Dealing with weeds that thrive in Roundup Ready crops

Monday, February 28, 2011

Using a soil-applied herbicide to control most of the annual weeds and then following with glyphosate as late as possible is one strategy that can help control weeds like perennial sow thistle in Roundup Ready corn and soys

by PAT LYNCH

Some perennial weeds are becoming more prevalent because of Roundup Ready crops. This is because of less tillage and time of glyphosate spraying. The worst of these weeds is perennial sow thistle. A few annual weeds also thrive if you do not use a soil residual herbicide.

When you work the ground, you break up the root system of perennial weeds. Once these roots are broken up, you have a higher percentage of the dormant buds starting to grow. If there is no tillage, then only some of the buds start growing. You spray and control some of this growth, but the dormant buds on the roots allow regrowth.

Spraying glyphosate on perennial weeds when they are small is like trying to kill them with a hoe. You get rid of the top growth, but the roots regrow. You have to spray them when they are translocating to the roots. This is generally near flowering.

One example is perennial sow thistle. It normally does not start to grow early. If you have it in winter wheat, it generally has very little topgrowth by the time you spray. If you spray it early in corn or soys, you get some top burn off but there are enough root reserves to allow regrowth. If you spray glyphosate early and then late, you can be sure it will come back. The early application sets it back. When you make the second application, it is not far enough advanced to get effective control.

However, you can control perennial sow thistle if you grow Roundup Ready corn and Roundup Ready soys. One strategy is to use a soil-applied herbicide to control most of the annual weeds and then follow with glyphosate as late as possible. In corn, this will probably be the six-to-seven leaf stage and with soys in late June, just before flowering. In both cases, you are trying to get as much top growth as possible to deplete the root reserves.

In conventional soybeans, Classic herbicide applied just before flowering is very effective. This is presuming you used a soil-applied herbicide. Some post-emergent herbicides will set it back enough to reduce the control with Classic. In conventional corn, there are a number of non-glyphosate products you can use.

You can get rid of most perennial weeds with a planned program. This is different from annual weeds that you will probably be spraying for years once they are in a field. If you are tenacious about perennial weeds, you can get rid of them, but it will take two to three years of spraying a perennial at the right time to eradicate it from a field.

If you are in a corn/soy/wheat rotation, you must control the weed in all three crops. For most perennial weeds, the best time to spray in winter wheat is pre-harvest. This should be followed up with a post- harvest spray before the weed goes dormant.

There are some annuals which germinate late that also get by on one or two applications of glyphosate. These include chickweed and annual nightshade. The way to deal with these weeds is to use a residual herbicide with glyphosate. Chickweed is becoming a bit of a bother. We know that soybean cysts and nematodes can increase when chickweed thrives. It has also been linked to an increase in cutworm numbers. If you have a heavy chickweed pressure, it is almost impossible to seed alfalfa in the summer. BF

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