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Barnyardgrass becoming monster in soybean fields

I wrote last week about the increase in barnyardgrass readily observed in soybean fields in Arkansas. I have noted a steady increase in the amount of barnyardgrass escapes over the past couple of years. Several factors can be contributing to the increase.

First, more farmers appear to be delaying the first glyphosate application. Some of the reason this year may be environmental. However, I have to wonder how much of the delay is due to the glyphosate price increase.

Glyphosate can be such a forgiving herbicide that it has often made weed control too easy. We used to emphasize making the first glyphosate application at 10 to 14 days after emergence (DAE) and repeating it 10 to 14 days later. I wonder now how many farmers are delaying the first application to “let all the weeds come up” and hope to get by with one application to save money.

Sometimes this can be successful. However when the weed mix includes heavy barnyardgrass pressure, it is not a good program.

The second big reason I see for failure is drought stress. While you may say, “Doc, I can’t help it if it doesn’t rain,” drought stress and application timing are often closely related.

One reason I have often favored timing based upon days after emergence instead of weed size is it gives a better measure of environmental conditions — especially soil moisture. If you have enough soil moisture to get good weed germination at planting, you usually will not have drought-stressed weeds at 10 to 14 DAE.

If timely rain does not fall, however, when you delay the application you have bigger weeds and drought-stressed weeds. A solid infestation of barnyardgrass will suck the moisture out of the ground at a rapid rate.

I see a lot of applications made three to four weeks after emergence to drought-stressed barnyardgrass. You will not kill it.

A third factor influencing control is herbicide rate. Some may not realize that while glyphosate is considered to be an excellent grass herbicide, it is weaker on barnyardgrass than most other grasses. You cannot mess around with rate and other factors that influence herbicide activity and expect to control barnyardgrass. The rate needs to be at least 22 ounces per acre of the concentrated formulations or equivalent.

Often, however, you cannot overcome improper timing and drought stress with rate. You need early timing, good conditions plus a solid recommended rate to be successful. Therefore, if you are thinking, “I will just wait and let everything come up, jack my rate up and get everything in one shot,” you will likely fail.

A fourth factor that can result in failure to control barnyardgrass is tank mix antagonism. With glyphosate being somewhat weak on weeds such as the morningglories combined with the increase in pigweed resistance, more growers are adding herbicides such as Flexstar to glyphosate. While this may help on the broadleaf weeds, it can result in a reduction in barnyardgrass activity.

Tank mix antagonism often is not clearly understood — often by many of us so-called experts. It does not always happen and whether of not it is evident often depends on the weeds present. If it is all weed species both herbicides have activity on, you may see no antagonism or perhaps an increase in activity.

It has been my experience with glyphosate that antagonism will show up on barnyardgrass first.

Any one of the factors mentioned above can result in failure. However, when you combine two or more of these factors, failure is nearly certain. We are creating a monster with barnyardgrass that has me very concerned. When we build the seed bank to the extent we are in some soybean fields, it makes the weed much more likely to overpower your rice weed control program.

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