Managing Glyphosate-Resistant Palmer Amaranth
after State Updates
Palmer amaranth seed longevity: Published research indicates that 3 years of complete control would reduce the seedbank of redroot pigweed and common waterhemp by 99%. However, even 1% of the seed can repopulate the infestation. Seed survival and dispersal of Palmer amaranth need to be studied, probably through coordinated, collaborative projects.
Palmer amaranth male to female ratio: In non-resistant populations throughout Arkansas, the female to male ratio is approximately 1:3. In Mississippi County Arkansas, the resistant population appeared to have a skewed proportion of male plants after treatment with glyphosate. Some plants that appeared to be male (based on inflorescence characteristics and pollen production) produced seed, albeit less than would be expected for female plants. Researchers were not aware of any ‘fitness penalty’, but acknowledged the need for determining the possibility in Palmer amaranth.
Glufosinate in Liberty Link cultivars: Activation of preemergence herbicides was poor in 2007 in some areas of the mid-South. In Arkansas, glufosinate (Ignite®) applied at the full rate (29 oz/A) to 2- to 3-leaf soybean adequately controlled Palmer amaranth. It was suggested that glufosinate is generally more effective in the mid-South than in the Southeast because plants in the sandy soils in the Southeast are often under moisture stress, which makes them difficult to control. Application timing is important, and the average grower finds it difficult to cover all fields in a timely fashion. A Bayer Crop Sciences representative suggested that increased rates of glufosinate may overcome this deficit.
KIH-485: Kumiai’s KIH-485 has residual activity on grasses and small-seeded broadleaf weeds. Work in Mississippi indicates that metolachlor has 3 to 4 weeks of residual activity, whereas KIH-485 is effective at 4 to 5 weeks. Colorado research indicated that KIH-485 is less adsorbed to soil than is metolachlor and may be activated with less moisture. However, Dan Poston of Mississippi observed that more moisture seemed to be needed for activation of KIH-485 than for metolachlor (Dual®). Researchers who have looked at the compound agreed that it may be a good tool for control of Palmer amaranth. Crop tolerance is an issue in the Southeast
Flumioxazin (Valor®): Attitudes about flumioxazin differed. In Tennessee, flumioxazin plus glufosinate applied under hoods looked better than glufosinate alone. In Arkansas, the combination controlled 2-, 4-, and 12-inch Palmer amaranth if applied over-the-top, but post-directed applications were not as effective because of inadequate coverage. In Mississippi, flumioxazin is used primarily as a residual herbicide in burndown or layby applications. However, the PPO (protoporphyrinogen oxidase) herbicides, including flumioxazin, may be the next resistance issue.