Ornamental Plants -- Annual Reports and Research Reviews 1998

Special Circular 165-99

Postemergence Herbicide Efficacy on Crabgrass: 1997

John R. Street and Renee M. Stewart

Introduction

Postemergence crabgrass (Digitaria spp.) control for many years was primarily limited to organic arsenicals (MSMA, DSMA), which:

• Require repeat applications for effective postemergence crabgrass control.
• Are most efficacious on seedling crabgrass.
• Can cause some phytotoxicity to desirable turf grasses, especially in hot weather.

Acclaim has shown good-to-excellent efficacy for postemergence crabgrass control. However, some discoloration and stunting of Kentucky bluegrass may occur, and efficacy drops off under droughty (dry) soil conditions. Acclaim Extra, the new isomer of Acclaim, provides similar efficacy to Acclaim. Dimension also provides early postemergence crabgrass control.

Methods

Various herbicides and rates were evaluated for postemergence crabgrass control on an established stand of Kentucky bluegrass. The trial area was verticut in mid-April 1997 and over-seeded with one pound of crabgrass seed per 1,000 square feet (ft²). In July 1997 herbicides were applied to crabgrass at two different stages of growth -- early postemergence in the three- to five-leaf to one-tiller stage (Table 1) and late postemergence in the two- to four-tiller stage (Table 2).

Irrigation

Each herbicide treatment in both growth stages was irrigated:

• Within one hour after herbicide application (watered in).

Or:

• Irrigation was delayed for several days (not watered in).

All liquid applications were made with a CO₂-pressurized sprayer at 88 gpa.

Mowing Height

The Kentucky bluegrass stand was maintained at a mowing height of 0.75 inches until three weeks prior to herbicide treatment. A mowing height of 1.75 inches was maintained for the remainder of the postemergence study.

Nitrogen Application

An annual total of three pounds of actual nitrogen per 1,000 ft² was applied during the growing season. Irrigation was provided several times per week throughout the season to encourage crabgrass germination and seedling development. Treatments were replicated three times in a randomized complete block design with a plot size of 3' x 8'. Treatments were monitored for crabgrass control (percent crabgrass cover) at periodic intervals (seven to 10 days) after herbicide application. (See Tables 1 and 2 for information on early and late postemergence crabgrass control, respectively.)

Discussion

Acclaim

In previous Ohio State University research, Acclaim has exhibited good-to-excellent efficacy for postemergence crabgrass control up to the three- to four-tiller stage where soil moisture was adequate and crabgrass was not under stress. Efficacy has been found to drop off dramatically under droughty (dry) soil conditions, sometimes causing erratic and variable results in the field. Efficacy is significantly reduced when Acclaim is used in combination with phenoxy herbicides, such as 2,4-D.

Acclaim is a foliar-absorbed herbicide, making adequate foliar coverage essential. This entails the following:

• Mowing prior to treatment to open the canopy for maximum contact of liquid spray with crabgrass foliage, and
• Spraying with sufficient water volume and proper nozzles to assure good foliar coverage.

The authors' research showed stunting and discoloration of Kentucky bluegrass may occur after most applications during the early season, when bluegrass is rapidly growing. This research also showed that Acclaim applications severely discolor bentgrass. As a result, the authors recommend light, multiple applications (see the label). Iron and/or nitrogen will help mask the discoloration symptoms without any negative effect on efficacy. The recommended rate range for Acclaim is 0.12 to 0.25 lbs. ai/A (active ingredient per acre), the higher rate targeted for more mature crabgrass.

The authors' research showed Acclaim's activity rate to be moderate: Crabgrass kill typically occurs in two to three weeks. Three to five days after herbicide application, crabgrass turns orange or purple, a color response that lasts for seven to 10 days. Eventual browning and necrosis occur over a total period of two to three weeks.

Acclaim Extra (fenoxyprop p-ethyl)

This new isomer formulation of Acclaim also provides good-to-excellent control of crabgrass in postemergence treatments at rates ranging from 0.06 to 0.125 lbs. ai/A. In 1997, Acclaim Extra provided good-to-excellent efficacy of crabgrass both early (Table 1) and late (Table 2) postemergence at 0.06 and 0.09 lbs. ai/A rates. More consistent and reliable control of mature tillered crabgrass occurs at the upper end of the rate recommendation range of 0.06 to 0.125 lbs. ai/A. The fenoxyprop herbicides are foliar-absorbed materials. Therefore, it is not surprising that irrigation shortly after application significantly reduces efficacy (see Tables 1 and 2, Watered In column). The authors' research showed no noticeable difference in the rate of crabgrass kill between Acclaim 1EC and Acclaim Extra.

Preclaim

(3.09 EC; American Cyanamid premix formulation of Acclaim Extra and pendimethalin)

In general, Preclaim has provided similar efficacy to Acclaim Extra alone for postemergence crabgrass control. As a premix formulation of pre- and post-herbicide products, it provides a wider window for preemergence herbicide applications in the spring. In 1997 Preclaim provided good-to-excellent efficacy on crabgrass in both the early and late postemergence trials at rates of 2.06 to 3.09 pounds ai/A (Tables 1 and 2). Preclaim activity at the 1.545 lbs. ai/A rate was good and fair/poor in the early and late postemergence stages, respectively. When watered in shortly after herbicide application, Preclaim activity for postemergence crabgrass control was significantly reduced.

Dimension (dithiopyr)

Dimension is a relatively new commercial herbicide that exhibits both pre- and postemergence herbicide activity on crabgrass. Dimension, therefore, provides a wider window for application of preemergence herbicides in the spring. Postemergence activity of Dimension is slow. Total kill typically ranges from three (untillered crabgrass) to five weeks (tillered crabgrass). Dimension does, however, stunt crabgrass in 10 to 14 days, making its presence in the turfgrass canopy less noticeable. The stunted crabgrass is initially hidden within the canopy, then eventually dies over a period of three to five weeks. During the stunting phase, crabgrass turns yellow, then purple, and is followed by necrosis.

In 1997, Dimension 2EC provided good to excellent early postemergence control of crabgrass at the 0.38 to 0.50 lbs. ai/A rates (Table 1). Dimension 0.164FG (AD 445) also provided good to excellent early postemergence crabgrass control at the highest rate of 0.5 lbs. ai/A. Lower rates of Dimension 0.164 FG were not as efficacious as equivalent rates of Dimension 2EC.

The authors' past research has shown that Dimension EC, G, and FG formulations generally do not differ in postemergence crabgrass efficacy, except for a slightly slower activity rate with G and FG formulations. In 1997, all formulations and rates of Dimension exhibited significantly poorer late postemergence crabgrass efficacy compared to early postemergence activity (Tables 1 and 2). These efficacy results are consistent with efficacy data from past early and late postemergence trials at Ohio State University. These results characterize Dimension as a good to excellent early postemergence crabgrass herbicide and a poor late postemergence crabgrass herbicide.

Root absorption is considered to be the principle uptake mechanism for the preemergence activity of Dimension. It was hypothesized that irrigation shortly after Dimension application to move the herbicide to the soil root zone might improve either the overall efficacy and or the rate of activity. Irrigation did not appear to have any significant negative or positive effect on the overall activity or efficacy of Dimension postemergence (Tables 1 and 2).

Combinations of Dimension with Daconate (MSMA) in previous Ohio State University research have been shown to increase the rate of crabgrass kill and to enhance efficacy. Dimension 1EC rates of 0.25, 0.38, and 0.30 lbs. ai/A in combination with 1.0, 0.50, and 0.25 lbs. ai/A rates of Daconate 6F, respectively, have proved effective in enhancing the efficacy and rate of Dimension activity. Dimension in combination with Daconate is not extremely effective beyond the three- to four-tiller stage. It may cause stunting of crabgrass, but kill may not occur.

Drive (quinclorac)

Drive is an excellent postemergence crabgrass herbicide. BASF continues to have an interest in acquiring a turfgrass label for this product. Liquid and granular formulations of Drive will be evaluated in 1998. Previous Ohio State University research has shown that Drive typically kills crabgrass in seven to 10 days or less at rates of 0.25 to 0.75 lbs. ai/A. Drive has also provided excellent efficacy of mature crabgrass at rates of 0.5 to 0.75 ai/A. Unlike Acclaim, Drive efficacy does not appear to be as sensitive to soil moisture.

In 1997, Drive exhibited good-to-excellent efficacy on crabgrass in the early postemergence stage (Table 1) and excellent efficacy in the late postemergence stage at rates of 0.50 and 0.75 lbs. ai/A. Crabgrass browning and kill occurred within a week after herbicide application. Drive activity rate was similar between early and late postemergence applications. It is important that an adjuvant (0.50% by volume) be used with the Drive 75 DF formulation. These results on both efficacy and activity rate are consistent with Drive research at Ohio State for the past five years. Drive is considered to have both foliar and root activity and is used in some crops preemergence. Surprisingly, Drive efficacy was significantly reduced where irrigation occurred shortIy after herbicide applications (Tables 1 and 2).

Table 3 provides a ranking of the relative activity rate and specific activity rate of the four postemergence crabgrass herbicides discussed based upon five years of Ohio State University research.