Soil temperatures need to be at least 5C before nitrogen application can boost spring grass growth. However, the date that soil temperatures reach this critical point varies from year to year.
In some years, 1kg of nitrogen (N) has the ability to grow 10-15kg of grass dry matter during February, while in other years there can be little or no grass growth response to it because of prolonged cold weather going into March.
This year would be classed as one of the later years and figures show soil temperatures above 5C only in the past week.
Until recently, the recommendation has been to apply nitrogen fertiliser six weeks before your expected turnout date.
However, as farmers move towards turning out smaller groups of cattle at intervals and starting with an earlier turnout date, this recommendation no longer applies and a more targeted approach is needed.
Teagasc experts Michael O'Donovan, Emer Kennedy and Pearse Kelly outlined some guidelines for nitrogen application on spring grass.
They advise that paddocks or fields that have heavy covers of grass -- more than 10cm -- built up on them since the previous autumn and over the winter should be grazed first before applying N.
Paddocks with little or no grass covers should receive cattle slurry first and N at a later date. These will be the last paddocks to be grazed in the first rotation.
They also recommend applying no more than 23 units of N/ac for the first application and waiting until soil temperatures are at least 5 C and rising.
Farmers should target the earliest nitrogen applications on the paddocks and fields that have the greatest production potential. These are paddocks that are predominately ryegrass swards, paddocks with 5-8cm of grass and those with good fertility -- when phosphorus, potassium and lime levels are good.
Given the rising cost of fertiliser, farmers should use urea for spring applications wherever possible to reduce costs because urea is cheaper per kilogramme of N than CAN.
"Remember not to spread N fertiliser when there is heavy rainfall forecasted or most of it can be washed away," warns Moorepark's Emer Kennedy.
The amount of nitrogen that should be spread for first-cut silage will depend on several factors, but most fields should be targeted to receive 90 units.
Where slurry is applied to the silage ground first, the nitrogen requirement can be reduced by up to 10 units N/1,000ga spread per acre. For example, silage ground that was spread with 2,500-3,000ga of slurry will need 24 units less of nitrogen.
The amount of ryegrass in the sward affects the response to nitrogen. Old pastures with low levels of perennial ryegrass should receive a maximum of 80 units -- from both slurry and chemical N fertiliser. In contrast, new reseeds will show a response up to 100 units/ac.
Where fields have received N in the weeks coming up to closing for first-cut silage, one-third of this N can still assumed to be available and this should also be taken off the requirement (see table).