Timing of postemergence micro-rate applications based on growing degree days in sugarbeet

Postemergence (POST) herbicides must be applied in sugarbeet when weeds are less than 2.5 cm tall, or poor weed control will result. Multiple POST applications are usually required because of continued weed emergence. Frequent POST applications may injure sugarbeet and increase herbicide/application costs; however, too few applications may result in poor weed control and reduced sugarbeet yield and quality. This research determined if growing degree days (GDD) could be used to time postemergence herbicide applications. Common lambsquarters (Chenopodium album L.) and redroot pigweed (Amaranthus retroflexus L.) were controlled in the growth chamber by three microrate herbicide treatments applied on seven day intervals, every 97 or 125 GDD (base temperature of 1.1o C). Two micro-rate applications also provided control of common lambsquarters and redroot pigweed when the first application was at 194 GDD (97 + 97 GDD), followed by the second application at 97 GDD (Table 1). Neither weed species was controlled by three micro-rate applications at 152 GDD, or when the first treatment was delayed until 250 GDD and only two micro-rates were applied. In the field, sugarbeets were planted April 5, April 19, and May 2 in 2001 and April 7, April 17, and May 1 in 2002. Micro-rate