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Weed Science

Safety of Preemergence Herbicides on Ryegrass

Summary:

1.  Chemical treatment which totally killed overseeded perennial ryegrass of all ages and at all rates:  Sencor® 75 (postemergence herbicide, not labeled for cool-season grasses).
2.   Chemical treatments which were moderately injurious to overseeded perennial ryegrass:  Surflan® A. S. (not labeled) and Pendulum® WDG (labeled with warnings), both preemergence.
3.   Chemical treatments which caused little or no injury on older (46- and 67-day) overseeded perennial ryegrass:  Pendulum® WDG and Dimension® (labeled with sufficient warning on age of grass), both preemergence.

4.   Chemical treatment which caused no injury on older (46- and 67-day) overseeded perennial ryegrass:  Drive® 75DF (labeled with warning on age of grass), postemergence.

5.  Chemical treatment which caused little or no injury on perennial ryegrass of all ages: Prograss® EC (labeled with warning), postemergence herbicide.

Methods

A thin 'Tifgreen' (328) bermuda was overseeded on various dates in strips with Lesco Double Eagle Blend perennial ryegrass, at 8.5 pounds per thousand square feet.  Research was conducted at University of Florida Fort Lauderdale Research and Education Center.  Dates of overseeding were 27 Dec. 1999, 17 Jan. 2000, 1 Feb. 2000, and 12 Feb 2000 which were 67, 46, 31, and 20 days, respectively before herbicide treatments on 3 Mar. 2000.  On the day of herbicide treatment a dense stand of perennial ryegrass covered all areas, except the 20-day-old ryegrass was thin.
Treatments were made with a 4-nozzle-boom, hand-held CO2 -pressurized (40 psi, at the central distribution tube upstream from the boom) backpack sprayer using nonoverlapping matched precipitation even flat fan nozzle tips, two TeeJet 9502EVS (even flat spray tips) for the center two nozzles, and two TeeJet UB8501SS (underleaf banding spray tips) for the outside two nozzles. Nozzles were 30.5 cm (12 inches) apart on the boom and with TeeJet 4193A strainer (100-mesh screen) and check valves, delivering an average of 36.2 ml per second for the boom.  Walking speed was 3 miles per hour, and the total effective spray swath was 37.25 inches, thus spray volume was 30.5 gallons per acre.

Herbicides were mixed accordingly to provide the following rates of application:  Dimension® 1.5 fluid ounces per 1000 square feet; Barricade® 65WG, 1.125 pounds per acre; Pendulum® WDG, 2.5 pounds per acre; Prograss® EC, 3 fluid ounces per 1000 square feet; Sencor® 75 0.333 pounds per acre; Surflan® A. S. T/O 1 ounce per 1000 square feet; and a water-only "untreated control" (UTC).  The rates were typical rates that were labeled for use in turf, but several of the products are not labeled for use on perennial ryegrass.  The Sencor rate was 0.5 the maximum label rate.  For purposes of discussion here, however, the preceding rates are considered the "1 x" rate.
Applications were made at 7:30 am on 3 Mar. 2000, on a sunny day with little breeze.  There was no rain for 24 hours after treatment.  Plots were evaluated for percent ryegrass cover on 17 Mar. and 3 April 2000.  Herbicide injury was calculated as the average reduction from 100%, averaged across the two dates of evaluation  (Table 1).

Results

The range of injury varied from 0% for the untreated control (UTC) to 100% for Sencor® (metribuzin).  Triazine herbicides such as Sencor® are known to be highly injurious to cool-season grasses, so this was not surprising.  While there was no replication in this experiment, it was analyzed by analysis of variance with the three-way interactions (chemical x rate x days) serving a pooled error.
Besides the varying effects of different chemicals, age of grass was extremely important in predicting injury, even more so than the rate of application.  Averaged across the products, the more days since overseeding, the less was the injury, with the biggest jump between 31 and 46 days, and very little difference between 46 and 67 days.
Prograss® (ethofumesate) was the least injurious product, showing 10% injury when applied at the 2 x rate on 20-day-old grass.  Drive® 75DF (quinclorac) showed unacceptable injury at all rates of application to grass that was 20- and 31-days-old, but showed 0% injury to 46-day and older grass.
The preemergence herbicides were surprising.  Barricade® 65WG (prodiamine), which was quite injurious in another experiment to perennial ryegrass planted after chemical treatment, was safe on the oldest 67-day-old ryegrass,even at the 2 x rate.  This was approximately the same level of phytotoxicity as Dimension® (dithiopyr).  In contrast, Surflan® A.S. (oryzalin) and Pendulum® WDG (pendimethalin) were moderately to highly injurious to ryegrass at all rates of application and all days since overseeding, with the exception of Surflan® at the 0.5 x applied to the older (46- and 67-day-old) ryegrass.

How good are the labels?

The labels on the herbicides were sufficiently cautious to have prevented injury.  The label for Prograss® EC Herbicide (ethofumesate) says the product may be applied "to overseeded bermudagrass in late fall, one to two weeks after emergence of overseeded perennial ryegrass to control annual bluegrass."  The label on Drive® 75 DF says that perennial ryegrass is highly tolerant, and Drive® 75DF can be applied to it, but "do not apply within 4 weeks after seedling emergence of perennial ryegrass."
The label for Barricade says, "do not apply to overseeded turf within 60 days after seeding or until after the second mowing, whichever is longer."  The label on Dimension® is similar, "the grass must have a good root system and a uniform stand, and have received at least two mowings following its seeding . . . ."
The Pendulum WDG label says, "applications made to overseeded warm-season turfgrasses may cause thinning or injury of the overseeded species."  The label for Surflan AS says, "in bermudagrass areas that have been overseeded with winter grasses, a spring application of Surflan AS will thin the overseeded grasses, and elsewhere it says, "do not apply Surflan AS to cool season turfgrass species."  The label for Sencor 75 Turf makes it clear that the product is only for bermudagrass turf.

These results were obtained at a time of year when perennial ryegrass dies naturally, so they probably portray a worse-case scenario for injury than would normally be encountered in the field.