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 ethofumesate (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.

Field trials were conducted to determine if tillage and soil-applied herbicides had an effect on weed control and sugarbeet growth with a micro-rate herbicide program. Sugarbeet emergence was earlier in the moldboard plowed system compared with the chisel plowed system at three of four sites. Conditions were dry and sugarbeets emerged 5 d later in the moldboard plowed system compared with the chisel plowed system at the fourth site. Even though the rate of sugarbeet emergence differed between tillage systems at all four sites, final sugarbeet populations did not differ at two of the four sites. Sugarbeet injury from PRE treatments of S-metolachlor, ethofumesate, and ethofumesate plus pyrazon, followed by four POST micro-rate applications, ranged from 11 to 27% and 1 to 18% in the chisel and moldboard plowed systems, respectively, 6 wk after planting (WAP). Under wet conditions, sugarbeet stand was reduced and injury was greatest from PRE applications of S-metolachlor. Common lambsquarters, pigweed (redroot pigweed and Powell amaranth), and giant foxtail control in mid-August was consistently higher when a PRE herbicide was applied prior to micro-rate herbicide treatments. Even though there were differences between PRE and no-PRE treatments with respect to sugarbeet injury and weed control, recoverable white sucrose yield did not differ between herbicide treatments. However, recoverable white sucrose yield was greater in the moldboard plowed treatments compared with the chisel plowed treatments at three out of the four sites.