maize topdressing

By David Jones, Broadacre Agronomist, Msc. Agriculture

Understanding the key growth stages of Maize is crucial to getting nutrients applied at the correct timing. As many crop reach top dressing timing, we look at the main requirements of the crop.

maize nitrogen topdressing

In some countries where equipment and application techniques are not developed or available, all of the nitrogen is applied at around the 3-4 leaf stage with a broadcaster, leaving the nitrogen vulnerable to leaching when the crop does not require it.

Nitrogen – and other nutrients – are not taken up at a constant rate through the crop life. The table below summarises the important growth stages and the amount of nitrogen in particular that the crop takes up at each stage.

maize nitrogen topdressing

You can see now that applying at 3-4 leaf stage leaves probably a 4-6 week gap before the crop is really taking up the fertiliser. Even slow release fertilisers do not overcome this in trials that I have been involved with, especially in seasons with well over 600mm of rainfall like we are experiencing now.

  What is happening in the crop? Kg of N taken up
V1 Crop emerging
V3 stage 3 leaf stage, crop established 12 kg/ha N
V6 Growing point and tassle at ground level 25 kg/ha N
V8-V10 Rapid top growth period 80 kg/ha N
V12 Number of rows of kernals determined. Rapid nutrient uptake 100 kg/ha N
R1 140 kg/ha N
R3 Little further N uptake 250 kg/ha N
R6 Physiologically mature

 

Of course, nitrogen takes time to get into the crop but in most cases, the optimum is around the 8-10 leaf stage for top dressing. There were certainly a number of crops in the North Rift that I looked at last week that were getting short of nitrogen at this stage where topdressing was delayed.

Leaf Tests are a Crucial Guide

Do not forget, leaching has been significant this year with the high rainfall. This is why leaf tests are an important tool to take a snap shot of what is happening inside the plant. They will also highlight other nutrients and I use them to guide:

  • Have I applied enough phosphate in the seedbed – do I need to change next year?
  • Is Boron at levels that might limit kernel numbers?
  • How much and how soon to apply nitrogen top dressing?
  • Comparing any trials in the field – has seedbed Boron or Zinc actually got into the plant and is it needed?

When we have decided how much to apply, consider splitting the application. This reduces the risk if the weather turns either very dry or so wet that run of and leaching occurs, allows you to get around the crops faster to ensure the later fields are not short of nitrogen, and as a general rule, the little-and-often approach means N is more efficiently taken up by the plant.

Now if we plot N uptake onto the growth stage chart, and look at the greatest risk of leaching at the early stages, we can see the logic of delaying nitrogen topdressing.

How To Apply Topdressing

Broadcaster / spinner

Pros

+ Fast and accurate

+ Minimises wheelings and therefore compaction

Cons

– Needs to be tray tested to get the spread pattern right

– Requires good quality fertiliser to get the spread

– Prills or granules can land in the leaf whorl and burn the plant

– Three point linkage on smaller tractors may not be sufficient to get the spreader to the correct height above the crop.

Side dresser with drop pipe

Pros

+ Accurate – no issue with spread pattern

+ No fertiliser prills/granules falling into leaf whorls and causing damage

Cons

-More wheelings and compaction

-Slow work rate

Side dresser injected

Pros

+ Far less risk of volatilisation with urea products

+ Accurate, and you only have to do in between every other row

Cons

-Slow work rate

-Soil disturbance, especially in notill situations. Also disrupts the herbicide layer.

Foliar urea

Pros

+ Fast and no leaching losses

+ Can combine with a fungicide in many cases

+ Often see yield responses above even a high topdressing dose. Very useful if topdressing has leached

Cons

– Prone to scorch and leaf damage

-Requires high clearance sprayer

-Requires some basic equipment to mix the product

 

Whatever you do, always try to apply a double and a nil (half rate) rate strip to understand the response that the crop is giving you.

Till next time,

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.