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Sub-Lethal Rates of Dicamba (07/13/17)

Recent published research shows Palmer amaranth plants that survive sub-lethal rates of dicamba can pass a greater level tolerance to successive generations.

Sub-Lethal Rates of Dicamba

Recent published research shows Palmer amaranth plants that survive sub-lethal rates of dicamba can pass a greater level tolerance to successive generations. Registration of dicamba-resistant crops will provide an alternative management option to control many weeds, including waterhemp and herbicide-resistant Palmer amaranth populations, including those with resistance to herbicide Groups 2, 3, 5, 9, 14, and 27. However, repeated use of sublethal doses of dicamba may lead to rapid evolution of herbicide resistance, especially in Palmer amaranth—a species with a strong tendency to evolve resistance.

Selection experiments with dicamba were conducted on Palmer amaranth using sublethal doses. In the greenhouse, a known susceptible Palmer amaranth population was subjected to sublethal dicamba doses for three generations (P1–P3). Susceptibility of the plants to dicamba was evaluated, and susceptibility to 2,4-D was also characterized. Researchers exposed dicamba-susceptible pigweed to low doses of dicamba for three successive generations where surviving weeds were allowed to cross-pollinate with one another, similar to a natural environment. The initial population was called PO and successive populations were named P1, P2 and P3. The labeled rate of dicamba on dicamba-resistant soybean is 0.5 lb ai/A (8 oz ai/A) and was applied 4 to 5 leaf pigweed. The first generation (PO) was exposed to ¼ dose, P1 a ½ dose, P2 a ¾ dose and P3 a full dose as per the table below.

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Results show exposing pigweed to three successive low doses of dicamba led to a roughly three-fold increase in resistance. Approximately 25% of the P3 generation survived the label rate of dicamba.

The pigweed selected with dicamba in this study were also tested with 2,4-D, another auxinic herbicide and surprisingly, reduced susceptibility of the P3 generation to 2,4-D was also evident. Approximately, a 2-fold resistance to 2,4-D was observed for the P3 generation with 25% of pigweed in the P3 generation surviving a rate of 2,4-D that gave 100% control of the PO population.

It is likely that low-dose recurrent selection leads to accumulation of “genes with small additive effects”. Low-dose recurrent selection is much slower in self-pollinated species such as wild oat and fastest in species that must cross pollinate such as Palmer amaranth and waterhemp.

Reference: Recurrent Sublethal-Dose Selection for Reduced Susceptibility of Palmer Amaranth (Amaranthus palmeri) to Dicamba. Parsa Tehranchian, Jason K. Norsworthy, et al. Weed Science 2017 65:206–212.

Application of this information: Several have asked if NDSU has conducted studies similar to this done by Norsworthy in Arkansas. NDSU has not conducted similar research although growers have been unknowingly doing this same selection pressure for decades by spraying low dicamba rates in corn (4 fl oz/A) and wheat (2 fl oz/A). There is ample evidence that low doses of herbicides (not just auxins) often produces resistant populations from surviving plants within a few generations. Positive news is that few weeds have developed resistance except kochia. Dicamba resistant kochia has been identified but increase in geography and level of resistance has been slow and small. We don't really understand why this is, so it is difficult to predict what a large-scale change in dicamba usage will produce. Plant (weed) specie response to dicamba will be weed specific – some weeds, like Palmer amaranth and possibly waterhemp may express higher levels of resistance faster than others, like kochia. To delay resistance it is assumed, based on basic population genetics, that using multiple applications of dicamba at high rates in one cropping season (only 1 year of 2 or more years) will LIKELY delay weed resistance more than using just one dicamba application in each growing season (example: once in wheat, once in DT soy, once in corn, etc.). Multiple applications in one season will likely kill more of the same population thereby decreasing the risk of higher levels of resistance to be passed on to surviving progeny. This confirms my motto: Dead weeds don't make seed.

Summary: This research was conducted on Palmer amaranth but waterhemp is also much like Palmer amaranth. Waterhemp is dioecious (male and female flowers grow on separate plants and they must cross-pollinate to produce seed) and waterhemp has become resistant to most herbicide classes, similar to Palmer amaranth. Waterhemp is well established in eastern North Dakota, with spread into central North Dakota. Dicamba-resistant soybean may become a dominant soybean technology used by growers in ND and the U.S. It would be accurate to anticipate that waterhemp surviving dicamba in dicamba-resistant soybean may develop an increased tolerance to dicamba and 2,4-D. This information validates many of the Weed Management Strategies we recommend on page 97 in the ND Weed Guide, namely:

1. Scout fields… before and after application and at the end of the season.

5. Apply effective POST herbicides… that include multiple mechanisms of action in tank-mix or in sequential applications.

6. Use high herbicide rates and effective adjuvants. Full rates kill weeds with low-level resistance and dead plants cannot produce resistant progeny. Reduced rates allow plants with low-level resistance to survive, hybridize, and produce progeny with elevated resistance. Hybrid plants (>1 resistance gene) express a higher level of resistance and require even higher herbicide rates to kill the plant. Dead weeds means zero tolerance (no seed production, zero resistant progeny) and is effective resistance weed management.

7. Spray small annual weeds. Generally, small weeds (<3 inches) are more susceptible to herbicides than large weeds. Even weeds with low level herbicide resistance are more susceptible at 1 inch than at larger growth stages.

8. Practice Zero Tolerance... seed from escaped weeds will contribute to the weed seedbank and will require diversified weed management strategies of mowing, cultivation/tillage, and hand weeding to achieve near 100% weed control.

9. Control weeds in field perimeters, drown out, and non-crop areas. Weeds surviving a partial herbicide dose on field borders can be a repository for the introduction of resistant weeds into a field.

10. Rotate herbicides with different mechanisms of action in consecutive years. Diverse crop rotations can introduce herbicides with different mechanisms of action to delay herbicide resistance.

12. Evaluate weed management at the end of each season and revise to improve weed control the next year.

Rich Zollinger

Extension Weed Specialist

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