Several calls have come in lately about a “new” weed that hay producers are seeing. It is not new, but it may be new to them. Thistles are winter annuals or biennials depending on the particular species you have. There are several different thistles that we see in our area; bull thistle, milk thistle, and yellow thistle to name a few. They can be tricky to control in pastures and hayfields. Because they are easiest to control when they are small or in the rosette stage. During the late winter to early spring as they start to grow, we often overlook them. Once they bolt up and begin to flower in late spring to early summer we notice them and at this stage and size they are harder to kill.
Here are a few notes on thistle control from Patrick McCullough our Extension Weed Scientist:
Regular mowing is recommended to suppress the growth and competition of established thistles, particularly at bolting or flowering stages, with pasture grasses. Mowing in late winter and spring can prevent seed production and disrupt the reproductive cycle. Mechanically suppressing thistles will not completely control established plants.
In perennial grass pastures, apply 2,4-D at 1 qt per acre for controlling thistles at the rosette stage. Apply 2 qt per acre of 2,4-D if thistles are in the bolting stage. Fall applications are recommended over winter or spring timings if possible. Other Group 4 herbicides that will control thistles include aminopyralid (Milestone, others), triclopyr (Remedy, others), and combination products containing these active ingredients (Grazon Next, others). In bermudagrass pastures, metsulfuron (Cimarron, Escort, others) will control thistles in the rosette stage but may require repeat applications or tank-mixtures with other herbicides for controlling thistles at the bolting or flowering stages.