Maintenance applications of P throughout the season can boost grass yields by up to 12%, even at target soil index 2, research has revealed.
Trials, conducted by fertiliser manufacturer GrowHow at Reaseheath College last year, examined the use of phosphorus in a three-cut system taken in May, July and September.
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All grassland received 240kg/ha of nitrogen; split 90:90:60 for each cut.
But phosphorus application rates varied between 0kg/ha, 80kg/ha and 120kg/ha. P2O5 was applied at the same time as the nitrogen.
Results showed at index 2 there was a yield benefit from applying a maintenance dressing of 80kg/ha of P2O5 , but there was no additional benefit of increasing the amount of phosphorus applied to 120kg/ha.
Overall, the accumulative yield increase seen in May, July and September, resulted in an extra 1.24t DM/ha being available to feed livestock – an increase of 12%.
GowHow’s farm adviser Hefin Llwyd says it could offer real value in terms of the extra feed provided.
“Some people, seeing a soil index of 2, may be tempted to skip phosphorus applications and think it won’t cause a big loss in yield.
“But the results show that failure to apply phosphorus means farmers could see a reduction in silage yield.
“Based on a triple super phosphate cost of £250/t, 80kg P2O5 costs about £45.
“The actual feed value of the additional dry matter depends on the silage quality and how it is stored and used, but the figures show most farmers can reduce their feed bill by getting phosphorus applications spot on, even at soil index 2.”
For example, for silage containing 11.5MJ ME/kg DM, an extra 1.24t of silage dry matter delivers 14,260MJ ME, he says.
“You won’t use all of this, but if you make and store the silage well, you can expect 85% to be available, which is about 12,000MJ ME.
“The amount of energy produced could replace about £250 of bought-in feed, and is equivalent to over £500 of milk. So this isn’t a case of marginal gains – the difference between £45 spent on phosphorus in fertiliser and £250 on feed is a healthy 5.5:1 return on investment.”