Jan 05, 2009
Herbicide Resistant Weeds Becoming Larger Problem In Brazil
Author: Michael Cordonnier/Soybean & Corn Advisor, Inc.
For the last several years, the farmers in southern Brazil have become increasingly concerned about invasive weed species that compete for light and nutrients with soybeans. The weeds are becoming resistant to Roundup Herbicide. The weed of greatest concern is a member of the conisa family, which includes more than 50 species. Common members of conisa family in the U.S. include horseweed, butterweed, and fleabane. The common name for the weed in Brazil is buva. The weed is becoming a major problem in Parana and Mato Grosso do Sul and its range is expanding all throughout southern Brazil.
The weed is a bigger problem where on-till continuous soybeans are produced. The weed is a prolific seed producer. Research shows that a single plant can produce as much as 200,000 seeds that are very light in weight and easily dispersed by the wind. Many farmers report that the weed has become a major problem only one or two years after it appeared in their fields.
The most effective way to control the weed is through a combination of chemical treatments and cultural practices. Researchers from Embrapa have found that a combination of herbicides, besides Roundup, can offer acceptable control. The herbicides must be applied before the plant reaches 10 centimeters in height. The effectiveness of the herbicides declines quickly on larger plants. Adjustments in cultural practices can also help to control the weed. Crop rotation would be beneficial, but only a small percentage of Brazil's soybeans are rotated to other crops. A winter cover crop of wheat or oats also helps to hold down germination of the seeds.
Brazilian farmers have been essentially growing a monocrop of soybeans for many years, so it's no surprise that these sorts of problems are starting to appear. Any time a farmer plants the same crop year after year and only uses one herbicide to control weeds; herbicide resistant weeds will start to develop. The same thing occurs with diseases and pests. Brazilian scientists have already noticing that the fungicides used to control soybean rust are loosing some of their effectiveness and they have had to use a combination of chemicals to control the disease.