It's that time of year when many pastures in Missouri become dominated by annual grass weeds like crabgrass and yellow foxtail. In the past few seasons, we have been finding another, much less common foxtail species called knotroot foxtail (Figure 1). Knotroot foxtail [a.k.a. knotroot bristlegrass or perennial foxtail (Setaria parviflora)] looks very similar to yellow foxtail. It has a short hair-like ligule (Figure 2) and a typical foxtail seedhead (Figure 3), but unlike yellow foxtail this species has short, knotty rhizomes (Figures 4 and 5) and is a perennial.
Although it is known that this species has been present in Missouri for some time, historically I have not encountered it before in Missouri pastures, or ever received any calls about it from growers or consultants. In the past two seasons, we have identified knotroot foxtail populations that were sent to us from Bates, Cape Girardeau, and Moniteau counties. However, according to the Flora of Missouri, its distribution is across the state in approximately two-thirds of Missouri counties.
As with just about any other perennial grass weed that we encounter in tall fescue pastures, the herbicidal options for the control of knotroot foxtail are very limited. Very little information is available and/or very little research has been conducted on the control of this species, and what has been done indicates that selectively removing this grass from tall fescue stands after it has already emerged just isn't possible with our current herbicide options. Glyphosate products (Roundup, etc.) can be used as a spot-treatment on knotroot foxtail, but any contact of glyphosate with the desirable grass or legume forages will likely kill those species as well. Some weed scientists in Georgia found that spring applications of Prowl could provide some control of knotroot foxtail plants emerging from seed, but this treatment will not provide any control of perennial plants sprouting from rhizomes.
For growers who have only sporadic infestations of this grass, probably the most effective method of eradication will be mechanical removal of the entire plant and rootstock, or spot treatment with glyphosate, preferably before viable seed are produced. Producers should also consider cultural control methods that will help prevent the establishment of knotroot foxtail in pastures. These practices include maintenance of a thick, competitive forage through proper pH, fertility, and grazing management. Additionally, it is important to monitor fields, fencerows, rights-of-way, and roadsides for the presence of this grass and avoid moving equipment, vehicles, or livestock through any fields when seedheads are present. And finally, be sure to check any hay that you may have purchased as it may be contaminated with knotroot foxtail seedheads.
If you encounter this species on your land, we'd be glad to know about it so that we can better understand the distribution of this species in Missouri.