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The Importance of Autumn Fertiliser

The Importance of Autumn Fertiliser

In August soils release their own organic N naturally.

On lowly stocked farms (< 170kg organic N/Ha) that have enough winter forage this may provide sufficient N to build grass covers. However, if you are short on silage applying 20-30 units N cross August and September.

Heavier stocked farms (<190kg organic N/Ha) may need to consider applying a blanket application of nitrogen (c. 25-30 units per acre in August and September).

The amount to apply will depend on how much of your full years allowance you have left, the overall grass supply on the farm and if extra silage is needed.

Response to N:

Research from Johnstown Castle shows response to 30kg N/Ha (24u N/ac) fertiliser spread at start of Aug, Sept and Oct over three years in four locations.

Every kg N applied gave a response of 27kg for Aug, 19kg for Sept and 10kg for Oct of Grass DM.

This means you are getting nearly 1.5 times more grass by spreading N in Aug compared to Sept.

August: 30kg N x 27kgDM = 810 KgDM/Ha Grass, stocked at 2.5 this means 20 days grazing per cow.

Sept: 30kg N x 19kgDM = 570 KgDM/Ha Grass, stocked at 2.5 this means 14 days grazing per cow.

Should I apply slurry?

Approximately 85% of the economic fertilizer value of slurry is due to its P and K, of which the K content is c.70%.

The remaining 15% is Nitrogen.

Nitrogen availability from slurry is c.40% lower in the summer than in the spring. 1000 gals per acre of cattle slurry applied using a splash plate in the spring will supply 6 units N per acre. In the summer the N supplied is reduced to 3.6 units per acre. Using low emission slurry spreading (LESS) equipment increases the N supplied by c. 25-30%. Average available Nitrogen in 1000 gals of cattle slurry applied to soils with low P and K indexes (1 or 2) in either spring or summer

  Spring (units per acre) Summer (units per acre)
Splash Plate 6.3 3.6
Trailing Shoe 9 5.4

Time of the year has little effect on P and K utilisation. By spreading slurry in the autumn you will realise most of the value. To realise the most value apply the slurry to low index P and K fields.

Nitrogen and slurry applications in autumn;

  • Apply slurry to low P & K paddocks.
  • Apply low levels of nitrogen rather than missing N on paddocks.
  • Apply a blanket application of nitrogen before the end of September