Influence of Environmental Factors on Broadleaf Signalgrass (Brachiaria platyphylla) and Crowfootgrass (Dactyloctenium aegyptium) Germination and Antagonism of Clethodim by CGA 362622 and Imazapic.

Abstract

Laboratory and greenhouse studies were conducted to determine the effect of temperature, solution pH, water stress, and planting depth on broadleaf signalgrass and crowfootgrass germination. Onset, rate, and total broadleaf signalgrass germination (87%) was greatest in alternating 20/30 C temperature regime, while onset, rate, and total crowfootgrass germination (95%) was greatest in an alternating 20/35 C temperature regime. For both species, germination decreased as solution pH increased, with greatest germination occurring at pH 4 and 5. Germination of both species decreased with increasing water potential, and no germination occurred below -0.8 mPa. Germination decreased with burial depth. No seed of either species emerged from 10 cm. Field and greenhouse experiments were conducted to evaluate clethodim applied alone, in mixture and sequential treatments of clethodim plus CGA 362622 or imazapic for control of broadleaf signalgrass, fall panicum, goosegrass, and large crabgrass. Clethodim alone controlled broadleaf signalgrass, goosegrass, fall panicum and large crabgrass >93%, regardless of rate, while CGA 362622 did not control grasses in greenhouse or field experiments. Imazapic provided <82% control of grass weeds. When CGA 362622 or imazapic were applied in mixture with clethodim the effectiveness of the graminicide was reduced for all annual grasses. Clethodim applied 7 d before or after CGA 362622 or imazapic controlled the four grass species as well as clethodim applied alone. Therefore, while antagonism was variable, it usually occurred when &#8804;3 d separated applications of clethodim and CGA 362622 or imazapic. CGA 362622 or imazapic did not affect absorption, translocation, or metabolism of 14C-clethodim by goosegrass. When CGA 362622 was applied to goosegrass, fresh weight accumulation stopped for a period of 4 d compared to nontreated plants. Growth resumed 4 d after application of CGA 362622. One day after treatment, the photosynthetic rate in plants treated with CGA 362622 or imazapic was less than non-treated goosegrass. Photosynthetic rate of goosegrass was less for a period of 6 d after treatment with CGA 362622. Goosegrass photosynthetic rate did not recover when treated with imazapic. These data suggest that the antagonism of clethodim by CGA 362622 and imazapic may result from CGA 362622 or imazapic altering the sensitivity of ACCase to clethodim.