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FERTILISER PRIORITIES FOR THE AUTUMN
Final N fertilizer application of the year
The aim of the final N fertilizer application of the year is to boost grass growth in order to build grass covers for the Autumn. Many farmers are completing their final blanket spread of N in the last week of August/ first week of September compared to waiting until the deadline day of the 14th September. By doing this they are taking advantage of the superior growth rates now compared to mid-September and getting a greater return from the final blanket spread of N fertilizer. Recent Teagasc trials clearly demonstrate that delaying the final N fertilizer application in autumn significantly reduces the grass dry matter yield response to fertilizer N (Figure 1).
The units N/acre applied will vary across farms and should be decided upon based on the remaining fertilizer N allowance of the farm for the year and on the amount of grass on the farm at that time. If rotation length and average farm cover is on target or ahead of target an application of 20-25 units will be sufficient (See GRASS MATTERS for grass targets). However, if rotation length and average farm cover is behind target an application of 30-35 units/acre should be applied. Check overall N farm allowance available before spreading.
The maintenance P and K requirements of a paddock In any given year will depend on the yield of the paddock and weather the paddock was primarily grazed or if there were, surplus bales cut on the paddock. Figure 1 highlights the amount of P and K removed from a cover of 1,500 grass DM/ha. Up to 10 times more K can be removed from a paddock when it is cut for bales vs grazing. When grazed, cows will recycle 90% of the K In the grass back on to the paddock in dung and urine. If baled, 100% of the K In the grass is removed from the fleld. A typical bale of silage contains 10 units of K and 1.7 units of P.
For example, if a paddock was grazed 6 times so far this year and will be grazed another 3 times before the end of the year the maintenance requirements can be worked out roughly as follows:
9 grazing x 3 units K/grazing = 27 units K/acre 9 grazing x 1.7 units P/grazing = 15.3 units P/acre
This changes significantly where surplus bales are taken. For example, If the same paddock was grazed 8 times in the year and cut for surplus bales once at a yleld of 4 bales/acre the maintenance requirements for K are as follows:
8 grazing x 3 units K/grazing = 24 units K/acre 4 bales/acre x 10 units K/bale = 40 units K/acre Total = 64 units K/acre
These figures can be used to work out the K and P maintenance requirements of your Individual paddocks. An application of 2,000 gallons of slurry will provide anywhere from 40 to 65 units K/acre and 8 to 12 units P/acre. Using slurry is the cheapest way to meet these P and K maintenance requirements, but often there is not enough slurry to go around the entire farm. Now is a good time to assess what has been applied P and K wise year to date and determine If sufficient P and K has been applied via slurry or fertiliser.