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Can we grow more grass with less nitrogen?





The Farm to Fork strategy which is part of the European Green Deal has set a target of reducing our chemical nitrogen by 20pc by 2030.

The challenge is to now to figure out how we are going to grow as much grass, or more, with less nitrogen.

Nitrogen use efficiency (NUE) is a new phrase that we are going to become very familiar with over the next few years. So what is it? NUE calculates all the nitrogen that goes out the gate of your farm and divides it by the nitrogen that is coming in the gate.

On a typical Irish dairy farm, the majority of nitrogen (N) that comes onto our farms is from five sources;

  • Chemical fertiliser
  • Purchased concentrate
  • Purchased livestock
  • Purchased forage
  • Imported slurry

Chemical fertiliser is an obvious source, but there are less obvious sources on our farms.

When a cow eats grass, the nitrogen in that grass can end up being contained in the milk, being converted into animal tissue, or passing through the cow and being excreted as slurry or urine; or that grass could be cut for silage and the N is locked up in a bale or pit of silage.

The primary vehicles for taking N off our farms are:

  • Milk
  • Livestock sales
  • Forage sold
  • Slurry exported

To calculate our nitrogen efficiency, simply divide the N going out the gate by the N coming in the gate.

The National Farm Survey shows that the average Irish dairy farm has a nitrogen efficiency of 25pc. This means that for every 4kg of nitrogen that comes onto our farms, 1kg is recovered. So where does the other 3kg go?

A high percentage is lost back into the atmosphere through volatilisation of ammonia from fertiliser/slurry or nitrogen gas directly from the soil.

A small percentage (6pc) is lost through leaching to groundwater.

25pc recovery is our current figure for NUE, and the industry target is 35pc.

So what can farmers do to reach this target? Two things: try to recover more of the N that we have already on our farms, or bring less N onto our farms.

There are a good number of options to recover more nitrogen. Spreading slurry with low-emission equipment (such the trailing shoe and dribble bar) is one easy way, and switching from CAN-based fertiliser to protected urea is another easy option.

Or we can simply reduce the amount of chemical N we purchase.

But can we grow the same amount of grass to feed our cows if we do this? Research in Teagasc Moorepark would suggest we can. The clover study trial underway is examining the differences between three treatments:

  • Grass-only swards receiving 250kg chemical N/ha.
  • Grass and clover sward receiving 150kg chemical N/ha.
  • Grass and clover sward receiving 100kg chemical N/ha.

Up to the end of June, the three treatments had grown 6.4, 6.5 and 6.7 ton of dry matter/ha respectively. So the treatment receiving the lowest level of N is growing the most. This means the clover is doing its job by harvesting N from the atmosphere and supplying it to the grass plant.

Another way of bringing less onto our farms is by feeding less meal or by feeding a lower-protein ration, which has a lower N content.

So there are numerous ways we can increase our Nitrogen Use Efficiency, and the good news is that most of them will also save us money.

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It is now time to be proactive and recover more N from our systems. The next time you ring the contractor to spread slurry, ask him to spread it with the trailing shoe. If you are thinking of ordering CAN from your fertiliser merchant, ask for protected urea instead. If filling the meal bin up in next few weeks, ask for a 14pc ration instead of 16pc. If you cut your second-cut silage last weekend, why not get a bag or two of clover seed and spread it while the ground is bare?

There are plenty of options available. Which ones will you implement?

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