Silage swards have higher potassium (K) requirements than grazed swards, according to Teagasc’s Mark Plunkett, who spoke at the recent Teagasc Soil Fertility Conference.

K plays a key role as it is necessary for the formation of sugar, starch and protein. Sufficient plant available, K is required during peak growth to drive nitrogen (N) efficiency for yield and reduce possible N losses through leaching or de-nitrification.

K deficiency will reduce grass yield rapidly in a silage sward, he noted, compared to a grazing situation, where K is recycled during dung and urine deposition.

Nationally, the application of fertiliser K on grassland farms is below the levels needed to replace K offtake by the silage crops and for maintenance of soil K levels (K index 3).

Plunkett said: “Intensively-cut grass silage removed significant quantities of nutrients at harvest time and may reduce soil fertility.

“Now is a good time to review soil test results and develop a fertiliser plan in time for the year ahead. Autumn is also a good time to apply lime and to replenish soil K levels – especially where extra grass cuts have been taken.

“This will reduce the likelihood of issues with grass tetany and N loss occurring where urea or slurry is applied in spring.”

Plunkett also noted that, over the last decade, the levels of K fertiliser applications on grassland farms have been insufficient to meet K offtakes from grazing and silage systems annually.

These low levels of K applications have been reflected in soil test results from grassland farms where the percentage of soils at index 1 continues to increase.

To produce annual grass silage requirements, he outlined, it is essential that recommended rates of K are applied at key times during the growing season.

This can be achieved by recycling the nutrients in cattle slurry back to silage fields and by selecting suitable compound fertilisers to top the crop’s K requirements.