and cattle slurry

Table 2. Nitrogen fertiliser and slurry application plan for the early spring period on heavy soil, less intensive and/or later turnout farms (flexibility in application is essential on heavy land)

¹Assumes slurry at 6% DM, adjust application rates based on slurry DM%

²Some of this area will be silage ground

³Combination of Protected Urea and cattle slurry

Table 3. Closed Periods for the application of organic & chemical fertilisers 2024