Zachary J. Reicher

Metabolism of 2,4-dichlorophenoxyacetic acid contributes to resistance in a common waterhemp (Amaranthus tuberculatus) population: Enhanced 2,4-D metabolism in A. tuberculatus

BACKGROUND Synthetic auxins such as 2,4-dichlorophenoxyacetic acid (2,4-D) have been widely used for selective control of broadleaf weeds since the mid-1940s. In 2009, an Amaranthus tuberculatus (common waterhemp) population with 10-fold resistance to 2,4-D was found in Nebraska, USA. The 2,4-D resistance mechanism was examined by conducting [14 C] 2,4-D absorption, translocation and metabolism experiments. RESULTS No differences were found in 2,4-D absorption or translocation between resistant and susceptible A. tuberculatus plants. Resistant plants metabolized [14 C] 2,4-D more rapidly than did susceptible plants. The half-life of [14 C] 2,4-D in susceptible plants was 105 h, compared with 22 h in resistant plants. Pretreatment with the cytochrome P450 inhibitor malathion inhibited [14 C] 2,4-D metabolism in resistant plants and reduced the 2,4-D dose required for 50% growth inhibition (GR50 ) of resistant plants by 7-fold to 27 g ha-1 , similar to the GR50 for susceptible plants in the absence of malathion. CONCLUSION Our results demonstrate that rapid 2,4-D metabolism is a contributing factor to resistance in A. tuberculatus, potentially mediated by cytochrome P450. Metabolism-based resistance to 2,4-D could pose a serious challenge for A. tuberculatus control because of the potential for cross-resistance to other herbicides.

Weed Control and Turf Safety of Single and Sequential Applications of Herbicides Over Spring Seedings

Weed control is important during establishment of cool-season grasses, especially when seeded in spring near summer annual weed germination. Our objectives were to determine (i) turf safety and weed control from sequential applications of newly-released herbicides on tall fescue at three seeding dates (May, June, July) and (ii) turf safety and weed control from newly-released herbicides applied at seeding or shortly after emergence of spring-seeded Kentucky bluegrass or tall fescue. No herbicides tested negatively affected turfgrass establishment and usually improved turf establishment by reducing weed competition. Mesotrione consistently provided effective weed control and turf cover from either single or sequential applications over different locations, years, seeding dates, and turf species. Quinclorac or siduron applied as single or sequential applications or quinclorac+carfentrazone as a single application provided effective weed control and turf cover depending on seeding date and application regime. Although spring seeding can be achieved with high maintenance, it is strongly recommended to seed cool season grasses at the optimal time from late summer to early fall.

Fall or Late Winter Applications of Preemergence Herbicides Rarely Provide Season-long Control of Crabgrass ( Digitaria spp) in Nebraska

Fall and late winter applications of preemergence herbicides (PREs) for crabgrass control have been evaluated in Illinois (Fermanian and Haley, 1994), Pennsylvania (Reicher et al., 2011), Indiana (Reicher et al., 2011; Reicher and Throssell, 1993), and Maryland (Dernoeden, 1993). The authors generally concluded that fall and late winter applications provide season-long control with higher application rates in areas with low/moderate crabgrass pressure. Many of these studies used older formulations of current active ingredients. None of these studies were located in the northwest edge of the transition zone where cool-season grasses dominate because of winter survival, but summer temperature extremes can thin even irrigated cool-season stands. Our objective was to determine if fall or late winter applications of PREs can provide season-long control of crabgrass in Nebraska.