According to a recent report from Brazil's agricultural research institute, Embrapa, over 8 million acres of soybeans and corn have been infected with horseweed (Conyza Canadensis) this season.
The weed, which has grown resistant to glyphosate, is particularly common in the south of the country. The big problem with horseweed is that it spreads very quickly and easily. Each plant can produce up to 200,000 seeds, which are very light and can be easily carried by the wind up to 40 miles away.
Populations of horseweed have been found that can resist up to 12 times the normal dosage of glyphosate. A weed is considered resistant when it can support up to twice the recommended dosage and when this resistance is passed on to future generations.
Agronomists recommend that farmers carry out strict control before planting time. Good soil cover, including a second crop and pasture during the inter-harvest period will prevent horseweed seeds from germinating as they need direct contact with soil.
The only other solution is to manually remove plants after they have reached a certain size, as they compete with soybean plants for light and nutrients and can reduce yields by up to 30 percent.
Apart from horseweed, other weeds that have shown resistance to glyphosate and affect Brazil's soybean crop include Ryegrass (Lollium multiflorum,
Milkweed (Euphorbia heterofila) and Sourgrass (Digitaria insularis).
Glyphosate and other chemical resistance can build up due to continuous use over a number of years, lack of crop rotation, and farmers using less than the recommended dosage.
Brazil is the largest user of herbicide in the world, spending over $7 billion in 2008, an increase of 40 percent on the previous year.
