Smutgrass is a real problem weed in pastures in our area. Sporobolus poiretii is the name that botanists know it by. Smutgrass is a perennial plant, meaning that it re-grows from the roots each year. It is an erect, bunch forming grass with narrow, pointed leaves that are rough to the touch. These bunches or clumps can reach three feet in height. It gets the name “smutgrass” from the seeds that it produces. These seeds are borne on a stem and are often infected with an ergot fungus that turns the seed black in color. These tiny, black seed give the impression that the stem is covered with “smut."
The seeds are one characteristic that make the plant so much of a problem. It produces many thousands of them. They will stick to animals and humans that brush against the plant and are transported from one part of the pasture to the other.
The rough leaves are also a problem. Because of this and other characteristics, livestock will not eat the plant after it reaches any height at all. It grows very fast in the spring. Like all plants, smutgrass has to compete with other plants for sunlight, water, nutrients and space. If the livestock eat the other plants in the pasture and leave the smutgrass alone, it has an advantage and will continue to grow and spread. This selective grazing is most evident with animals that feed close to the soil surface such as sheep and horses. Cattle will also graze pasture “into-the-ground” if they are short on forage due to drought or overstocking. Smutgrass always seems to be worse in pastures with more livestock than the grass can support. Even though this may be a temporary situation, once the smutgrass gets established it will continue to spread even if the pasture conditions improve or the stocking rate is corrected.
How do you control this weed? Not very easily or well, I am afraid. Because it is a perennial plant, smutgrass does not respond well to mowing. It will simply produce more tillers, re-grow and the clump will become larger. There are limited herbicides available for its control. Velpar is labeled for smutgrass control. Velpar is not inexpensive. It also must be applied correctly and under the right conditions to effectively control the smutgrass. Most of the herbicidal action of the Velpar is due to root uptake by the smutgrass. For proper root uptake, there must be adequate soil moisture at the time of application. It is also beneficial to receive a light to moderate rain within a few days of the Velpar application. The rain will move the herbicide into the soil zone where it will be absorbed by the roots. Velpar also is most effective on rapidly growing smutgrass. If the smutgrass is stressed by drought or other weather conditions, the herbicide will be less effective. Velpar will also injure the Bermuda grass or other summer forages that you want to grow. Recently, label restrictions for grazing following an application of Velpar have changed. Be sure to follow all directions and restrictions for any pesticide you apply. Some producers have had very favorable results with Velpar. Others haven’t. I believe that the main reason for this inconsistency is due to the weather conditions at the time of its application and immediately after application.
Round Up (or other glyphosate herbicides) will kill smutgrass. Unfortunately, they will also kill the desirable grasses that are growing in the pasture. If you “spot spray” the smutgrass clumps with glyphosate, you will kill the entire spot, leaving a bare area. Often other weeds will emerge in the bare spot and you have only traded one weed for another.
One cultural practice, plowing, can be effective. Again, the environmental conditions must be right for plowing to work well. If the smutgrass is plowed in a period of dry weather, the roots will desiccate and the clump will die. The soil must be dry or the smutgrass clump will merely re-root and begin to grow again. If this can be accomplished in the late summer or early fall, ryegrass can be planted into the pasture. The ryegrass will emerge, grow through the winter, provide grazing for livestock and prevent the smutgrass from emerging in the early spring. Ryegrass is very competitive with other grasses in the spring. Until it matures in May or June, no other grasses will emerge. If properly timed and with cooperative weather, the fall plowing-ryegrass establishment method is very effective and economical.
Smutgrass is a very costly pasture weed, but with proper technique and good luck, it can be managed over time.