Summer Weeds of Pastures and Hayfields


Now that hay season is (mostly) over, it’s time to start thinking about getting pastures and hayfields into the best possible condition going into the summer and fall. 

Several things need to be considered: fertility, avoiding overgrazing, stockpiling, overseeding legumes, and maybe even a complete pasture renovation.  One management practice that should not be overlooked is summer weed control. 

1. Musk Thistle – Musk thistle is a common weed that nearly everyone recognizes when it’s in bloom this time of year.  It’s also the weed that extension specialists receive the most calls about, typically from individuals complaining that their neighbors are not doing anything to control the weed.  Unfortunately, there’s not a whole lot to be done about thistles when they are in bloom. Spraying at this time does not provide good control.  Mowing may only spread the seed  further. Once option is to cut the flower heads off by hand and   destroy them but this is extremely time consuming and inefficient.  The best thing for this time of year is to hope that the musk thistle weevil is working on the plant.  Musk thistle control is best achieved by spraying rosettes in the spring or fall. 

2. Johnsongrass – Is johnsongrass a weed or isn’t it?  Early in the spring, when johnsongrass is young, it has a forage quality similar to that of tall fescue.  As it matures, it becomes more of a problem.  It is capable of accumulating high levels of nitrates on fields that have been heavily fertilized. In the fall, in can produce high levels of prussic acid when stressed due to freezing temperatures.  It is also an aggressive spreader that can quickly take over larges sections of pastures. Unfortunately, there are not selective herbicide options on cool-season grass pastures.  Outrider is often mentioned as an option but there are specific label restriction regarding its us on certain forages. Glyphosate used as a spot spray or with a weed wiper is effective, but has limited utility.

3. Sericea Lespedeza – Sericea lespedeza is rapidly becoming a major problem throughout the region.  A single plant has dozens of stems and each of those stems can produce up to 10,000 seeds, making a small problem this year a big problem in future years. There are two times when sericea lespedeza can be effectively controlled with herbicides: when it 12 or more inches in height (June) or when it is in the bud to flowering stages (late August to early September).  Sericea lespedeza should not be sprayed when the plant is under drought stress, as the herbicides will not be effective.  Seed in the ground will make multiple years of spraying a necessity to achieving good control. 

4. Poison Hemlock – Poison hemlock is not a summer weed, but it is a weed that was especially prevalent this past spring.  Much like musk thistle, poison hemlock is a biennial and is best controlled by spraying the rosette in the fall or early spring. Poison hemlock is of somewhat greater concern than musk thistle because of its toxic properties. Ingestion of a relatively small amount can easily kill a cow.  Grazing animals are unlikely to selectively consume the plant in a pasture setting, as long as other forage is available. However, they will eat it in a bale of hay and the plant remains toxic long after the hay has been put in the barn. 

A lot of farmers will tell me that they don’t want to spray their pastures because they don’t want to kill their legumes.  If weed pressure is severe enough, I tell them not to worry about the legumes as they are relatively cheap and easy to reestablish.  However, depending on the herbicide that was used, there may be plant-back restrictions of several months up to a few years for legumes and other forage species.

 For more information, see MU Extension’s Pasture Weed and Brush Control Guide or contact your local county extension office.

Source: Travis Harper, Agronomy Specialist