Maize Nitrogen Topdressing & Fall Armyworm Update

By David Jones, Broad Acre Agronomist

We had a lot of discussion last week within our agronomy team about the problem of getting Nitrogen (mainly Urea) into the plant in dry weather. So, I start this week with a really useful graph that I often use as a reminder of “why not to give up on topdressing!”

Maize Topdressing

Maize typically takes up around 23 Kg of Nitrogen for every ton of grain yield. Therefore a 9 ton/ha (40 bag/acre) crop removes:

9 tons x 23 Kg N = 207 kg of nitrogen

The soil typically contains 80-150 kg of Nitrogen at the start of the season, so a further 60-120 kg needs to be applied as fertiliser to make up the shortfall.

Because plants only take up around 60-70% of nitrogen available to them, we need to apply proportionally more:

100 kg / 60% efficiency = 166 kg of N, in this example.

We can see from the graph that for the first 40 days of the crops life, very little N is needed.  In fact it is only when the crop reaches V9 (9 leaves) that N demand increases, and starts to exhaust the 80-150kg that are typically in the soil and supplied in the starter fertiliser at planting.

Crop demand increases sharply towards R1 stage, when the Silks emerge. The Nitrogen requirement still increases after this point, but as the graph shows this is much slower.

In an ideal situation, Nitrogen such as Urea is applied at 5-6 leaves, so that it has time to convert into Ammonium, then Nitrate and into the plant. Yield will be partly reduced by any delay, but as the graph shows, even if rains are delayed to V10 or beyond it is still worth applying nitrogen.

Simple Steps:

  • Calculate N in the soil and calculate crop requirement based on yield potential
  • Apply a small amount in the seedbed at planting
  • Check the plant population and adjust the nitrogen rate accordingly before topdressing
  • Apply the balance at the 5-6 leaf stage, as long as soils are moist
  • Do NOT apply nitrogen too early. In a wet season the nitrogen can leach before the crop has time to use it. This will result in reduced yield.

In the past I have experimented with foliar feeds, to increase nitrogen supply late in the crops life. Remarkably, despite the scorch seen in the picture below from applying Foliar Urea, this plot recovered and slightly out yielded the plot beside it which had none!!!

Scorch from applying foliar Urea

Fall Armyworm; what we’ve learnt so far

We are entering a critical time for Fall Armyworm control. Tall crops that cannot be sprayed, difficulty in scouting the crop thoroughly, and larvae that are hidden well within the plant. It is crucial to keep scouting and to remember the following advice:

  1. Use effective products rotating chemical groups
  2. Spray even if low numbers are seen – zero tolerance
  3. Larvae will exit the leaf whorl for a short period when the tassels emerge, leaving themselves exposed before moving to feed on the developing ear. This is a key time to spray.
  4. Avoiding using spray adjuvants – you want the droplets to roll down into the leaf whorl and not stick on the upper leaf surfaces.
  5. Aim for a coarse quality to increase droplet size and energy
  6. Spray in the morning or evening when larvae are likely to be more exposed on the plant
  7. For small areas of crop consider using a concentrated solution in a squeezy-bottle shown above. This can be directed into the whorl, only requires a few drops of chemical, and is much safer for the operator than spraying vapour from a nozzle at head height.

Click the video below for a live demo I did recently on controlling Army worms:

Till next time,

Take care!

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.

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