Making the decision whether or not to apply a pre-emergence herbicide treatment to Roundup Ready soybeans may be based more on grower preference than on any difference in weed control performance.
Jim Griffin, Louisiana State University weed scientist in Baton Rouge, La., related three years of study on the topic to those growers attending the 2001 Northeast Research Station Crop Production and Pest Management Field Day in St. Joseph, La.
Griffin, who is testing the efficacy and economics of using pre-emergence herbicide treatments in Roundup Ready soybeans, says which system a grower chooses may come down to time availability.
Of his study of pre-emergence herbicide treatments in a Roundup Ready soybean production system, he says, “We're working with pre-emergence herbicides at both half and full labeled rates, in order to make the most economical treatment possible for growers.
“So far, none of the pre-emergence treatments provided 100 percent control of the weeds present, but the treatment decreased the number of weeds that came up and delayed the growth of those weeds,” he says. “We have also discovered that whether we use the half rate or full labeled herbicide rate, we can increase a producer's Roundup application window by three to five days. Whether that's important or not is a grower's personal choice.”
Griffin is also studying whether treating Roundup Ready soybeans with a pre-emergence herbicide avoids the need for a second Roundup (glyphosate) application.
What he has found is that in two out of the three years in his study, the pre-emergence treatment did make a second glyphosate application unnecessary for weed control. However, Griffin says, the need for a second glyphosate treatment depended on a variety of factors, in addition to the pre-emergence treatment, including environment, row spacing and crop planting dates.
“Our harvested soybean yields and the weed control we achieved were the same either with a pre-emergence treatment or without. The number one benefit we found with the application of a soil-applied pre-emergence herbicide, was a three- to five-day increase in time,” he says.
Which glyphosate product was used did not affect the results of the study, according to Griffin. “At the same rate of active ingredient per acre, we cannot separate the long-term control between glyphosate products,” he says.
Despite his findings, Griffin says he is concerned with the development of weed resistance to the herbicide glyphosate (Roundup).
“We're moving towards using a single herbicide in multiple crops year after year. And, anytime you start putting that type of pressure with a particular mode of action year after year, you're going to start to see both a shift in weed species present in a field and the development of weed resistance to the product being used,” he says.