A tonne (FW) of 30 per cent dry matter (DM) silage contains 2.1kg of phosphate and 7.2kg of potash. This can be used to calculate off-takes and what nutrients need to be returned to land that has been cut for silage. It also shows the importance of potash in silage systems, so grass yields can be maintained.
Recommendations from section 3 of the Nutrient Management Guide (RB209) in the table below shows the variation in off-takes and how soil indices can be used to assess phosphate and potash off-takes for silage. For a field that has a P Index of 1 and K Index of 2-, the recommendations for what should be applied for second cut would be 25kg of phosphate and 90kg of potash.
Recommendations for phosphate and potash for silage from the Nutrient Management Guide (RB209) (Section 3, Grass & Forage crops).
If the field has a K index of 2+ or below, it is recommended that for a one or two cut system an extra 60kg of potash is applied following the last cut or by the autumn. This can be split into two 30kg doses in grazing after the cuts.
Nitrogen recommendations are more complicated. Generally, 1kg of N will grow between 10-15kg of grass DM. 50kg N will grow an extra 600kg of DM at a 1kg:12kg response. Swards containing high levels of ryegrass will be more responsive to nitrogen than swards containing weed grasses, such as Yorkshire Fog or Creeping Bent. The response will also depend on the timing, season, soil fertility and moisture.