Midseason Weed Control Issues
Posted: June 16, 2015
We are receiving calls about weeds breaking through soil-applied herbicides. With all of the recent rainfall and especially if reduced herbicide rates were used, a postemergence herbicide may be necessary to clean up some of the escaped weeds. However, keep in mind there are crop height restrictions on many of the post herbicides. Application restrictions can be found in the Penn State Agronomy Guide or online online (see Table 2.2-14 - corn).
Horseweed/marestail in soybean: Use the highest rate possible of Classic, Synchrony (use highest rate on STS bean only), and FirstRate and full adjuvant systems (see label) may help control glyphosate-resistant marestail. However, there are populations in PA that are both ALS- and glyphosate-resistant. In these cases, the above products will not provide control. High rates of Liberty can be effective postemergence on marestail if Liberty Link soybean varieties were planted. According to Mark VanGessel, University of Delaware, horseweed plants are generally not very tolerant of shade and most soybeans will begin to canopy over the horseweed and outcompete them. Additional glyphosate applications at higher rates will provide some suppression of horseweed and sometimes the soybeans have a chance to outcompete them. It is always best to treat the horseweed plants soon after they start regrowing from the burndown application.
Pokeweed can be controlled with several herbicides in corn, including glyphosate, 2,4-D, Banvel, Status, and Callisto + atrazine. These herbicides can provide at least 80% control by the end of the season. In soybeans, similar control can be achieved; however, there are less effective options than in corn. Glyphosate is effective (90% control) and should be used as a foundation of spray programs when controlling pokeweed in soybeans. Applying glyphosate mid to late summer is more effective than in the spring due to greater translocation during flowering. The ALS-inhibitor herbicides (Classic, Synchrony, FirstRate, Harmony, etc.) provide 60% control or less when sprayed alone and should be used in combination with glyphosate. Using a residual herbicide will help to control the pokeweed seedlings, which can emerge throughout the growing season. (Pokeweed management summary by Kelly Patches)
Be cautious of herbicide drift: Certain herbicides especially, glyphosate and the PGR herbicides (dicamba, 2,4-D, etc.) can cause problems outside the field boundaries. To help reduce drift, use drift reducing nozzles such as venturi or air-inductions (AI) style tips. Most manufactures now make these tips as extended range models in order to reduce the spray pressure down to 15 psi. Also consider the difference between particle drift and vapor drift. Particle drift occurs when small droplets actually move and deposit onto leaf surfaces (this can be prevented by choice of nozzle, pressure, spray volume, application time, etc.). Vapor drift is related to the function of the herbicide formulation (e.g., ester vs. amine) and does not matter what kind of tip or pressure is used. It is impacted by temperature and relative humidity. Visit the Purdue website for a more detailed discussion on spray drift and ways to reduce it.
However, keep in mind, if using herbicides that require good spray coverage (e.g., Liberty, Gramoxone, Cadet, Cobra, Reflex, etc.), AI tips may not be the best option unless certain adjustments are made to allow for better coverage including, higher spray volume, pressure, and boom height.
Proper sprayer cleanout is important especially when moving from one crop to another. There are still many reports of PGR herbicide injury on soybeans when applicators don’t properly clean out the sprayer after a corn application. Dicamba residues left in the sprayer still pose one of the biggest threats to soybean (and certain vegetable crops) when not thoroughly cleaned out. Dicamba-containing products include: Banvel, Clarity, DiFlexx, Sterling, Status, NortStar, Yukon and others. To get the most effective sprayer cleanout, simply rinsing with water will not work especially with plastic tanks and rubber hoses. Usually it requires a few steps with inclusion of ammonia and/or tank cleaner. Sprayer cleanout details can be found on the Missouri Extension website.
Hot weather and crop injury
Since the past week or so has been rainy and cloudy and the upcoming forecast is for hot/humid weather, there is the potential for crop injury when post herbicides are applied. The general rule of thumb is to allow a couple sunny days or so to pass after coming out of a rainy, overcast period before applying herbicides. Since the plants are stressed, this allows them time to build up a thicker leaf surface and to get their metabolic processes functioning at a faster pace to detoxify the herbicide. Also, with all the moisture and sunlight the plants will be growing very quickly and are succulent, so consider using nonionic surfactant (NIS) instead of crop oil concentrate (COC) or methylated seed oil (MSO) as the spray additive.