Crops: The Lynch File - The dream of achieving one-pass weed control

Monday, April 5, 2010

A one-pass weed control program is feasible, but you will give up something – either yield, cost or weed control

By PAT LYNCH

Growers dream about many things. You dream about 200 bushel-an-acre corn or renting land cheaper than anyone else. You dream about weed-free fields. And you might dream that you can get these weed-free fields in one pass.

All of these dreams are possible but not probable – unless you are willing to give up some things.

You can get 200 bushel-an-acre corn, but you will have to do certain things. You will have to plant early into a good seedbed. You must have adequate fertilizer. And you will have to make sure the crop is never out of water.
You can also have weed-free fields, but you may have to use extra herbicides and scout on a regular basis.

You can have a one-pass weed control program, but you will give up something. You will give up yield, cost or weed control.

We used to have a one-pass weed control program in corn. It was four pounds per acre of atrazine along with one gallon of corn oil and it was very effective. Sometimes we burnt the corn pretty badly. You had to wait until the quackgrass was emerged. This meant spraying at the five-to-six leaf stage. Now we know that, by waiting that long, we gave up significant yield. But we did have one-pass weed control. And you had to plant corn the second year. You gave up crop rotation flexibility.

If you are planning a one-pass weed control program in your Roundup Ready corn, there are two popular choices. You can wait until the weeds are all up and by then you have lost yield. You can spray early and get the first flush, and the next flush will cost yield if not controlled. Both of these methods leave something to be desired.

An alternative method is to mix a residual herbicide in with your glyphosate. This kills the first flush and controls the next flush of susceptible weeds. This system has a higher probability of working than glyphosate alone. You should make this application as soon as you see any weeds emerge.

But you really should not blindly bet on a one-pass-weed control program working. You need to scout the field. This is where most growers fail with their one-pass-weed control program. And when you scout the field, you must check all areas of the field.

There are numerous possibilities for weeds to get through a one-pass-weed control dream. These include weeds that emerged after you sprayed and are not controlled with your residual herbicide and weeds that are resistant. The chances of you having weeds that are resistant to glyphosate in 2010 are small. You do, however, have a good probability of having weeds resistant to your residual herbicide.

A program that works well in Roundup Ready crops is a soil-applied residual product at or before planting. You can include a glyphosate in this spray mix if there are emerged weeds. Then you apply a glyphosate treatment early in the crop stage.

The timing and rate will depend on you scouting the weeds or having someone scout the field for you. This is not a one-pass weed control program, but it does have a high probability of working. BF

Consulting agronomist Pat Lynch, CCA (ON) formerly worked with the Ontario
agriculture ministry and with Cargill.