Glyphosate efficacy on sourgrass biotypes with suspected resistance collected in GR-crop fields/Eficacia do glyphosate em biotipos de capim-amargoso com indicio de resistencia coletadas em cultivos RR

In Brazil, infestations of crop areas with glyphosate-resistant (GR) sourgrass (Digitaria insularis (L.) Fedde) biotypes has risen significantly, increasing crop production costs. Glyphosate efficacy on three biotypes (GO, BA and MT) of sourgrass with suspected resistance was evaluated. A susceptible biotype (MG) was used as the control. The results confirmed that the MG and GO biotypes were susceptible to glyphosate (control > 90%). The MG biotype exhibited growth reduction and mortality by 50% ([GR.sub.50] and [LD.sub.50], respectively) with mean glyphosate doses of 243.7 and 431.6 g ae [ha.sup.-1]. The resistance index of the biotypes with suspected resistance ranged from 2.8 to 6.1 in relation to [GR.sub.50] and between 1.4 to 26.7 in relation to [LD.sub.50]. The glyphosate susceptibility ranking of the sourgrass biotypes was MG < GO < MT < BA. The MT and BA biotypes demonstrated high glyphosate resistance levels, and the GO biotype had a high potential to develop resistance. Farmers should avoid the application of glyphosate overdoses to minimize the selection pressure on weeds. Keywords: chemical control, Digitaria insularis, dose response, resistance factor. No Brasil, as infestacoes de areas agricolas com biotipos de capim-amargoso resistentes ao glyphosate, resultam em aumento significativo nos custos de producao das culturas. Avaliou-se a eficacia do glyphosate em tres biotipos de capim-amargoso com indicio de resistencia (GO, BA e MT) a este herbicida. Um biotipo suscetivel (MG) foi utilizado como controle. Constatou-se que os biotipos MG e GO foram suscetiveis ao herbicida (controle > 90%). O biotipo MG apresentou reducao do crescimento e mortalidade em 50% ([GR.sub.50] e [C.sub.50], respectivamente) nas doses de glyphosate de 243,7 e 431,6 g [ha.sup.-1]. Os fatores de resistencia dos biotipos com indicio de resistencia oscilaram entre 2,8 a 6,1 em relacao ao [GR.sub.50]; e entre 1,4 e [greater than or equal to] 26,7 em relacao ao [C.sub.50]. A ordem de susceptibilidade dos biotipos de capim-amargoso foi MG < GO < MT < BA. Estes ultimos apresentaram elevado nivel de resistencia ao glyphosate, e o biotipo GO tem potencial para desenvolve-la. Portanto, os agricultores devem evitar a aplicacao de superdose de glyphosate a fim de reduzir a pressao de selecao das plantas daninhas. Palavras-chave: controle quimico, Digitaria insularis, dose-resposta, fator de resistencia.