The “micro-rate” application, a POST combination of desmedipham plus phenmedipham at 0.045 0.045 kg ai/ha (desphen) or desmedipham plus phenmedipham plus ethofumesate3 (1:1: 1 ratio) (desphenetho) at 0.09 kg ai/ha plus triflusulfuron at 0.004 kg ai/ha plus clopyralid at 0.026 kg ae/ha plus 1.5% methylated seed oil received registration in 1998 and 2000 in North Dakota and Michigan, respectively. Herbicide rates are reduced by 80%, compared to standard-split applications, and growers typically apply the micro-rate three to five times to very small weeds that are 1 cm or less in height. In standard-split applications, growers make two sequential applications, the first when weeds are 1.5 cm tall and the sequential application usually 10 to 14 d later. Research was conducted in small plots and large grower plots in 2001 and 2002 to determine the effect of PRE herbicides on weed control and sugarbeet injury from micro-rates compared to standard-split POST herbicide applications. Sugarbeet populations were reduced in the cycloate treatment compared to all other PRE and the no-PRE treatment in 2001 and in the S-metolachlor compared to the ethofumesate treatment in 2002. Sugarbeet injury was 6% or less from POST-only treatments in 2001. Control of common lambsquarters and Amaranthus spp. by desphen and desphenetho treatments was similar. Sugarbeet injury in 2002 was 29 to 43% from POST-only treatments. The standard-split of desphenetho was more injurious than the standard-split of desphen. Common lambsquarters control was greater in both the standard-split and micro-rate of desphenetho compared to the standard-split of desphen in 2002. However, sugarbeet populations and recoverable white sucrose per hectare did not differ among POST herbicide treatments in either year. No herbicide program provided 100% control of all weeds in both years. In the seven large production fields, PRE herbicide treatments did not reduce sugarbeet populations or recoverable sucrose per hectare compared to the no-PRE control. Weed control from four POST micro-rate applications only was similar to weed control in instances in which PRE herbicides were applied prior to the POST micro-rate applications.
Nomenclature: Cycloate, pyrazon, ethofumesate, desmedipham plus phenmedipham, triflusulfuron, clopyralid, Chenopodium album L. #4 CHEAL; Amaranthus species # AMASS.
Additional index words: Micro-rate, standard-split.
Abbreviations: DAT, days after treatment; desphen, desmedipham phenmedipham; desphenetho, desmedipham phenmedipham ethofumesate; fb, followed by; RWSH, recoverable white sucrose per hectare.
