Farm Ireland

Monday 22 January 2018

Apply nitrogen to fresh grass

Liam Fitzgerald

At this time in a normal year, or at least in more recent years, I would be telling farmers to start gearing up to apply early nitrogen. As I look at pastures today, they are a mixture of pale green to light brown in colour, with no sign of fresh growth.

Average soil temperatures are up to 1°C below normal and most soils are saturated. Soil temperature is the main factor that determines the start of spring growth. Grass needs a soil temperature of 6°C for growth and, over much of the country, this does not occur until early March.

Recent winters have been exceptionally mild, with periods of several weeks when the average temperatures were in the range of 6-8°C. This gave a good accumulation of grass over the winter months where land was closed from late October.

The provision for early grazing in February and March comes from grass grown between October/November and February, rather than any substantial response to early nitrogen. Where there is good soil fertility there is adequate background nitrogen to provide for the low-level growth that occurs during the closed period.

Unfortunately, to date this year, not only has there been no growth but the pasture covers that were there in December have diminished by around 200kg DM/ha, or 60-70pc of what we might currently expect to be available. Still, a couple of mild weeks would get growth started and we could end up with 'normal' grass covers on rested land by mid-March.

Apart from southern counties and coastal regions, growth rates in February are likely to be low or non-existent due to prevailing temperatures. Therefore, the yield response to applied nitrogen will be low.

Over most of the country, temperatures rise steadily from early March and so does the response to nitrogen. Because early grass is valuable there is an advantage in going out with a small amount in advance of when growth takes off. Also, there is a lag of a week or so between the spreading of nitrogen and its effect on growth. Even if temperatures are lower than that required for growth, it takes a while for the nitrogen to get through the soil and into the plant roots, and this occurs at 1-2°C lower than that for growth.

If weather conditions are suitable from next week onwards, then start spreading nitrogen where there is a reasonable cover of fresh grass. A reasonable cover might be a grass height of 3-6cm of fresh growth -- not a decayed stubble.

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The application of nitrogen in spring always involves some speculation on growth conditions. If there is a prolonged period without growth, a proportion of the nitrogen will be lost, mainly through de-nitrification, but on average you can expect to get a response of 8-10kg of grass DM/kg of nitrogen applied from mid-February to late March.

However, response will depend on several factors -- weather, soil type, sward composition and topographical aspects. Where there are high grass covers (not common this year) it would be preferable to graze them off and then apply the nitrogen to boost the regrowth.

At a response of 8:1, using CAN, one tonne of grass dry matter costs €125, while using urea, one tonne of grass DM costs €87.50, assuming 80pc of the grass is used. Therefore, urea is 30pc cheaper than CAN for spring grass when CAN costs €220/t and urea costs €320/t.

Apply only the amount of nitrogen that can be used by the sward: in the region of 0.5 bags urea or 0.75 bags CAN/ac. This rather light dressing will also help to avoid excess nitrogen loss in the event of adverse conditions.

An alternative to chemical nitrogen is to apply a light dressing of slurry -- not more than 22,000l/ha (2000ga/ac). This should supply up to 20 units of nitrogen per acre, similar to the above recommendations for chemical nitrogen.

Slurry should not be spread on high grass covers as it will contaminate the grass and delay grazing. There should be 4-5 weeks between the application of slurry and expected date of grazing.

The phosphate in slurry can give early grass an extra boost where P levels are marginal, as early grass benefits from higher P levels.

Irish Independent