Avoid applying inorganic nitrogen when water is the limiting factor

Tuesday, 13 June 2023

There are increased concerns around whether the current grass growth rates on farm will be enough for livestock grazing and forage production this season.

According to GrassCheck GB soil moisture deficits are ranging from 101.7– 65.9 cb, indicating that soils are extremely dry and grass growth is likely to be significantly reduced, impacting on yield potentials.

Depending on the soil moisture deficit in your area the advice would be to withhold fertiliser applications as there is no point in spreading fertiliser N until rain is in the short-range weather forecast. For those with lower soil moisture deficit you should match your application to grass growth rates.

There should be enough nitrogen in the ground to provide growth when the rain does come. Any inorganic N applied during or just before you last had rain is still there and has not been washed out or lost from the soil. Grass needs sunlight, oxygen, water, and nutrients to grow and at the moment water is the limiting factor not nutrients.

Do not be fooled by the dew - it might be enough to make the fertiliser pills or granules disappear from a dry soil, but the way the grass roots take up nitrogen in the form of nitrate dissolved in water means that transpiration needs to happen.

Transpiration is the movement of water from the soil, through the root, into the above-ground growing point, stem, and leaves – and that takes moisture in the soil, not just dew.

Retain moisture where you can

Bare ground after silage or grazing is at higher risk of burning off.

At this point it is advised to keep a little cover on the fields to minimise the amount of moisture being evaporated.

In terms of second cut silage ground, there will be lots of questions around weather it should be used for grazing or kept out of the rotation for silage.

Advice around management decisions is to act quickly, put a plan in place and do not just wait for the rain to come.

We are early enough in the season for things to recover but we do not know what is going to occur later in the season.

Start to build resilience in your soils

For heavy land that has retained moisture and not experienced a drastic reduction in growth rates, the best fertiliser to apply right now might be lime.

With limited access at the start of the season, this is the perfect opportunity to apply. This in turn will help soil chemistry and ensure you are getting the most from your fertilisers. This will start working its way into the soil until the rain comes to wash it in.

A key nutrient during the dry period and especially for those on lighter soils is potash.

It has a key role in water turgor and water regulation in the plant. Fields or swards that are low in potash will wilt a lot quicker and growth rates will drop faster.

Ensuring your soil indices are on target for potash will help you build resilience during the season and through dry periods.

Organic matter levels are also needed to help build resilience against a lot of this but especially drought.

Organic matter has a huge water holding capacity compared to sand, silt, or clay.

For those who are introducing arable land into the grazing rotations, organic matter in these fields tend to be low — a way to increase those levels is to put livestock on them or apply organic manures.

On a weight-for-weight basis, organic matter is said to hold four times as much water than just sand, silt, or clay particles. Organic matter will also help store nutrients and in turn helps build resilience in your swards.

Use it or lose it!

If the water is not there and the fertiliser is applied, there is a risk of loss of nitrogen from that fertiliser to the atmosphere, through the process of volatilisation.

This is the loss of ammonium-N from the soil as ammonia gas, and the conditions that favour volatilisation most are warm and drying soils, particularly with a breeze.

Volatilisation losses from urea can be much higher than from ammonium nitrate because urea must go through an ammonium stage before it becomes nitrate, and available for grass root uptake.

So, if you are applying urea at this time of year, do not – the risk is too high.

Protect your average farm covers

Over the last two weeks we have lost substantial grass growth that we are unlikely going to get back now because we are in the highest point of grass growth in May and June.

Unfortunately, we need to accept we might not grow as much grass as we would than a "normal" year meaning the budget and demand will be under pressure going into the rest of the season.

The key is not to let the average cover on the farm drop too low. To do this you will need to extend the rotation for the next few weeks, by supplementary feeding where needed but it is important that you try to extend the rotation now.

Further information

Limited rainfall could be problematic

Future planning for dry-weather resistance

Improving resilience to drought through grazing management

Forage For Knowledge (interactive grass growth dashboard)

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Katie Evans

Senior Knowledge Exchange Manager - National Specialist (Grass Forage & Soil)

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