What is the impact of reducing nitrogen rates on grassland yields?

Each year, most farms through experience will apply roughly the same amount of nitrogen fertiliser to grow the forage requirements for the coming seasons.

Yara nitrogen graph new

Some years due to favourable weather you might grow more grass for the same amount of nitrogen and other years it might be a lot less. For any farmer thinking of reducing nitrogen fertiliser applications, it could be worthwhile to calculate what effect any decrease in nitrogen rates will potentially have on grass growth.

Nitrogen is an important and major nutrient required by grass. It is the key to achieving high dry matter yields and is often strategically used to increase production. The key to achieving high yields is to apply the correct amount of nitrogen, from the right source, at the right time.

Nitrogen response

This chart shows the typical nitrogen response from applications of nitrogen. This is based on a trial site (SRUC Aberdeenshire 2015) but is typical of similar Yara trials conducted over many years. The precise optimum will tend to vary depending on site location and grass growth potential.

How much dry matter per kg of nitrogen should you expect?

From studies in the UK and Ireland, we estimate that on a typical 1st cut with good yield potential and soil fertility, harvested between the 1st and 20th of May, we can expect to grow 25 kg of dry matter (DM) per kg of applied N. Late May to early June harvested 1st cuts will grow closer to 30 kg of dry matter per kg nitrogen.

For grazing on intensively stocked farms, nitrogen responses can be very variable in early spring, with 10 kg of DM per kg of applied N for that late-February to March period considered good. The response increases (20 – 30 kg of DM per kg N) very quickly in April with improving weather and soil conditions, rising to 30 – 40 kg DM per kg N in May.

The important question is

As we all know on any livestock farm homegrown grass will always be the cheapest source of feed.

It's therefore a balancing act and for farmers who are thinking of reducing their fertiliser applications this coming seaon, it's important to note the knock on effects not only on grass growth but forage yields, diets and purchase feed.

A prime example is:
A farmer plans to mow their 1st cut on the 10th of May and is considering reducing the N rate from 120 kg/ha to 100 kg/ha because of an increased fertiliser prices. The yield loss is likely to be 500 kg/ha of DM (25 kg of DM x 20 kg N). The question is, can the farmer replace this 500 kg of DM for less than the saving on the fertiliser?