At the present time, any change that can help reduce input costs must be welcome. Changing from 'NPK' to 'NK' fertiliser for first cut silage has the potential of reducing fertiliser costs without sacrificing grass yield. In the past the most common fertilisers were either straight N or NPK compounds. Inputs of phosphate on intensively managed farms have considerably exceeded outputs for many years.
Farm animals retain less than one-third of the phosphate in their feed.
Thus most of the phosphate brought on to the farm in feedstuffs is excreted, making slurry a rich source of this nutrient. Fifty years ago, soils were typically very low in available phosphorus. Fertiliser recommendations in the past were based on this low soil P status and were set not only to supply plant needs but also to build up soil reserves. As a result of repeated fertiliser and slurry dressings over the years, most soils now have adequate phosphate for grass production. Slurry can maintain these levels without the need for additional fertiliser phosphate.