By Dwight Lingenfelter and John Wallace
As we proceed through the growing season, there are a few problem weeds that tend to become evident. Below are some considerations for control and other factors.
As we proceed through the growing season, there are a few issues surrounding weed control and herbicide use in corn and soybean fields. Below are some items to consider:
Escaped weeds and rescue applications
With all 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. For a listing of additional herbicides and maximum corn heights and information on maximum weeds sizes for these products please refer to Table 2.2-15 in the Penn State Agronomy Guide and check the most recent herbicide label for specific use guidelines. (Or refer to the herbicide label for additional use information — www.cdms.net or www.greenbook.net).
Horseweed/marestail in soybean
There are populations in PA that are both ALS- and glyphosate-resistant. In these cases, here are some suggestions. In Xtend soybean, Xtendimax, Engenia, FeXapan, and Tavium can be used to control marestail. If LibertyLink or LLGT27 beans are planted, glufosinate (Liberty, others) at higher rates (32-36 fl oz) is a viable option and more so if tankmixed with 2,4-D choline (Enlist One/Duo) in an Enlist E3 system. In a standard Roundup Ready system, a 2x rate (1.5 lb ae) of glyphosate or a couple of in-crop applications of glyphosate usually stunts marestail. It is always best to treat them soon after they start regrowing from the burndown application. This will not control them but might suppress them somewhat if they are sprayed early enough. Keep in mind, marestail plants are generally not very tolerant of shade and most soybeans will begin to canopy over the marestail and outcompete them. In general, the post PPOs (Reflex, Cadet, Cobra, Ultra Blazer, etc.) are not effective on marestail, but the pre’s (Valor, Authority, etc.) are.
Palmer amaranth and waterhemp in soybean
Populations of these weeds continue to be found around the state. Make sure to scout fields early to determine what kind of pigweed species are in your fields. The sooner you ID them the easier it is to manage them. There is still plenty of time for them to emerge this season and cause problems. Once Palmer or waterhemp reaches 4 inches tall they are very difficult to control and they grow quickly. Aside from Xtend or Enlist soybean systems, consider the following options. Assuming glyphosate will still be used, the addition of a Group 14 (Reflex, Cobra, Ultra Blazer, etc.) plus a Group 15 (Dual/Prefix, Zidua/Anthem Maxx, Warrant/Warrant Ultra, etc.) will be necessary for adequate control through the season. The use of Liberty in LibertyLink, Enlist E3, or LLGT27 varieties provides very good control of Palmer and waterhemp. More details about these noxious weeds can be found at " Herbicide Recommendations for Noxious Pigweeds ."
Pokeweed management
Pokeweed can be controlled with several herbicides in corn, including glyphosate, 2,4-D, dicamba, Status, and Callisto + atrazine. Tankmixing provides the best control. These herbicides can provide at least 80% control by the end of the season. In soybean, similar control can be achieved; however, there are fewer 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. The contact herbicides (Reflex, Cobra, Cadet, etc.) only provide initial “burn” but then the weed recovers. Using a residual herbicide (atrazine, metribuzin, Authority, Canopy, Python) will provide about 85-90% control of pokeweed seedlings, which can emerge throughout the growing season.
Other issues related to midseason weed control include:
Unstable weather, crop injury, and tank-mixing
With unsettled weather patterns, 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. Keep in mind also that certain pesticide formulations (e.g., EC) can increase crop injury. And finally, with the lack of spray days due to poor weather conditions, it is tempting to put “everything” in the tank at once (i.e., other pesticides, various adjuvants, foliar fertilizers, etc.) to reduce the number of trips across the field but this can sometime cause serious crop injury since the plants are sensitive and can’t fully metabolize and handle all of the products at once.
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. For a more detailed discussion on spray drift and ways to reduce it, see "Reducing Spray Drift from Glyphosate and Growth Regulator Herbicide Drift Caution."
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.
Sprayer cleanout
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 (or Xtend and Enlist soybean) application. Dicamba residues left in the sprayer still pose one of the biggest threats to non-Xtend soybean (and certain vegetable crops) when not thoroughly cleaned out. Dicamba-containing products include: Banvel, Clarity, DiFlexx, Xtendimax, Engenia, FeXapan, Tavium, Sterling, Status, 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. For more details on sprayer cleanout and risks with certain hose types see "Removing Dicamba Residues from Your Sprayer: A Tricky Task" and "Cleanout and upkeep of the sprayer: don't get complacent."
Source : psu.edu