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Previous greenhouse studies have demonstrated that photosynthesis in some cultivars of first-(GR1) and second-generation (GR2) glyphosate-resistant soybean was reduced by glyphosate. The reduction in photosynthesis that resulted from glyphosate might affect nutrient uptake and lead to lower plant biomass production and ultimately reduced grain yield. Therefore, a field study was conducted to determine if glyphosate-induced damage to soybean (Glycine max L. Merr. cv. Asgrow AG3539) plants observed under controlled greenhouse conditions might occur in the field environment. The present study evaluated photosynthetic rate, nutrient accumulation, nodulation, and biomass production of GR2 soybean receiving different rates of glyphosate (0, 800, 1200, 2400 g a.e. ha -1 ) applied at V2, V4, and V6 growth stages. In general, plant damage observed in the field study was similar to that in previous greenhouse studies. Increasing glyphosate rates and applications at later growth stages decreased nutrient accumulation, nodulation, leaf area, and shoot biomass production. Thus, to reduce potential undesirable effects of glyphosate on plant growth, application of the lowest glyphosate rate for weed-control efficacy at early growth stages (V2 to V4) is suggested as an advantageous practice within current weed control in GR soybean for optimal crop productivity. : Aminomethylphosphonic acid, a metabolite of glyphosate, causes injury in glyphosate-treated, glyphosate-resistant soybean.

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Aims:  Glyphosate‐resistant (GR) soybean production increases each year because of the efficacy of glyphosate for weed management. A new or ‘second’ generation of GR soybean (GR2) is now commercially available for farmers that is being promoted as higher yielding relative to the previous, ‘first generation’ (GR1) cultivars. Recent reports show that glyphosate affects the biology and ecology of rhizosphere micro‐organisms in GR soybean that affect yield. The objective of this research was to evaluate the microbiological interactions in the rhizospheres of GR2 and GR1 soybean and the performance of the cultivars with different rates of glyphosate applied at different growth stages. Methods and Results:  A greenhouse study was conducted using GR1 and GR2 soybean cultivars grown in a silt loam soil. Glyphosate was applied at V2, V4 and V6 growth stages at three rates. Plants harvested at R1 growth stage had high root colonization by Fusarium spp.; reduced rhizosphere fluorescent pseudomonads, Mn‐reducing bacteria, and indoleacetic acid–producing rhizobacteria; and reduced shoot and root biomass. Conclusions:  Glyphosate applied to GR soybean, regardless of cultivar, negatively impacts the complex interactions of microbial groups, biochemical activity and root growth that can have subsequent detrimental effects on plant growth and productivity. Significance and Impact of the Study:  The information presented here will be crucial in developing strategies to overcome the potential detrimental effects of glyphosate in GR cropping systems.

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RESUMOEste trabalho teve como objetivo estabelecer curvas de acúmulo de macronutrientes na cultura do girassol. Para isso, foi instalado um experimento em campo sobre Latossolo Vermelho distroférrico de textura muito argilosa, localizado na fazenda experimental da Embrapa Soja, em Londrina/PR. As parcelas foram constituídas de 14 linhas com 25 m de comprimento cada e espaçamento entrelinhas de 0,70 m, resultando em área total de 245 m 2 . Cada parcela foi repetida quatro vezes. A adubação de semeadura foi de 300 kg ha -1 da fórmula 5-20-20, aplicada a lanço, antes do plantio. A adubação de cobertura foi parcelada em duas aplicações: 25 e 1 kg ha -1 de N e B, respectivamente, sendo a primeira aos 21 e a segunda aos 35 dias após a semeadura. O híbrido utilizado foi o BRS-191, e a densidade final de plantas foi de 40.000 plantas ha -1 . Amostras de plantas foram coletadas em intervalos de 14 dias após a emergência e separadas em pecíolos, folhas, caules e, quando existentes, em capítulo e em aquênios. Após secagem, cada parte da planta foi pesada e moída para, em seguida, determinarem-se os teores de N, P, K, Ca, Mg e S. A partir desses nutrientes e da matéria seca de cada parte da planta, foram obtidas as curvas de acúmulo. Verificou-se que, para obtenção de produtividades superiores a 3.000 kg ha -1 , o híbrido BRS 191 extrai aproximadamente 150, 24, 286, 116, 42 e 24 kg ha -1 de N, P, K, Ca, Mg e S, respectivamente, resultando na seguinte ordem de extração: K > N > Ca > Mg > P = S. Em relação à exportação, a ordem dos nutrientes foi a seguinte: N > P = K > Mg = S > Ca. Portanto, atenção especial deve ser dada à manutenção da adequada disponibilidade de N, K e Ca, devido à alta demanda da cultura por esses nutrientes. Termos de indexação:Helianthus annuus, crescimento, acúmulo.

Global production of glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.]

Plants of Sumatran fleabane [Conyza sumatrensis (Retz.) E. Walker] were identified in a field with an unusual rapid leaf-injury herbicide symptoms after application of 2,4-D in mixture with glyphosate. The objectives of this study were to confirm the occurrence of resistance to 2,4-D herbicide and to characterize the occurrence of rapid necrosis as the mechanism associated with the herbicide resistance in C. sumatrensis. The studies performed were an initial screening, effect of 2,4-D alone and associated with glyphosate, cross- and multiple-resistance evaluation, effect of commercial formulation and analytical product, and rate of H2O2 evolution. The Marpr9-rn accession was identified with rapid necrosis symptoms and survival to 804 g ae ha−1 of 2,4-D. The resistance factor to the herbicide 2,4-D was 18.6 at 49 d after spraying. The analytical product 2,4-D and the commercial formulation resulted in similar symptoms of rapid necrosis. This symptom did not occur for the six other auxinic herbicides (dicamba, florpyrauxifen-benzyl, fluroxypyr, halauxifen-methyl, picloram, and triclopyr), indicating absence of cross-resistance. Multiple resistance to the herbicides paraquat, saflufenacil, and ammonium glufosinate was not identified in the Marpr9-rn population. However, survival following treatment with the herbicides glyphosate and chlorimuron-ethyl occurred. The evolution of H2O2 began at 15 min after application and was less pronounced in low light. These results indicate the first case of resistance to 2,4-D and occurrence of rapid necrosis in C. sumatrensis.