By Dwight Lingenfelter
Milkweed and hemp dogbane are very aggressive perennials that are difficult to control. In forages, hemp dogbane is considered toxic to animals in both the fresh and dry forms. The seedlings of common milkweed and hemp dogbane germinate initially from seeds, but once the plants are established, they can emerge from adventitious buds on creeping roots or rhizomes. Infestations in crops typically arise from regrowth of established vegetative propagules beneath the soil, which have been reported to form within a month after seedling emergence. Mechanical control by repeated removal of top growth (e.g., mowing) is possible where cultural practices allow. Mowing weedy areas before seeds ripen prevents milkweed and dogbane from setting seed and spreading. As far as herbicides, none are that exceptional, including glyphosate. In general, they only provide about 60-80% control depending on the product, rate, and application timing. In grass pastures/hay, a combination of routine mowing during the growing season and a herbicide application, especially in the fall, is best. 2,4-D (1-2 quart/acre) + Clarity/dicamba (1-2 pint/a) or Crossbow (3-4 quarts/A) are the best options in that setting. Starane Ultra (fluroxypyr) has activity on hemp dogbane but is weak on milkweed. Adding a surfactant and liquid nitrogen or AMS can improve weed control. Keep in mind, it is best to allow perennial weeds to regrow about 12-15 inches or so (usually about a month after cutting) before a herbicide application. This allows adequate foliage to be present to uptake a critical amount of herbicide and to optimize the herbicide's translocation into the root system. Therefore, it is critical to appropriately manage the late season mowing so there is enough time for regrowth and a systemic herbicide application before the weeds start to go dormant. Spring herbicide applications are possible, but milkweed must be at least 12 to 15 inches tall for control. Spring applications must be followed by routine mowing and a fall herbicide application for best control. When possible, glyphosate may be spot applied to manage these weeds.
Another option is to use a sponge/ropewick applicator. With this tactic the herbicide is wiped across the leaves and stems of weeds. Wiper applicators (weed wipers) are devices that physically wipe a concentrated solution of herbicide directly onto weeds by taking advantage of sufficient height difference between the weeds and crop canopy. The height difference in pasture can be increased by grazing the forage prior to wiper application and allowing some days for trampled weeds to recover. Sufficient contact from the wiper applicator while preventing contact with the crop controls weeds selectively is the goal. Since only the top several inches of the weeds are contacted with herbicide solution, a translocated herbicide such as dicamba or glyphosate is usually required for effective control. Repeat applications may be required to manage certain perennial weeds. A healthy actively growing crop that can develop a canopy over suppressed weeds is important to the success of wiper applications. There are several types of wiper applicators on the market. Wiper applicators must be designed and operated so that the rope, roller, sponge, or panel remains moist enough to transfer a sufficient amount of herbicide to the weed while not allowing drips to contact the crop. Keep in mind, university data shows that wiper applications have not been as effective as broadcast or spot spray applications on perennial weeds in pastures. However, they can provide some necessary suppression of weeds if spraying is not possible. The two most common types are wick and roller applicators.
Wick applicators use short braided ropes or appropriate fabric to "wick" the herbicide solution from inside a pipe to the plants that are wiped. With roller applicators, herbicide is manually sprayed intermittently onto a carpet-covered roller that is rotated with a motor or a ground-driven shaft. The main advantage of the roller applicator is that more herbicide solution can be maintained on the roller because the centripetal force from the rotation inhibits dripping. Compared to wick applicators, roller applicators provide consistent weed control. Bidirectional application is recommended to provide thorough control. Herbicides labeled for wiper application will usually provide specific application instructions on the label. In addition, research trials have indicated that a 5 percent solution applied in a wiper applicator is somewhat comparable to a broadcast application rate of 1 quart per acre. Or use a mix ratio of 1/3 glyphosate to 2/3 water in the tank or this can be tankmixed with Crossbow, 2,4-D, or dicamba.
In conclusion, since milkweed or dogbane populations likely have been growing for many years in a field, it will take a few years or more to get it under control with these repeated measures. Therefore, effective control in grass hay/pastures is primarily based on routine mowing and timely systemic herbicide applications. But over time these tactics can drastically reduce or eradicate these weed populations.
Source : psu.edu