Atrazine has been used for control of many weeds, primarily broadleaf weeds, in U.S. corn fields since 1957. Recently, the adoption of glyphosate-resistant corn hybrids have led to glyphosate eclipsing atrazine as the most commonly used herbicide in corn production. However, the evolution and spread of glyphosate-resistant weeds is a major concern. Atrazine use in Wisconsin is prohibited in 102 areas encompassing 0.49 million ha where total chlorinated residues were found in drinking water wells at concentrations > 3 μg L−1. Atrazine has been prohibited in many of those areas for > 10 yr, providing an opportunity to evaluate weed community composition differences due to herbicide regulation. In question, has the abundance of broadleaf weeds increased, coupled with an increased reliance on glyphosate, where atrazine use has been discontinued? To answer this, an online questionnaire was distributed to Wisconsin growers in June and then weeds present in 343 fields in late July through mid-September in 2012 and 2013 were counted. Data were summarized for frequency, uniformity, density, and relative abundance to compare weed community composition in fields with discontinued vs. recent atrazine use. Growers used glyphosate in 70 vs. 54% of fields with discontinued vs. recent atrazine use, respectively (P = 0.021). Moreover, broadleaf weeds were found more frequently, (73 vs. 61%; P = 0.03), they had 50% greater in-field uniformity (P = 0.002), and density was 0.4 vs. 0.19 plants m−2 (i.e., twofold greater; P < 0.0001) in discontinued vs. recent atrazine-use fields. Changes were most evident with troublesome glyphosate-resistant broadleaf weeds such as Amaranthus species and giant ragweed. In conclusion, weed community composition consisted of more broadleaf weeds in fields where atrazine has not been used in the recent decade coupled with greater glyphosate use. These results provide evidence of negative long-term implications for glyphosate resistance where growers increased reliance on glyphosate in place of atrazine.
Nomenclature: Atrazine; glyphosate; giant ragweed, Ambrosia trifida L. AMBTR; corn, Zea mays L.
Atrazine ha sido usado para el control de muchas malezas, principalmente malezas de hoja ancha, en campos de maíz en los Estados Unidos desde 1957. Recientemente, la adopción de híbridos de maíz resistentes a glyphosate ha hecho que glyphosate eclipse el uso de atrazine como el herbicida más usado en la producción de maíz. Sin embargo, la evolución y diseminación de malezas resistentes a glyphosate causa gran preocupación. El uso de atrazine en Wisconsin está prohibido en 102 áreas cubriendo 0.49 millones de hectáreas, donde residuos clorinados totales fueron encontrados en pozos de agua potable a concentraciones > 3 μ L−1. Atrazine ha estado prohibido en muchas de esas áreas por > 10 años, lo que brinda una oportunidad para evaluar las diferencias en la composición de la comunidad de malezas debido a la regulación del uso de herbicidas. ¿Ha incrementado la abundancia de malezas de hoja ancha en combinación con la mayor dependencia en glyphosate, cuando se descontinuó el uso de atrazine? Para responder esta pregunta se distribuyó un cuestionario en línea a productores de Wisconsin en Junio y después se contaron las malezas presentes en 343 campos desde el final de Julio hasta la mitad de Septiembre en 2012 y 2013. Los datos fueron resumidos en frecuencia, uniformidad, densidad, y abundancia relativa para comparar la composición de la comunidad de malezas en campos con uso de atrazine descontinuado vs. reciente. Los productores usaron glyphosate en 70 vs. 54% de los campos con uso de atrazine descontinuado vs. reciente, respectivamente (P = 0.021). Además, las malezas de hoja ancha fueron encontradas más frecuentement
