Nutrient inputs for second and third cut silage

Published 25 May 12

DairyCo extension officer, Chris Duller

With the first cut safely in the clamp it is now important to focus on ensuring that subsequent cuts have enough nutrients to produce high yielding, high quality crops.

At 25% dry matter a silage crop will remove 1.7kg of phosphate (P) and 6kg of potash (K) for every tonne of fresh material - meaning that a fairly decent crop of 18t/ha (around 7t/acre) will remove 30kg of P and 108kg of K per hectare. Without nutrient inputs, either as fertiliser or muck/slurry, then the crop has to draw on soil reserves to sustain its growth - if the soil reserves are low then the crop will under yield.

Potash supply impacts on the way the grass plant accumulates water in its cells and affects its turgidity and also the way the crop uses nitrogen and builds proteins. Visually it is hard to identify a potash deficient grass crop - yellowing leaves can often be confused with drought stress or wind damage.

To avoid luxury uptake of potash it is recommended that you apply it as required for each cut, rather than one large dose ahead of the first cut.  Even at target index 2 there is still a requirement for 90kg/ha of potash to supply crop demand for second cut and maintain soil index. It is only at high indexes of 4 and above that soil reserves will completely supply crop demand and mean you can use straight nitrogen.

Fertiliser options for second and third cuts will normally be based around 'aftercut' type products (26:0:13), as in many situations winter and spring slurry have supplied enough phosphate for the whole year. However unless you are at K index 3 or above it's unlikely you'll be able to satisfy crop demand with this approach - to top up you may consider using muriate of potash (0:0:60), or more likely an application of slurry or FYM.

The key issues with using slurry and muck between cuts are the poor utilisation of the nitrogen they contain and the risk of crop contamination.  In warmer, drier weather conditions there is greater nitrogen loss as ammonia and a chance that slurry will stay on the grass leaf through to harvest time, or that the harvester will pick up muck in the base of the sward.

Slurry injection is an excellent way to minimise these risks, with clear research evidence to support both cleaner crops and better nitrogen uptake

As well as P and K the other key nutrient to consider for second and third cuts is sulphur. With ever decreasing levels of deposition more crops are becoming sulphur deficient, reducing nitrogen use efficiency and leading to lower proteins.  At most risk are low rainfall areas and sandy soils. Test for sulphur deficiency with a herbage test not a soil analysis. Take your grass sample a few days before first cut to establish if subsequent cuts will require a sulphur input. Recommendation for a sulphur deficient crop is 40kg SO3/ha as a sulphate containing fertiliser after each cut.