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Optimise nitrogen application rates

Grassland nutrition in the lead up to first cut was in the spotlight during a recent Yara organised webinar. Hannah Park reports.

 

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Optimise nitrogen application rates

The single biggest factor contributing to the cost of producing silage is the yield of the crop.

 

This was the message from Yara agronomist Philip Cosgrave who, speaking in a recent Yara webinar, said the primary challenge for farmers producing silage was to simultaneously maximise silage yield while maintaining feed quality and optimum D-value.

 

With nitrogen taken up by grass species quicker than it is incorporated into proteins and other structures, optimum efficiency requires sufficient time between application and cutting or grazing.

 

A useful guide highlighted on the webinar in terms of nitrogen (N) rate and harvest date was that a grass crop typically uses up 2.5kg N or two units per day to achieve optimum efficiency, and with grass response to nitrogen highest in April and May, the economic return on nitrogen application is greatest on first cuts.

 

Mr Cosgrave advised, according to Yara trial work, an optimum nitrogen application rate of between 120-130kg/ha for first cut for newer, high performing swards and for older, more permanent swards a rate of about 100kg/ha.


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Philip Cosgrave, Yara agronomist
Philip Cosgrave, Yara agronomist

These swards, he explained, were still capable of producing high quality silage if cut at the correct growth stage, but because of their lower yield potential, they did not require these higher nitrogen rates necessary where high yield potential exists.

 

Mr Cosgrave also discussed the role nitrogen has on developing yield and protein levels.

 

He said: “As the growth stage of the plant matures, the protein levels in silage starts to drop. So to produce that high quality silage, cutting date needs to be early.

 

“Although it does not have the same impact as cutting date, optimising nitrogen application is worth bearing in mind to produce high quality silage.

 

“It may be worth considering applying the optimum rate on fewer hectares rather than lower the rate on more, as this will dilute the amount of protein in the crop by not optimising N application.”

 

Sulphur application was also discussed in the webinar, and Mr Cosgrave explained its key role as a ‘complementor’ to nitrogen to drive protein percentage.

 

Any sulphur deficiency will decrease nitrogen use efficiency and so reduce yield, he explained.

“It is widely known now that atmospheric supply of sulphur is inadequate and for first cut silage the national recommendations for application rate is 40kg/ha SO3,” he said.

Low soil phosphorus and potassium will reduce herbage yield and N use efficiency. Cattle slurry can supply a large portion of this requirement with the balance coming from purchased fertiliser.

 

“Trials and farmer experience have proven the benefits of using NPKs to insure against the unpredictable nature of slurries and if we could measure grass yields routinely we would see this.

 

“As a stand-alone feed, silage is expensive to produce, but looking at as part of an integrated grassland management system, grass silage is a competitive feed and is over 2.5 times cheaper than concentrate feeds.”

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