Second cut silage: potash and sulphur

May 2017

When cutting grass for 1^st cut silage, with a view to getting a 2^nd cut, there is an urgent need to get the fertiliser on for the regrowth right away, while the soil is still damp, to feed that growth. Some say that silaging the field is not complete until the fertiliser is on for the 2^nd cut! Any delay leads to a lower yield.

While Nitrogen is the main driver for this urgency, the quick application of Potash and Sulphur are important too. The large uptake of Potash in the 1^st cut will mean that the readily available Potash (“the exchangeable K”) in the soil will be depleted temporarily until a new equilibrium is reached. Inadequate Potash supply will slow the Nitrogen uptake by the regrowth. Also, the atmospheric deposition of Sulphur is nowadays so low that all silage cuts need an application to help create enough protein in the grass. This helps the rapid growth of the plants as well as the feed quality of the silage.

Many farms will have slurry to apply and it is right to put it on silage fields, because this is where a lot of Potash is removed over the season, and they contain Sulphur. However, care must be taken to avoid smothering the plants by over-application in this regrowth period, especially in a dry time – thinner slurries are the least problematic. The second issue, as always, is to avoid damage to the sward and soil structure by the application machinery itself.