Midseason Weed Control: Rescue or Too Late?

With the recent rains and as we proceed through the growing season, there are several issues surrounding weed control and herbicide use in soybeans, small grains, and pastures. Below are some items to consider:

Horseweed/marestail in soybean: There are populations in PA that are both ALS- and glyphosate-resistant; in these cases, here are some suggestions. For varieties that have the LibertyLink trait, including Enlist E3 and XtendFlex platforms, 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 E3 system. If your populations are not ALS-resistant, then FirstRate (0.3-0.6 oz/A) can still provide adequate control of marestail. In a Roundup Ready system, a 2x rate (1.5 lb ae) of glyphosate or a couple of in-crop applications of glyphosate usually stunts marestail. In a Plenish system, a combination of FirstRate, Classic/Synchrony, and glyphosate tends to provide some suppression of these populations. 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/Flexstar, 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 spread 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. And now that it has rained, germination will be accelerated. Once Palmer or waterhemp reaches 4 inches tall, they are very difficult to control, and they grow quickly. Consider the following options – assuming glyphosate will still be used, the addition of a Group 14 (Reflex/Flexstar, Cobra, Ultra Blazer, etc.) plus a Group 15 (Zidua/Anthem Maxx/Perpetuo, Dual/Prefix, Warrant/Warrant Ultra, Enversa, etc.) will be necessary for adequate foliar and residual control through the season. The use of Liberty (glufosinate) in LibertyLink, Enlist E3, or XtendFlex varieties provides very good control of Palmer and waterhemp. University research has shown that a tank mix of Liberty and Enlist One in E3 soybeans provides effective control of these problem pigweeds.

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:

Select Max 0.97EC: <12" tall corn – 6 fl oz; 12-24" – 9 fl oz

Clethodim 2EC: 4-12" tall corn – 4 fl oz; 12-18" – 6 fl oz

Assure II 0.88EC: 10-12" tall corn – 4 fl oz; 12-18" – 5 fl oz; 18-30" – 8 fl oz

Fusilade DX 2EC: <12” tall corn – 4 fl oz; 12-18” – 6 fl oz

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 that 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. It is well documented that products like 2,4-D (e.g., Enlist One) can definitely antagonize the activity of these post-grass herbicides, so it might be best to apply them separately, if possible.

It’s Too Late for Weed Control

On another note, there are certain weeds that have matured beyond their prime for effective management. We typically get calls this time of year inquiring about recommendations for control of weeds such as poison hemlock, buttercup, roughstalk bluegrass, perennial pasture weeds, and others. The simple answer is – NO, now is not the time for effective management of these weeds. Just because they are most noticeable this time of year does not mean it is the best time to manage them. Below are some considerations about proper herbicide application timing for these weeds.

Buttercup management: If your pasture is "yellow" with buttercup flowers, it's too late to get effective control from herbicides. In general, the best time to apply herbicides to control buttercup is in the early spring (Mid-March thru April). Herbicides such as 2,4-D (2-3 pints/acre), 2,4-D (1 quart) + dicamba (1 pint), metsulfuron/MSM (0.2-0.3 oz), NovaGraz (24-48 fl oz), or Crossbow (2-3 quarts) are commonly used to control buttercup.

Poison hemlock control: When it is flowering and 3-7 feet tall, herbicide applications are not effective. Poison hemlock is a biennial, so it forms a rosette in the fall. Herbicide applications are most effective when they are sprayed in the fall when poison hemlock is in the rosette stage or before it bolts in the spring. Effective herbicides include 2,4-D + dicamba, Crossbow (2,4-D+triclopyr), NovaGraz (24-48 fl oz), or glyphosate as a spot treatment. 

Roughstalk bluegrass management: Roughstalk bluegrass can get up to 3 feet tall or more and is very evident in crop fields now. Once it starts to set seed, no management tactics, especially herbicides, will provide adequate control. This weed is becoming more of a problem in small grain and forage fields across the state. It is a cool-season perennial with short stolons, but it mostly spreads by seed. As it matures, it typically turns reddish-tan and sets seed prior to summer. It then goes dormant until fall and remains green during the winter months. Fall and early spring are the best times to apply effective herbicides. In studies at Michigan State University, Osprey, Axial XL/Bold, and PowerFlex HL provided control of roughstalk bluegrass. However, Osprey applied to roughstalk (1-2 inches tall) in the fall or early spring provided the most consistent control.

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 "set up" 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 weeks or more application window can exist for Canada thistle and quackgrass, because of their cool-season habit of growth.