Weed Control: Miscellaneous Midseason Matters

By Dwght Lingenfelter 

This article is this week’s Agronomy Highlight, recorded live on Fridays at 9:00 am. The Agronomy Highlight is an opportunity for readers to ask the author questions and hear updates from around Pennsylvania. Join the Agronomy highlight live on Facebook or zoom or join by calling +1 646 876 9923, and when prompted enter the webinar ID: 946 6516 7271.

June 30 application deadline approaching for XtendiMax and Engenia

Cut-off dates for application of XtendiMax, Engenia, and Tavium are quickly approaching. These herbicides cannot be applied to Xtend platform soybeans after June 30, other growth stage limits may apply depending on the product. Alternative herbicides will need to be used after June 30, depending on the type of soybean that you planted. Consider using products such as glyphosate, glufosinate, Pursuit, Raptor, Classic, FirstRate, Reflex/Flexstar, Cobra, Ultra Blazer, and several others. However, keep in mind these products also have limits on how late they can be applied, either by soybean growth stage or days prior to harvest. (On a side note, if you have Enlist E3 soybeans planted, the Enlist products can be applied through R2 (full flower) soybean growth stage, with no calendar date restrictions.)

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 platforms, Xtendimax, Engenia, 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 tank-mixed with 2,4-D choline (Enlist One/Duo) in an Enlist E3 system: XtendFlex soybeans also are tolerant to glufosinate. If your populations are not ALS-resistant, then FirstRate (0.3-0.6 oz/A) can still provide adequate control of marestail. 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 (glufosinate) in LibertyLink, Enlist E3, or XtendFlex, 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. Tank mixing 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 if possible. The contact herbicides (Reflex, Cobra, Cadet, etc.) only provide an 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.

Controlling volunteer corn in soybeans

As the soybean crops continue to mature, volunteer corn is rearing its head in some fields. Regarding management, we assume that most of the volunteer corn is glyphosate- and glufosinate-resistant (Roundup Ready/LibertyLink) so we will not spend time going over all the options for killing the various types of volunteer corn. In general, the most logical choices to control volunteer corn in any kind of soybean are the post-grass herbicides (e.g., Assure II, Select, clethodim, Fusilade, etc.). Below are some rates to consider depending on the height of the volunteer corn:

Always include the necessary adjuvants otherwise control will be impacted. In most cases, this will require the addition of a crop oil concentrate (COC) or methylated seed oil (MSO). If crop injury is a concern, a nonionic surfactant (NIS) may be used in place of an oil with Select Max only. Keep in mind if you plan to tank-mix a broadleaf-specific herbicide, the rate of the grass herbicide may need to be increased to overcome possible antagonism.

Burndown suggestions for double crop soybeans

With barley harvest beginning and wheat coming off soon, double-crop soybean planting will soon follow in some areas of the state. One of the biggest weed concerns is in managing glyphosate-resistant marestail or horseweed. Some of these populations may also be ALS-resistant. Marestail that are cut off during small grain harvest will recover, branch out, and probably be even more difficult to control. Most farmers are not willing to wait the necessary 7 days after 2,4-D application in double-crop and there is potential for drift at this time of year. So, what are other options to control marestail?

• Add 1 oz of Sharpen (or equivalent rates of Zidua Pro, Optill, or Verdict) to glyphosate to control marestail stumps. Keep in mind, Sharpen at this rate helps with burndown but does not provide much residual control. Adding metribuzin improves residual control.
• Use Liberty at 32 to 43 fl oz in the burndown application. This is good on marestail, but not as effective on some other weeds such as large annual grasses and Canada thistle. You can tank-mix with other herbicides to improve performance and provide some residual control (Sharpen, metribuzin, etc.). Glyphosate (32 fl oz) can also be tank-mixed with Liberty to improve spectrum of burndown control.
• Gramoxone is a burndown option as well and has some activity on marestail as long as they are <3” tall, and good coverage is essential.
• Plant a LibertyLink, LLGT27, Enlist E3, or XtendFlex soybean variety and apply Liberty (or other labeled glufosinate product) post as needed.
• If planting an Enlist E3 variety, Enlist One/Duo could be applied and then soybeans planted immediately. But be cautious of drift to sensitive areas.
• Add FirstRate (or Authority First or Sonic) or a chlorimuron-containing product (Authority XL, Canopy, Envive, Synchrony, Valor XLT, etc.) to improve control if the population is not ALS-resistant. (Valor, Authority, and metribuzin provide residual activity; but not much burndown activity.) Also, root uptake of Valor XLT/Envive may help kill marestail stumps. Keep in mind that if you tank-mix other PPO (group 14) products that contain Valor or Authority with Sharpen, there is a 14 day wait period before planting. Valor and Authority-containing products provide residual control of emerging Palmer amaranth and waterhemp seedlings.
• For farmers interested in non-GMO soybeans, several post herbicides are still available, but many of the broadleaf products have some potential for soybean injury and they are not effective for control of marestail.

Perennial weeds in pastures: Is now the best time to spray?

In general, the answer is NO. Now is not the most optimal time to control perennial weeds such as Canada thistle, milkweed, hemp dogbane, and others in a pasture. During the late spring and summer, we typically suggest mowing pastures a couple of times or so to "setup" the weeds for a more effective herbicide application in the fall. Time the late-season mowing to occur early enough to allow perennial weeds to grow about 12-15 inches or so (usually about a month after cutting) before a herbicide application in the fall. Warm-season perennials such as horsenettle, hemp dogbane, common milkweed, smooth groundcherry, pokeweed, bindweed, poison ivy, and brambles should be sprayed with a systemic herbicide (such as 2,4-D, dicamba, triclopyr, etc.) from September 1 through October 15 or before a hard frost. In general, applications by October 1 may be more effective. In northern areas of Pennsylvania, consider making the application before October 1. An additional 2 to 4-week or more application window can exist for Canada thistle and quackgrass, because of their cool-season habit of growth.