Herbicide injury during and after emergence in soybean

Pre-emergence and post-emergence herbicides can injure soybean plants. This year, soybean injury symptoms following pre-emergence applications have been relatively slight for May 13 and May 19 planting dates. In the herbicide evaluation program, soybean injury symptoms that we typically observe are stunting, drawstring (puckering), chlorosis (yellowing), and necrosis. However, it is important to keep in mind that injury symptoms may be due to weather, soil conditions, or disease.

To diagnose an injury problem at or during emergence, first check the root system of the plant. Seedling root growth inhibitors, ALS inhibitors, and growth regulators change the root architecture in different ways. Next step, if the roots appear normal, take a look at the leaves for yellowing (chlorosis), bleaching, or drawstring (puckering). To help with diagnosis, University of Wisconsin Extension has a two-page guide, available for download and an online diagnostic tool.

After emergence, PPO inhibitor injury tends to appear on the leaves that receive the application but younger leaves will not show any injury symptoms. Typical damage includes yellowing (chlorosis) and browning of the leaf surface in spots (necrosis) (Fig. 1A). Researchers at Purdue University have a five minute video discussing PPO inhibitor and fluopyram (ILeVO) seed treatment injury.

It is important to remember that despite the damage, in most cases yield is not affected. Also, PPO inhibitors add another site of action to your resistance management plan and effectively control a variety of broadleaf weed species. Growth regulators cause leaf cupping or epinasty (downward growth habit) (Fig. 1B). The leaves of soybean plants with ALS inhibitor injury show chlorosis and distinctive reddish leaf veins (Fig. 1C).

injury_figure

Injury symptoms typically appear seven to fourteen days after application and will gradually decrease through the season when the plants resume normal growth. Phytotoxicity data are available in the WCWS research reports. Each trial contains a summary section that will mention if any phytotoxicity symptons were observed. If the injury differed by treatment and exceeded five percent then a bar graph is included. Crop injury resulting from an application made according to the label instructions usually does not cause a reduction in yield. “Pest Management in Wisconsin Field Crops” shows the relative risk of soybean injury from different herbicides on pages 143 and 144, available at Cooperative Extension’s Learning Store.

Herbicide resistance management for giant and common ragweed

Weed scientists across the Midwest and Midsouth have identified eleven species of weeds that are of most concern for herbicide resistance because of their ability to compete with crops and to develop resistance to different herbicide sites of action (SOA). They are common waterhemp, Palmer amaranth, horseweed (marestail), giant ragweed, common ragweed, common lambsquarters, kochia, Italian ryegrass, barnyard grass, johnsongrass, and giant foxtail. All of the species have shown resistance to between two and nine herbicide sites of action.

Herbicides are typically classified in two ways by (1) mode of action or (2) site of action. The mode of action is generally how the herbicide inhibits plant growth and development. The site of action is specifically which biochemical pathway in the plant that the herbicide disrupts. Each site of action is associated with a number and the number is located on the first page of most herbicide labels. The TakeAction herbicide classification chart clearly displays the herbicides and premixes by sites of action. A successful resistance management plan will include multiple effective sites of action applied at the full rate to weeds less than four inches in height.

In 2011 and 2013, University of Wisconsin weed scientists identified two separate giant ragweed, Ambrosia trifida, populations resistant to either glyphosate (SOA 9) or ALS inhibitors (SOA 2). In 2013, a population of common ragweed, Ambrosia artemisiifolia, was confirmed resistant to ALS inhibitors. Not sure what a glyphosate resistant giant ragweed plant looks like, check out a time lapse video from Purdue University comparing susceptible and resistant biotypes of giant ragweed, two minutes fifteen seconds in length.

At this point in the season, the most important task for herbicide resistance management is to scout fields several times before and after an herbicide application. Scouting after pre-emergence applications, prior to a post-emergence timing, can ensure that the post application is on target for weed species and size. It is important to pay attention to any weed species that is becoming more abundant across the field and then to double-check that your planned herbicide program is effective at controlling that species. Also, factoring in current weed size with weather forecasts is important for scheduling that next application. After applying herbicides, the next task is to follow-up with scouting at 14 days after the post application to verify adequate control. For more information about effective scouting, please visit the TakeAction website.

Both ragweed species reduce crop yields because early season germination and growth interferes with crop establishment prior to canopy closure. Giant ragweed can be difficult to control with a one-pass herbicide program because seedlings can emerge from deeper soil depths where a pre-emergence herbicide may not reach and some populations tend to emerge over an extended period. For herbicide resistance management recommendations, please consult the TakeAction fact sheet. Giant ragweed tends to be more of a problem in conventional tillage systems whereas common ragweed infestations are common in reduced and no-till systems. Management recommendations for common ragweed infestations in corn and soybean are available on the WCWS website.