21 March 1997

Over-applying N can have bad effect on silage

APPLYING too much nitrogen can have an adverse effect on the quality of silage and the performance of stock.

Trials at the Agricultural Research Institute of Northern Ireland, Hillsborough, have shown that excess N decreases herbage dry matter and soluble carbohydrate concentration, making it harder to ensile. Concentrations of nitrate N and crude protein are also increased, the rate of pH decline slows, and ammonia nitrogen accumulation rises.

Silages of similar good preservations, as measured by conventional laboratory analysis, but with varying crude protein contents, were fed to dairy cows in mid-lactation and supplemented by 5kg of concentrates a cow a day. Increased crude protein decreased silage intake and animal performance. The work by Tim Keady looked at the effect of lifting nitrogen over the range 72 to 168kg/ha.

This indicated that each extra kg/ha of nitrogen increased effluent production by 800 litres/1000t of grass. After the trial, Dr Keady recommends that where a single N application is used a maximum of 120kg/ha (100 units/acre) is put on by Apr 1 for first-cut silage.

There are two options for farmers who plan to delay N application until early April to take a first cut in late May. Where harvesting date is fixed applications should be cut by about 2.5kg/ha (2 units/ acre) for each days delay after Apr 1. Alternatively, the full dressing can be used and harvesting delayed. Uncompleted trials indicate that each one-week delay in harvesting after mid-May cuts D value by 4.1 units and DM intake of dairy cows by 1kg a cow a day. On farms where N is applied for early grazing, estimates suggest 35-45% will remain after grazing and be available for silage production.

Phosphorus and potash should be based on soil analysis, and lime, if required, should be applied after the last harvest of the season. &#42

Tim Keady… Too much nitrogen can reduce silage quality.