HOW TO: Plan nitrogen requirements

Nitrogen (N) is the most important nutrient for grassland because it has such a strong effect on yield.

Very generally, 1kg of N will grow between 10 – 15kg of grass DM. So 50kg N will grow an extra 600kg of DM at a 1kg:12kg response ratio.

There are several sources of N:

  • The breakdown of soil organic matter
  • Deposition from the air (usually included with that from soil organic matter as ‘soil N’)
  • Dung and urine deposited by grazing animals
  • Applied manures
  • Clover
  • Fertilisers

These all add up to the total N supply. However, only some of this will be readily available to the plants and it is this that controls grass yield. It is worth considering what other sources of N are being used before topping up with bought-in fertiliser.

The status of soil can be difficult to measure, but fields can be categorised depending on previous management. Low soil nitrogen supply (SNS) refers to grass that has previously received up to 100kg N per ha, medium SNS previously received 100 – 250kg N per ha or has substantial clover content and high SNS previously received over 250kg N per ha.

Table 1 and 2 show example nitrogen recommendations for beef and sheep respectively, with the aim of growing around nine tonnes of dry matter (DM) per ha.

Table 1: Example N recommendations for a beef herd

Nitrogen table

Table 2: Example N recommendations for a sheep flock

Nitrogen table

Remember that white clover can fix up to 150kg N per ha per year, so focusing on maintaining white clover swards will reduce bought-in fertiliser costs while improving animal performance as growth rates can increase by up to 20%.

It’s also important to remember sulphur (S). Fields are likely to become sulphur deficient later in the season, especially if silage is cut. Grassland will benefit from an application of sulphate-containing fertilisers at around 40kg SO3 per ha for each cut of silage. Sulphur deficiency in grass looks very much like N deficiency, but if grass looks yellow after the recommended amount of N has been applied, it is probably deficient in S. A tissue sample needs to be taken to confirm and a concentration of less than 0.25% S, or an N:S ratio greater than 13:1 in the grass DM indicates S deficiency. As well as reducing grass yield, sulphur deficiency affects nitrogen uptake and the formation of protein in grass.

Seek advice from a FACTS qualified adviser for precise recommendations.

For more information on nutrient management see the BRP manual Managing Nutrients for Better Returns.

New chapter for grass and forage crops

The crop nutrient manual (RB209) is being launched at Grassland & Muck 2017 on the 24th of May. It replaces the Fertiliser Manual, which was the eighth edition of the RB209. In the new version, there is a specific chapter for grass and forage crops.

Come and visit the AHDB stand at the event (stand 323) where you can find out about the new crop nutrient manual. Liz Genever (AHDB Beef & Lamb) and Paul Westaway (chair of the grass and forage crops technical working group) will be discussing the main changes at a briefing at the event on the 24th of May.