Recurrent selection for glyphosate tolerance in birdsfoot trefoil

Glyphosate [N-(phosphonomethyl)glycine] tolerant birdsfoot trefoil (Lotus corniculatus L.) would allow selective herbicide control of Canada thistle [Cirsium arvense (L.) Scop.] and other dicot weeds in seed production fields. The objectives of this research were to determine if recurrent selection can increase the level of glyphosate tolerance in birdsfoot trefoil and if increased glyphosate tolerance is associated with increased S-enolpyruvylshikimate 3-phosphate (EPSP) synthase activity. Two cycles of selection for glyphosate tolerance were made in three birdsfoot trefoil germplasms, ‘Leo’, ‘Norcen’, and ‘MU-81’ by selecting seedlings following treatment with 0.56 kg ae/ha (kg acid equivalents per hectare) of glyphosate. To evaluate tolerance, seedings with eight leaves of the selected and parental populations were either untreated or treated with 0.56 kg ae/ha of glyphosate plus surfactant in a greenhouse. Shoot fresh weights were measured 14 days after treatment (DAT) and regrowth was measured 35 DAT. Treated shoot weights of the three C₂ populations were from 44 to 85% greater than their C₀ populations, indicating increased glyphosate tolerance. The evaluation of regrowth weights also showed 44 to 127% increases in the C₂ populations. Tolerant plants from C₂populations had greater EPSP synthase-specific activity (the primary site of action of glyphosate) than susceptible C₀ plants. This suggested that tolerance was at least partially conferred by increased EPSP synthase activity. Genetic variance should allow continued progress from selection for increased glyphosate tolerance in birdsfoot trefoil.