Think Agronomy: PGRs and Maize weed control

This week my attention has been drawn to two interesting agrochemicals, and aerial imaging for crop scouting

I first started using aerial imaging a couple of years ago, when I decided that I needed to gather as much data as possible about some maize trials I was running. Sometimes, when you want to understand how a technique or a new product really works, yield data alone is not enough.

I was looking at fungicides in maize, and I wanted to see how disease levels – in terms of leaf area affected – and yield response were related.

Weed control is another interesting issue. It is all very well using a drone image to look at where weeds are growing, but you still need to be able to control them. It is very useful to look at where the weeds occur sometimes, so what you can start to ask questions about how they spread (from a contractors combine that came into a shamba for example). I once advised a client to STOP growing certain crops in one field, because the drone image showed us that the wind was spreading over the fence from his neighbours fields and was never going to be controlled properly in his existing rotation – meaning it would spread across the rest of farm on his machinery.

I will cover more ideas in future Think Agronomy updates on how to make drones work, and achieve more than just an exercise in flying model planes!

Two drone images of a maize crop; NDVI on the left (which reveals very little difference in this crop), and Digital Crop Height Map on the right which reveals far more differences in the crop.

From our trials in Nakuru. First photo is without PGRs (you can tell from the gaps between the rows). Second photo is with plant growth regulator – you can see how the crop has tillered much more and filled the rows.

From our trials in Nakuru. First photo is without PGRs (you can tell from the gaps between the rows). Second photo is with plant growth regulator – you can see how the crop has tillered much more and filled the rows.

Plant Growth Regulators

Plant Growth Regulators or “PGRs” are common place in other parts of the world, but are much less common for us in Kenya. The concept appears a contradiction at first, “why would I want to regulate the growth of my crops?!”

There are several reasons:

  1. Improve standing ability and reduce yield loss through lodged crops. This also means easier harvesting, and the ability to use high rates of nitrogen where appropriate without the crop falling over.
  2. Reduce apical dominance on the main tillers, which makes plant growth – and fungicide timings – more even. It can also reduce green tillers at harvest in cereals.
  3. Promote tillering, particularly in low plant populations, by stopping the plant sending its energy to the strongest tillers at the expense of others.
  4. Shorter straw means less residue after harvest. This can make planting the following crop far easier especially for no-till situations.
  5. Can plant at higher seed rates to increase crop competition against weeds.

There is a lot of work to do however; in lower rainfall areas overly thick crops are the enemy. We need to let the plant abandon tillers if there is not enough moisture available in the soil. And regulating too much can result in reduced yield.

As we look more internationally in the types of varieties we are growing in Kenya, we will need to get better at managing their growth in our local conditions.

Maize grassweed control

As I walk thought some impressive maize crops this year, one unfortunate problem stands out for me as well as Fall Armyworm; poor grass weed control.

The problem arises not just in continuous maize but also in rotations where grassweeds are well controlled in the other broadleaved crops. Volunteer cereals harbour root diseases, act as a source of inoculum of Septoria and Net Blotch to other nearby crops, and mean that the full benefit of the break crop is not achieved. This means that the 10% yield benefit from growing a barley or wheat after a break crop is lost.

The herbicide options available to us for grassweeds are (mainly) residuals, which means in dry weather especially there is a real difficulty in controlling grass weeds. This is clearly a priority which we need to solve by working with manufacturers to get appropriate active ingredients approved in the next few years. It could also take the pressure of “Fop and Dim” herbicides which are used extensively in other cropsand are at risk of resistance developing.

In the meantime, good fallow hygiene and using another break after maize to properly clean the grassweeds is a sensible option.

This coming week I am looking at some new sprayer ideas for improving the control of Fall Armyworm, and visiting the UK Cereals Event, one of the world’s premier arable farming shows featuring everything from crop variety plots, machinery, soil inspection pits and discussions on new farming techniques.

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