When discussing weed control in perennial warm-season pastures, the focus is usually on warm-season broadleaf weeds, such as ragweed or broomweed, or warm-season grass weeds, such as sandbur or johnsongrass. However, cool-season weeds that emerge in the fall and grow in the winter and early spring are becoming more problematic in these fields.
One example is annual ryegrass. Ryegrass can be a valuable forage or a difficult weed, depending on your situation. It is generally considered a weed in summer forages in two scenarios. One is horse-quality hay production. If a producer is trying to bale horse-quality hay, and the buyers do not want ryegrass in the bale, ryegrass is definitely a weed in the first and possibly the second cutting. The other situation occurs when fields contain more ryegrass than livestock can consume. Ryegrass is very competitive with perennial summer grass in late April and May. When ryegrass dies, it forms a mat that can prevent sunlight from reaching the ground. This can shade out summer grass, particularly bermudagrass. I have seen quite a few stands of bermudagrass lost to excessive ryegrass competition.
The most effective way to control annual ryegrass in warm-season perennial pastures is to spray a nonselective herbicide in the dormant season. This treatment is not recommended if there are desirable plant species actively growing at that time, such as cool-season legumes. The most commonly used herbicide for this treatment method is glyphosate (active ingredient in Roundup). This herbicide will kill or injure most plants that are green at application time, so all desirable plants must be completely dormant. Ryegrass has developed resistance to glyphosate in some areas. If this is true in your area, paraquat can be used. Take extreme caution when handling paraquat since it can be lethal to the applicator if ingested. It is a good idea to rotate glyphosate and paraquat to prevent resistance from developing, even if resistance is not confirmed in your fields.
Thistles can be very invasive and should be controlled during the winter or very early spring for best results. The glyphosate and paraquat treatments discussed earlier are effective against thistles if they are in the rosette stage (lying flat on the ground). If the thistles have bolted (begun to rise off the ground) or developed seed heads, they are much more difficult to control. Several broadleaf herbicides are effective against thistles if they are in the rosette stage. These include 2,4-D alone; 2,4-D with picloram, dicamba or aminopyralid; metsulfuron methyl; or a combination of metsulfuron methyl, 2,4-D and dicamba. The primary factor in achieving control is to spray before the thistles bolt.
Henbit is a plant that was not generally considered a pasture weed in the Southern Great Plains until the past few years, but now it can be a major competitor with bermudagrass in the early spring. 2,4-D alone is not highly effective against henbit. However, glyphosate in the dormant season; mixtures of 2,4-D and glyphosate; and mixtures of 2,4-D and dicamba, picloram, aminopyralid, and metsulfuron; are quite effective against henbit. Henbit should be sprayed when it is small for best results.
Winter weeds are not a problem in all perennial warm-season pastures and hay fields. Fields should be scouted to determine if a treatment is warranted. In most cases, controlling winter weeds in summer perennial pastures involves an additional application since it is unlikely that an application during the dormant season will control summer weeds. An exception is aminopyralid (sold as Milestone or formulated with 2,4-D and sold as GrazonNext HL). We have conducted research that showed aminopyralid applied in February gave season-long control of western ragweed.
Always read the label before handling, mixing or applying pesticides. Pay particular attention to safety information and follow all recommended safety practices. Remember, the label is the law.