A guide to applying nitrogen on grassland early in the season

Applying fertiliser in February can provide grazed grass at less than half the cost of the silage on a dry matter (DM) equivalent basis, providing the right choices are made on product, rates and field conditions.

Applied in conditions that remove environmental risks, the economic and production gains of early application can stack up, says grassland specialist Chris Duller.

“Going out with 30kg of nitrogen [N] at a response rate of 10-1 will give you 300kg DM of grass,’’ he calculates. Costed at £220/t, that works out at 64p/kg of N.

“For my 64p, I can grow 10kg DM of grass, which is equivalent to 6.5p/kg of DM, compared to 15p for silage and 20p for concentrates.”

See also: Is foliar fertiliser better than compound fertiliser?

That grass must be used – at a utilisation rate of 50%, costs double, yet that is still cheaper than the alternatives.

“But be careful about poaching while grazing in February, as you can lose all that gain and struggle for growth later in spring,’’ he adds.

Mr Duller advises considering the following when applying early N.

Chris Duller © Debbie James

1 Soil temperature

• The temperature needs to be recorded with a soil thermometer at 10cm below ground for a true result.
• Grass will only grow at soil temperatures above 5C, when soil biology starts to become active.
• At 5C, bacteria will use a proportion of what goes into the soil solution, such as fertiliser applied, and convert it to organic N.
• With three days of dry weather at 5C a lot of the fertiliser will go into the bacteria pool, but any lower and the N in the soil solution is at risk of disappearing, leading to a response rate of less than 20%.
• Apply fertiliser to south-facing fields with lighter soils first as heavier and wetter soils are slower to warm up.
• Avoid shady areas – in February, the sun is still low, leaving some parts of fields in shade for up to seven hours.

2 Only apply in appropriate weather conditions

Soil temperature is vital, but the driver for early fertiliser application in recent years has been ground conditions resulting from exceptional levels of rainfall in the winter and early spring.

• Don’t go out with the fertiliser spreader unless you can get the cows out to graze the grass.
• Monitor the long-term weather forecast for signs of  high pressure building – what the weather looks to be in 10 days’ time is crucial if you plan to spread fertiliser early in the season.

3 Recommended rate for early application

• To kick-start growth, apply no more than 20-30kg N/ha, to give a growth rate of 15kg DM/ha/day in February.

4 Expected response rate

• Even with perfect conditions, the response rate will be no higher than 30%.
• Losses will always be high, but N not used will be stored in the organic pool of bacteria and used up to 10-15 weeks later.

5 How to reduce losses

• Opt for urea when soil conditions are marginal or if rain is forecast, as it is less soluble than ammonium nitrate.
• Avoid spreading, or delay by three or four weeks, on fields that have been drained or where a mole plough or sward lifter has been used, as water can drain quickly from them.
• Run-off will be greater in fields with capped soils and poor ground cover, where livestock have been grazed over winter.
• Switch off the spreader near muddy gateways and feeding areas.
• Consider a 5m buffer zone from watercourses if it’s likely to rain.