Weed population dynamics after six years under glyphosate- and conventional herbicide-based weed control strategies

Field studies using glyphosate-resistant corn (Zea mays L.), sugarbeet (Beta vulgaris L.), and spring wheat (Triticum aestivum L.) were conducted at Scottsbluff, NE, Torrington, WY, and Fort Collins, CO, over 6 yr to evaluate weed population dynamics under glyphosate- and conventional herbicide-based weed control strategies. We report the response of common lambsquarters (Chenepodium album L.), wild buckwheat (Polygonum convolvulus L.), and all grasses combined to four management strategies imbedded in two crop rotations. There were no consistent or highly significant benefits from a diverse crop rotation versus continuous corn when weed control was considered. Rotating herbicide mode of action for general weed control provided no benefit above that provided by glyphosate at 0.8 kg acid equivalent (ae) [ha.sup.-1] applied twice each year. There was no evidence that any weed species developed resistance to glyphosate. The most striking finding of these studies was the buildup of common lambs-quarters and wild buckwheat that occurs when glyphosate is applied at 0.4 kg ae [ha.sup.-1] twice each year. Such a low-use-rate approach is to be completely discouraged since it appears to enrich the gene pool for individuals that survive low rates of glyphosate. The continuing decline in the real cost of glyphosate should eliminate any legitimate reason for growers to consider using less than the full labeled rate of glyphosate. All treatments that included glyphosate at all three locations over 6 yr provided significantly better grass control than the nonglyphosate conventional treatments.