urea vs can topdressing

By David Jones, Broad Acre Agronomist, Msc. Agriculture

When discussing types of fertiliser for Nitrogen topdressing you can always be sure of lots of debate about the merits of Urea vs Ammonium sources such as CAN. Urea remains the least expensive form of Nitrogen at around 90 Kes per kilo of N compared to around 115 kes in CAN. But what are the likely volatilisation losses, and is it possible to go some way predicting this?

Firstly, there is actually surprisingly more data out there than you might imagine given the intensity of the debate. The problem is, very little of this work has been carried out in Africa, in our conditions. Should we be worried?

The losses of ammonia measured in a GRDC trial in Australia in liquid fertiliser is summarised in the graph and shows what can happen over 5 days of unfavourable conditions – losses topped out at nearly 30% of the total N applied in the urea. Ammonium Sulphate lost only around 5% of its nitrogen.

Urea

Some very extensive work was carried out in the UK by ADAS in the 1980s which concluded that Urea was only a few percent less efficient that Ammonium Nitrate in most conditions.

More recent work suggests that while urease inhibitors and controlled release fertilisers do work effectively, the overriding views of independent researchers that I speak to is that losses from untreated urea are rarely large enough to make a coating / inhibitor worthwhile.

Looking at recent GRDC work in Australia the consensus is that a maximum of 20% of applied N is lost from Urea, and usually far less (link at the bottom).

That said, in the real world there are often practical reasons for using them as we cannot always spread large acreages in perfect conditions, and if the weather forecast is wrong having some protection on the urea to reduce losses it very desirable.

A study in Brazil showing how urea losses can be mitigated by using an inhibitor or side dressing 2cm deep

Ultimately it comes down to an individual farmer’s perception of risk, and farmers should remember that if you happen to be on alkaline soils they behave much differently, as research from Canada highlights (link at the bottom if this is relevant to your farm).

Factors affecting volatilisation losses

  • High soil temperatures
  • Low rainfall
  • Moist soil surface that breaks down prills, then dries out
  • Lots of surface residue
  • High soil pH
  • Organic Matter (correlation with Urease enzyme activity so faster conversion to ammonia)
  • Windy conditions
  • High lime content on the soil surface (chalk soils)

So in summary, don’t be afraid of using urea fertiliser but if you have large acreages to spread and conditions go against you, consider a urease inhibitor or controlled release product. Just don’t expect big improvements in most conditions.

Go into more depth…

Turnip Sawfly Larvae Threat to Canola crops

There has been a high incidence of Turnip Sawfly larvae in Canola and Brassica crops across many areas of the country recently.

The larvae are small, black worms with no distinct legs and a ribbed body. They rapidly fall from the plants when disturbed and can cause significant damage in a matter of days.

Turnip Sawfly Larvae

Turnip Sawfly Larvae

Provided crops are not flowering and bees are no present, pyrethroids such as lambda-cyhalothrin are effective and provide rapid knock down of the larvae and control of the adult flys. Normally when crops reach 5-6 leaf stage up to 50cm in diameter they outgrow the damage too, so make a judgement about whether the crop needs to be sprayed.

If there are pollinators in or around the field, a specific Bee-safe product such as flubendiamide or Lufenuron will be far safer. Normally the Sawfly only produces 3-4 generations per year so if control is good it is unusual – but not impossible – for damage to persist.

New certified seed, good planting and quick emergence through correct nutrition will reduce the period at which the crop is at risk.

Combating Ryegrass in Cereals

Just to show that troublesome Ryegrass can be well controlled in cereals, this is from one of our trial sites. Of course rotations are crucial to ensure reliable long term control, but chemistry is still effective!

On this particular population, pendimethalin has shown very little activity on Ryegrass. The addition of flufenacet provided a reasonable level of control up to 45% but a herbicide stack of four pre emergence herbicides has achieved up to 95% control.

The importance of good conditions for the pre em cannot be overstated; if it is likely to be dry around planting, leave the seeder in the shed. Ensure the seed is well covered by 30mm of settled soil to avoid herbicide damage, and if necessary to break clods and provide a level surface for the herbicide to coat.

Rye grass herbicide

Left – the four-way stack of pre emergence herbicides, right – untreated.

Center of Excellence for Crop Rotation Agventures

Farming for the future requires a change of approach. Monoculture, soil degradation and climate change and soil degradation are threats to the future of how we feed the planet.

Agventure Ltd set up the Center of Excellence for Crop Rotation to help farmers diversify cropping systems and introduce techniques which have a long-term outlook to improve soil health.

The Center of Excellence for Crop Rotation works extensively with Crop Nutrition Laboratory Services Ltd (CropNuts).

Till next time,

Happy farming,

David,

About David

David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.