Farm Progress

Check pastures and hay fields for weeds before fall.

August 4, 2017

3 Min Read
CLEAN PASTURES UP: Getting rid of weeds like musk thistle this summer will lead to better pastures next spring.RuudMorijn/iStock/Thinkstock

With hay season mostly over, now is the time to get pastures and hay fields back into condition.

According to University of Missouri Extension agronomy specialist Travis Harper, several things need to be considered, including fertility, avoiding overgrazing, stockpiling, overseeding legumes, and maybe even a complete pasture renovation.

However, Harper finds that there is one management practice that should not be overlooked — weed control. Here are four weeds he says farmers should control in pastures this summer:

1. Musk thistle. Musk thistle is a common weed that nearly everyone recognizes when it’s in bloom. 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, he says.

Unfortunately, there is not a whole lot to do about thistles when they are in bloom. He says spraying at this time does not provide good control. "Mowing may only spread the seed further," Harper adds. However, farmers can cut the flower heads off by hand and destroy them but this is extremely time-consuming and inefficient. Musk thistle control is best achieved by spraying rosettes in the spring or fall.

2. Johnsongrass. Is johnsongrass a weed? 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," Harper says. "It is capable of accumulating high levels of nitrates on fields that have been heavily fertilized."

In the fall, it 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 no selective herbicide options on cool-season grass pastures. Glyphosate used as a spot spray or with a weed wiper is effective, Harper says, but has limited utility.

3. Sericea lespedeza. Sericea lespedeza is rapidly becoming a major problem throughout the region, according to Harper. 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, or when it is in the bud to flowering stages. 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 achieve 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, Harper adds.

Source: University of Missouri Extension

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