Pennsylvania Corn, Soybeans: Midseason Control of Marestail, Palmer Pigweed, Waterhemp

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As we proceed through the growing season, there are a few issues surrounding weed control and herbicide use. Below are some items to consider.

Rescue applications

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.

Herbicide Resistance Info


For a listing of additional herbicides and maximum corn heights and information on maximum weeds sizes for these products please refer to Table 2.2-10 in the Penn State Agronomy Guide or Table 2-15 in the Mid-Atlantic Field Crop Weed Management Guide and check the most recent herbicide label for specific use guidelines. (Or refer to the herbicide label for additional use information at CDMS Corporate Site or the Green Book Data Solutions Site).

Horseweed/marestail in corn:

Control of marestail postemergence in corn is a little easier than in soybean. Certain herbicides that provided adequate control in some recent Penn State preliminary trials included: dicamba (5 oz Status-96% control; 8 fl oz Clarity-86%); 36 fl oz Liberty 280-92%; or a tank-mix of glyphosate + Callisto (3 fl oz) + atrazine (1 pt)->85% control.

Less effective herbicides included: 1 pt 2,4-D-75%; 2 pt Basagran-50%; and 0.9 fl oz Cadet-<20%. In general, these treatments were applied when marestail averaged 8 inches tall. For best results, control marestail when it is less than 6 inches tall.

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, Univ. 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.

Palmer amaranth and waterhemp in soybean:

We are receiving some reports about Palmer amaranth growing rapidly and blanketing soybean fields in some parts of the state. Some of these fields did not receive a PRE/residual herbicide application and Palmer is now over 4 inches tall. The best management option in this situation is to start over, otherwise adequate control of Palmer or waterhemp will not be obtained.

Consider terminating the entire field and replant with a LibertyLink variety to allow for additional herbicide options. Make sure to start clean and include a residual at planting followed by additional residual herbicides mid-season. Review the website for additional details on what herbicides to consider.

Also, 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. Once Palmer or waterhemp reaches 4 inches tall they are very difficult to control.