Ecophysiological response of Sorghum halepense populations to reduced rates of nicosulfuron Resposta ecofisiológica de populações de Sorghum halepense a doses reduzidas de nicosulfuron

The control and regrowth after nicosulfuron reduced rate treatment of Johnsongrass (Sorghum halepense L. Pers.) populations, from seven Argentinean locations, were evaluated in pot experiments to assess if differential performance could limit the design and implementation of integrated weed management programs. Populations from humid regions registered a higher sensibility to reduced rates of nicosulfuron than populations from subhumid regions. This effect was visualised in the values of regression coefficient of the non-linear models (relating fresh weight to nicosulfuron rate), and in the time needed to obtain a 50% reduction of photosynthesis rate and stomatal conductance. The least leaf CO2 exchange of subhumid populations could result in a lower foliar absorption and translocation of nicosulfuron, thus producing less control and increasing their ability to sprout and produce new aerial biomass. The three populations from subhumid regions, with less sensibility to nicosulfuron rates, presented substantial difference in fresh weight, total rhizome length and number of rhizome nodes, when they were evaluated 20 week after treatment. In consequence, a substantial Johnsongrass re-infestation could occur, if rates below one-half of nicosulfuron labeled rate were used to control Johnsongrass in subhumid regions.<br>Foram conduzidos experimentos sob condições controladas, a fim de se investigar os efeitos de doses reduzidas de nicosulfuron, no controle e rebrota de populações de capim massambará (Sorghum halepense), de sete localidades da Argentina, visando programas de manejo integrado. As populações das regiões úmidas registraram maior sensibilidade a doses reduzidas de nicosulfuron, em relação àquelas das regiões subúmidas. Esse efeito foi observado nos valores do coeficiente de regressão, das curvas de dose-resposta de modelo log-logístico, que relacionam a biomassa fresca com doses de nicosulfuron; tal efeito foi observado também no tempo necessário para se obter 50% da redução da fotossíntese e da condutância estomática. O menor intercâmbio de CO2, obtido nessas populações subúmidas, pode ter contribuído para a absorção e translocação mais baixa do nicosulfuron, tendo produzido menor controle e, conseqüentemente, tendo aumentado sua capacidade de rebrotar e produzir nova biomassa aérea ao final do ciclo. As três populações das regiões subúmidas, com menor sensibilidade ao nicosulfuron, apresentaram uma diferença substancial na biomassa fresca, no comprimento total e no número de nódulos dos rizomas, quando foram avaliadas vinte semanas após o tratamento com herbicida. Os resultados obtidos sugerem que se forem usadas doses de nicosulfuron menores do que as recomendadas, pode ocorrer uma severa rebrota do capim massambará em regiões subúmidas

Effects on weed and invertebrate abundance and diversity of herbicide management in genetically modified herbicide-tolerant winter-sown oilseed rape

We evaluated the effects of the herbicide management associated with genetically modified herbicide-tolerant (GMHT) winter oilseed rape (WOSR) on weed and invertebrate abundance and diversity by testing the null hypothesis that there is no difference between the effects of herbicide management of GMHT WOSR and that of comparable conventional varieties. For total weeds there were few treatment differences between GMHT and conventional cropping, but large and opposite treatment effects were observed for dicots and monocots. In the GMHT treatment, there were fewer dicots and more monocots than in conventional crops. At harvest, dicot biomass and seed rain in the GMHT treatment were one-third of that in the conventional, while monocot biomass was threefold greater and monocot seed rain almost fivefold greater in the GMHT treatment than in the conventional. These differential effects persisted into the following two years of the rotation. Bees and butterflies that forage and select for dicot weeds were less abundant in GMHT WOSR management in July. Year totals for Collembola were greater under GMHT management. There were few other treatment effects on invertebrates, despite the marked effects of herbicide management on the weeds