Glyphosate efficacy on sourgrass biotypes with suspected resistance collected in GR-crop fields

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 50 and LD 50 , respectively) with mean glyphosate doses of 243.7 and 431.6 g ae ha -1 . The resistance index of the biotypes with suspected resistance ranged from 2.8 to 6.1 in relation to GR 50 and between 1.4 to 26.7 in relation to LD 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.