Herbicide-resistant weeds are an ever increasing problem.

When most people hear herbicide-resistant weeds these days they think of glyphosate resistance. However, I think Group 2 resistance (or ALS-resistance) is just as bad, if not worse, for our area. Small grains is a crop that relies heavily on ALS herbicides for weed control, and over the years the products have been used heavily. We now have three weed species that are resistant to this mechanism of action. ALS-resistant common chickweed, annual (Italian) ryegrass, and horseweed (or marestail) are becoming more common. Before I talk about some options to control them, let’s look at how we got here. [Read more…]

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

For no-till fields, a non-selective herbicide needs to be used prior to planting. If grasses or perennial weeds are present, glyphosate is a better choice than paraquat. Fields worked with a vertical tillage implement for residue management still need a non-selective herbicide. These implements are not weed control tools.

There are few effective herbicides labeled for preemergence applications. Sharpen is labeled but we have limited data in the region. Valor can be used at 1 to 2 oz with the burndown application, but there must be a 7 day period between application and planting wheat. Valor is not labeled for barley.

Axiom and Prowl H2O can be used at crop emergence (Axiom at the spike stage and Prowl H2O at 1 leaf stage, winter wheat only). Neither provides control of emerged weeds but can have utility in situations of needing limited residual control shortly after planting.

Products that provide postemergence control include: Harmony, Harmony Extra, Starane Ultra, Osprey, PowerFlex, Axial XL, or 2,4-D. Other labeled herbicides with a limited fit include Finesse and Maverick.

Control of annual ryegrass has been good with Opsrey, PowerFlex, or Axial XL. However, ALS-resistant ryegrass has been identified in Delaware and these populations will not be controlled by Osprey or PowerFlex. Furthermore, Finesse will not control these populations. In situations where ALS resistance is suspected, Axial XL is the best postemergence option.

ALS-resistant common chickweed has been confirmed in Delaware. These biotypes are not controlled with Harmony Extra, Osprey, PowerFlex or Finesse. Currently, Starane Ultra has showed the best activity, often not killing common chickweed, but providing good suppression. Starane Ultra can be used either in the fall or spring.

A number of calls have come in about Palmer amaranth in soybeans and what can be done. First of all, if the plants have been sprayed with glyphosate and they did not die, then they are likely to be glyphosate-resistant and spraying more glyphosate will not kill them. Products like Pursuit or Reflex will not control Palmer amaranth plants larger than 5 to 6 inches. Options are limited to Gramoxone, handweeding, or mowing.

After corn harvest, some fields may need to be mowed or sprayed with Gramoxone to prevent late-season seed production.

Late-season management of Palmer amaranth to reduce or prevent seed production is not easy or convenient, but it will pay dividends in the future. Removing the plants prior to harvest will reduce the spread of the seed within the field and from field to field. If a single plant can produce 500,000 seeds; spread over an acre that is 10 seeds per square foot. You can quickly see how eliminating seed production and preventing the spread of seed with equipment is a tremendous help.

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
I have seen Palmer amaranth seedlings emerging in corn fields that had no preemergence herbicides applied.

There are Palmer amaranth plants in DE and MD that are resistant to glyphosate. So fields with Palmer amaranth present, or fields where you suspect it is present, need to be treated with an effective herbicide (or herbicide combination) that will provide postemergence control as well as residual control. Some considerations include atrazine, Callisto, Capreno, Impact, Permit Plus, Halex, Realm Q, Resolve Q. Other products such as Status or Liberty provide effective postemergence control, but will not provide residual control.

Palmer amaranth plants look very similar to smooth and redroot pigweed. However, Palmer amaranth leaves, stems, and petioles do not have hairs (smooth and redroot pigweed do have fine hairs). Palmer amaranth’s leaves have long petioles that are often as long, or longer, than the leaf blade. As a result, the leaves often droop. Occasionally, leaves will have a variegated “V” mark or watermark across the leaf blade. On-line photos can be found at http://extension.udel.edu/ag/files/2013/05/AMAPA_images_lg.pdf

Source: Read More from Weekly Crop Update

by Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

A number of corn herbicides have label precautions about use of an organo-phosphate insecticide at planting due to potential crop injury. This includes the insecticides Counter (terbufos), Lorsban (chlorpyrifos), and Fortress (chlorethoxyfos). The herbicides include both soil-applied and postemergence herbicides. Some of the common products include Callisto (Halex GT); Resolve (Steadfast, Basis); Accent; or Capreno. Many of the Group 2 (or ALS-inhibiting herbicides) have these precautions. The list of herbicides can be found at http://extension.udel.edu/ag/files/2012/08/CornWeedguide.pdf and go to Table 19. [Read more…]