Infestation of Italian ryegrass (Lolium multiflorum, a.k.a. annual ryegrass) is a serious problem in tall fescue sod production. Unsightly infestations decrease marketability of tall fescue sod (Figure 1). The Virginia Handbook for Seed Certification Standards states that certified tall fescue and Kentucky bluegrass sod in Virginia must have less than one weed per 100 square feet. As you may know certified tall fescue and Kentucky bluegrass seed has an allowance for other crop seed, including Italian ryegrass. Planting in the fall and harvest the next spring or fall helps producers avoid some basic weed problems. Italian ryegrass and tall fescue have very similar growth patterns, making selective control difficult. Producers can hope for Italian ryegrass to die from summer heat but in Virginiaís cool climate this usually occurs in late summer. By that time Italian ryegrass carcasses will leave behind thin areas of turfgrass with poorly developed roots that cause sod to rip and tear during harvest. Since selective herbicides arenít available to control Italian ryegrass in fescue, Virginia Tech researchers evaluated several possible products on turfgrass production fields in Virginia.
Overview of selected herbicides:
Diclofop and fenoxaprop are currently labeled for use in cool season grasses, however not in tall fescue. Fluazifop and aryloxyphenoxy propionate are herbicides that are used for selective grass control, therefore hold potential for Italian Ryegrass control in tall fescue. Chlorsulfuron, nicosulfuron, and primisulfuron are ALS (acetolactate synthase inhibiting) herbicides that are used to control annual grasses in crops. ALS herbicides are slow acting herbicides that tend to stunt and discolor susceptible plants. Metsulfuron (Manor) is another ALS herbicide that is registered for use in fine fescue and Kentucky bluegrass, however not in tall fescue. While the above products could hold potential for controlling Italian ryegrass they could also severely injure tall fescue in the process. Turfgrass injury in sod production is of little consequence given that the turfgrass fully recovers before cutting and sale. Since several herbicides used for Italian ryegrass control in crops exhibit only marginal safety in tall fescue.
Three studies were conducted in the spring following fall seeding of tall fescue. Sites were chosen based on Italian ryegrass infestation levels. Two locations were actual sod farms; special thanks to Enfield Sod Farm (ESF) in Aylett, VA and Virginia Turf Farms (VTF) in Baskerville, VA, and the third the Turfgrass Research Center (TRC) in Blacksburg, VA. Herbicides and rates as well as percentage Italian ryegrass control and tall fescue injury are listed in Table 1.
The TRC location was mown at 2.75 inches compared to 3.5 inches at ESF and VTF locations. Mowing heights may account for some of the variability in Italian ryegrass control and tall fescue injury between locations. Primisulfuron is a product that was thought to have great potential for Italian ryegrass control while not injuring tall fescue, but control in our trials was highly variable. Nicosulfuron at rates higher than 1.67 ounces active ingredient per acre controlled Italian ryegrass greater than 90% at the two sod farm locations and greater than 50% at TRC. Turf injury was severe for some treatments, in some instances lasting full season. Nicosulfuron, which had best control, had less than 35% turf injury season long, and injury was not significantly different from check plots by sod cutting time. The other products tested did not provide consistent control of Italian ryegrass, or had unacceptable injury of tall fescue.
Results from these experiments suggest that Italian ryegrass control in tall fescue and Kentucky bluegrass sod production can be achieved with nicosulfuron more often and with less injury to turf than other products tested. Nicosulfuron at any rate will injury tall fescue, however sod should recover within 10 weeks. Primisulfuron is much less injurious to tall fescue but inconsistent for Italian ryegrass control.