Agronomy Advice

The impact of reduced nitrogen rates on grassland yield

Most farms will apply through experience, roughly the same amount of nitrogen fertiliser each year to grow what forage is required on the farm. Some years you grow more grass for the same amount of nitrogen because the weather is favourable and other years you grow less. For any farmer contemplating reducing nitrogen fertiliser applications on grass, it would be worthwhile to calculate what effect any decrease in nitrogen rates will have on grass growth.

The impact of reduced nitrogen rates on grassland yield
The impact of reduced nitrogen rates on grassland yield

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

Nitrogen response curve for first cut silage
This chart shows the typical nitrogen response from applications of AN or Urea based on Yara trials conducted over many years. The actual optimum will tend to vary depending on site location and grass growth potential.

From Yara’s own trial work in both the UK and Ireland and other sources we can 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 – 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.

Example: A farmer who plans to mow his/her 1st cut on the 10th of May is going to reduce the N rate from 120 kg/ha to 100 kg/ha because of an increased fertiliser price. 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?

On any livestock farm homegrown grass will always be the cheapest source of feed

The highest and most reliable dry matter response year-on-year to applied nitrogen is in this April/May period, and any reduction in nitrogen rates particularly on 1st cut silage crops should be carefully considered. Because, a large portion of the cash costs associated with producing grass silage is charged on an area basis, so a reduction in yield pushes up the cost on a per tonne basis and any savings on fertiliser can be quickly eroded.

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