How will the wet weather affect your nitrogen use?

Thursday, 18 April 2024

What does the wet weather mean for your nitrogen applications and forage production this spring? In this article supplied by Arla we catch up with a strategic farmer and get some top tips from industry experts. 

North Yorkshire-based AHDB strategic farmer, Tom Pattison, says despite the wet weather he has been able to get out with the fertiliser spreader:

“Our crops in the ground don’t look too bad at the moment, which comes as some reassurance. We did manage to get fertiliser out on the winter wheat some three weeks ago and on the grass just a bit before that. 

“Before we knew quite how wet this year was going to be, we decided to try something a bit different with our crop rotations. We had 50 acres of wheat stubble which we put to Westerwolds and had planned to cut for silage at the end of April and then plant maize.

"We also had another 20-acre block which we had covered with grass for the winter for first cut before maize drilling. So, we have got 70 acres of grass to cut, turn over, muck and get into maize by May. I do think though the additional root structure provided by the grass has helped to soak up some of the moisture from the wet weather on our medium to heavy ground. 

“When it comes to slurry, we have two lagoons, only one of which is covered which hasn’t been helpful in recent months. Our storage is right, but the rainwater has meant it has taken us over. We have however been able to export slurry to other, more freely-draining farms.”

How much nitrogen has gone down the drain?

Independent Grassland Consultant, George Fisher, says it is tricky to know how much nitrogen may have leached but warns against over-fertilising:

“As the winter has been mild and wet, there will likely be nothing left of nitrogen applied late last season. Of the nitrogen added this season, some will have gone into plant growth, and some will still be available in the soil, but a lot will have been lost to leaching.

“However, mild conditions benefit the ability of microbes to mineralise nitrogen in the soil and release this for grass and crop growth. While making this an even more difficult picture to assess, we need to be aware we are in danger of excessive nitrogen applications.

“The problem is then, we can’t measure or assume how much has been lost and how much is left. We therefore need to proceed with caution. The risk of adding more could lead to grass with high levels of free nitrogen, making it problematic for silage, and therefore leading to reduced grazing intake. We can measure nitrates in grass, and this year it will be particularly important to do so – particularly in silage fields, where we want nitrate levels no higher than 500mg/kg DM as we cut.”

George’s advice for applications:

  • Example: If you applied 50 kg N per ha 4 weeks ago then most will be gone or used, then you can proceed with your application at the same rate
  • Example: If you applied 50 kg N per ha 2–3 weeks ago and you have had excessive rainfall, don’t replace this in total – be cautious and top up with 20–30 kg N per ha when possible. Then, measure grass nitrate levels. If below 500 mg/kg DM, you could apply a little more. If not, then wait
  • Consider a ‘little and often’ mindset – do not just replace what you think has been lost
  • Assess soil health: if it’s capped at the top, then spike aerate when you can. If there is compaction lower down, then aim to address this in autumn for the 2025 season – if you address this issue in wet conditions, you are in danger of making it worse 

AHDB Senior Knowledge Exchange Manager, Katie Evans, suggests some further actions to support forage production this season: 

  • In terms of nitrogen, a good way to obtain an accurate picture of your soil is to do an N-Min Test (£50–70) before any N applications. Implement split applications of nitrogen fertiliser to reduce loss potential and optimise uptake by crops
  • Encouraging nitrogen-fixing crops or legumes in rotation is a good way to enhance soil nitrogen levels naturally
  • Regularly walk paddocks and measure grass growth to detect signs of nutrient deficiency early. Monitor for weed infestations, particularly in poached areas or where poor residuals have occurred and treat as needed
  • Consider paddock rejuvenation and potentially reseeding if perennial rye-grass content falls below 60%
  • Rooting can be severely compromised following periods of heavy rain and waterlogging; this will impact nutrient uptake. Therefore, if conditions allow getting a product on to stimulate rooting can be beneficial. A better root system will also help to improve yields