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.
Injury from herbicides may reduce sugarbeet yield or sugar content. Previous research has shown a differential response of sugarbeet varieties to herbicides. We evaluated the growth response of fourteen sugarbeet varieties, and four USDA sugarbeet entries (three USDA experimental hybrids and their pollinator) to postemergence applications of the micro-rate of desmedipham plus phenmedipham (1:1 ratio) at 0.09 kg ai/ha plus triflusulfuron at 0.004 kg ai/ha plus clopyralid at 0.023 kg ae/ha plus methylated seed oil at 1.5% v/v. Sugarbeets were sprayed three times at weekly intervals beginning at the cotyledon growth stage. Sugarbeet varieties differed in their response to micro-rate herbicide applications. Leaf area, fresh weight and dry weight of Hilleshog E-17 and ACH 555, two diploid varieties, were not reduced by micro-rate applications, while the leaf area of Beta 5400 and Beta 5736, two triploid varieties, was reduced by 24 and 35%, respectively, compared to their respective untreated controls. In a second experiment, the leaf area and fresh and dry weight of Spartan and Hilleshog E-17 were not reduced by postemergence microrate applications in the growth chamber or field, compared to their respective controls. Hilleshog E-38, ACH 185, and Beta 5736 had significant reductions in leaf area and dry weight in the growth chamber and field, while RH-5 had significant reductions in the growth chamber only, and tolerance was not correlated with ploidy level. Among the USDA materials, reductions in growth measures were evident in all entries; however, reductions in the experimental hybrid with SP85576 cms were markedly less for all traits except growth chamber dry weight (27% reduction in fresh weight, 20% reduction in leaf area and 16% reduction in
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