With a lot of first-cut silage crops under wrapping now, most farmers who planned to cut before the end of May will also have planned on growing a second crop of silage. Here are some considerations and targets to ensure the crop gets sufficient nutrients to grow.
Nitrogen
Nitrogen (N) is the key driver of yield in a silage crop. N application rate for second-cut silage is slightly lower than first-cut to allow for the fact that the N uptake of grass in late summer is typically lower than it is in the peak growth period between late May and mid-June.
Based on a target yield of 4t DM/ha silage (8t/ac fresh grass yield), the target N application rate for second-cut silage is 100kg/ha (80 units/ac). Reseeded swards with high levels of perennial ryegrass should fully utilise 110kg/ha (90 units/ac).
If targeting a shorter growing time and a lighter yield, let’s say 3t DM/ha (6t/ac fresh grass yield), 75kg/ha (60 units/ac) is sufficient.
P and K
Phosphorus (P) and potassium (K) are essential nutrients to maximise silage-crop yields.
Again, based on a typical yield of 4t DM/ha silage, 16kg/ha (13 units/ac) of P and 100kg/ha (80 units/ac) of K is required.
A 3t DM/ha crop will need 12kg/ha (10 units/ac) of P and 75kg/ha (60 units/ac) of K.
But when looking at P and K for a second-cut, it is also important to consider the effect of the first cut. A silage crop will remove approximately 4kg/ha (3 units/ac) of P and 25kg /ha (20 units/ac) of K per tonne of grass silage.
So a first cut on 20 May, yielding 5t DM/ha (fresh grass yield of 10t/ac at 20% DM), will remove 20kg/ha (15 units/ac) of P and 125kg/ha (100 units/ac) of K.
Before getting alarmed, these crop off-takes should have been compensated for when fertilising for the first cut. Index 3 and 4 soils should have received up to those levels of P and K while index 1 and 2 soils should have got more to allow for soil fertility build-up, as well as crop offtake.
It is only if this wasn’t the case that the deficit in P and/or K should be compensated for in the second cut to prevent nutrient deficits.
Sulphur
Sulphur is an essential nutrient for grass growth and is closely associated with N uptake and efficiency. It is estimated that up to 30% of Irish soils would benefit from an application of sulphur.
To reduce the risk of impact from a sulphur deficiency, it is recommended to spread 20kg/ha (16 units/acre) per cut.
Meeting the demands
Slurry should be the go-to fertiliser when it comes to second-cut silage.
At this time of the year, the nutrient value of cattle slurry is 5 units N, 5 units P and 30 units K per 1,000 gallons.
Therefore, applying 2,500 gallons per acre will provide all of the crop P and K requirements, as well as a portion of the N requirements (13 units/ac N, 13 units/ac P and 80 units/ac K).
It is important to be vigilant with slurry spreading though. The weather forecast will dictate how heavy you can go with the slurry. If a prolonged dry period is forecast, a heavy coating of 2,500 gallons is not the way to go.
Also, those in derogation should remember that low-emission slurry spreading systems must be used after 15 June.
Protected urea or CAN should be used to make up the remaining N requirement. With a 65 unit/ac deficit, 2.5 bags per acre of CAN will be adequate. As already mentioned, consider getting sulphur in this fertiliser.
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