Bermudagrass control: Basamid treatment regimens to replace methyl bromide

Researchers in North Carolina and Texas have been testing the efficacy of treatment regimens for bermudagrass control that could be used as alternatives to methyl bromide.

^†Day 0 applications initiated May 25, 2015 in Raleigh, N.C., and June 15, 2015 in College Station, Texas
^‡Fluazifop (0.4 pound/acre) and glyphosate (3 pounds/acre)
Abbreviations: DAZ, dazomet; FLU, fluazifop; GLY, glyphosate; fb, followed by; NC, North Carolina; TX, Texas
Table 1. Dazomet treatment descriptions and incorporation methods for field studies conducted in Raleigh, N.C., and College Station, Texas.

Matthew D. Jeffries, M.S.; Travis W. Gannon, Ph.D.; Fred H. Yelverton, Ph.D.; and W. Casey Reynolds, Ph.D.

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Bermudagrass (Cynodon spp.) accounts for approximately 33% of managed turfgrass areas on golf courses in the United States (9). Because of its superior drought, heat and traffic tolerance, coupled with fine texture, dense canopy, and excellent performance at low mowing heights, it is widely used on golf course fairways and tees (11). Despite these positive attributes, bermudagrass areas are sometimes renovated, at which point the grass’s hardy, perennial growth habit can become a problem because it may contaminate the successive stand (4). Contamination from previously planted grasses can adversely affect aesthetics and functionality of the newly established turfgrass.

For decades, methyl bromide was the standard for fumigating turfgrass areas before renovation because it was very effective at controlling soil-borne fungi, insects, nematodes and weeds. Methyl bromide, however, is classified as a Class I ozone-depleting substance and was phased out for use in turfgrass systems in the early 2000s (5,12). Consequently, turfgrass managers today rely mainly on the nonselective herbicide glyphosate (Roundup, Monsanto) for bermudagrass control. Although control with glyphosate is possible, research has shown multiple applications are required over a period of months (8). The two- to three-month treatment period is not optimal on golf courses for various reasons, including revenue loss and reduction of the establishment period of a subsequent warm-season turfgrass.

Dazomet (Basamid 99 G, Amvac) is a granular soil fumigant designed to be soil-incorporated via mechanical equipment and/or water. Following incorporation, dazomet reacts with water to produce methyl isothiocyanate, a gas that controls various fungi, insects, nematodes and weeds (13). Under favorable conditions, methyl isothiocyanate formation is rapid (soil half-life < 24 hours), at which point it can volatilize and escape from the soil profile, reducing control (6,10). For this reason, application practices aimed at reducing atmospheric losses are recommended to end users.

Incorporating dazomet in the soil via cultivation is recommended for agricultural uses, but this practice is not desirable on wide-scale turfgrass renovations and can be problematic on certain topographies (for example, steep slopes). Additionally, incorporating dazomet into the soil via cultivation alone may provide unacceptable bermudagrass control (13). Previous research has shown dazomet efficacy may be improved if the treated area is irrigated and/or tarped to reduce volatility losses (10). Improved bermudagrass control has also been documented when glyphosate was applied before dazomet. Hybrid bermudagrass [Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt-Davy] cover 15 weeks after treatment was reported to be 71% when dazomet was applied alone, and 6% when glyphosate was applied at 4 pounds a.i./acre (4.5 kilograms a.i./hectare) two weeks before dazomet application (4).

Figure 1. Common bermudagrass (Cynodon spp.) cover at eight and 46 weeks after initial treatment (WAIT) in Raleigh, N.C. Different lowercase letters indicate significant differences among treatments at 8 WAIT; and different uppercase letters indicate significant differences among treatments at 46 WAIT. DAZ = dazomet; FLU = fluazifop (0.4 pound/acre); fb = followed by; GLY = glyphosate (3 pounds/acre).

Although previous research has shown that pretreating with herbicides, irrigation, tillage and tarping can improve bermudagrass control following dazomet application, minimal effort has been made to evaluate all of these practices concurrently. Because implementing such practices incurs additional expenses associated with product costs and labor, effective and efficient programs for carrying out these practices need to be identified. Furthermore, tarping and tillage can increase dazomet exposure to workers and are not feasible in some scenarios (2). The objective of this research was to determine the impacts of various combinations of pretreatment herbicides, irrigation, tillage and tarping on bermudagrass control following dazomet application.

Materials and methods

Site descriptions

Field experiments were initiated May 25, 2015 at the North Carolina State University Lake Wheeler Turfgrass Field Laboratory in Raleigh, N.C., and June 15, 2015 at the Texas A&M University Turfgrass Field Lab in College Station, Texas. The soil type in North Carolina was a Cecil sandy clay loam, measuring 6.4 in pH and 1.9% w/w organic matter. The soil type in Texas was a Zack fine sandy loam, measuring 6.2 in pH and < 1% w/w organic matter. At both locations, research was conducted on established common bermudagrass maintained at a 2-inch (5-cm) height of cut, with plots measuring 8 × 12.5 feet (2.4 × 3.8 meters).

Experiment initiation

Seven days before the initial dazomet application (June 1, 2015 in North Carolina and June 22, 2015 in Texas), select plots were treated with fluazifop (Fusilade, Syngenta) at 0.4 pound a.i./acre (0.4 kilogram/hectare) plus glyphosate at 3 pounds a.i./acre (3.4 kilograms/hectare) with a CO₂-propelled sprayer calibrated to deliver 45 gallons/acre (420 liters/hectare). Sequential fluazifop + glyphosate treatments were made at monthly intervals for plots receiving two or three applications.

Three days before the initial dazomet application, all plots were core-aerified (0.75-inch diameter × 3-inch depth [1.9-cm diameter × 7.6-cm depth]) to affect ≈10% of the trial area (7). All plots were irrigated daily from one to three days before the initial dazomet application to ensure sufficient soil moisture at dazomet application. One day before dazomet application, all plots were scalped to 0.5 inch (1.25 cm) to conform with label recommendations (1). Immediately before initial dazomet application, select plots were tilled twice to a 6-inch (15-cm) depth using a tractor-mounted rotary tiller. Dazomet was applied using a 3-foot-wide (0.9-meter-wide) drop spreader at rates of 262 pounds a.i./acre; 262 pounds a.i./acre followed by (fb) 262 pounds a.i./acre; or 525 pounds a.i./acre (291; 291 fb 291; or 583 kilograms a.i./hectare). After the initial dazomet application, various tillage, rolling, irrigation and tarping combinations were performed at prescribed timings (Table 1). Sequential dazomet applications were made five days after the initial application.

Figure 2. Common bermudagrass (Cynodon spp.) cover at five and 42 weeks after initial treatment (WAIT) in College Station, Texas. Different lowercase letters indicate significant differences among treatments at 5 WAIT; and different uppercase letters indicate significant differences among treatments at 42 WAIT. DAZ = dazomet; FLU = fluazifop (0.4 pound/acre); fb = followed by; GLY = glyphosate (3 pounds/acre).

Incorporating dazomet into the soil by tillage was similar to tillage performed when the trial was started. Irrigation was applied at zero, one, two and three days after dazomet application at 1, 0.5, 0.25 and 0.125 inch (2.5, 1.3, 0.6 and 0.3 cm) of water, respectively. Treatments including a second dazomet application and incorporation via irrigation were carried out using the same protocol. Rolling, which was performed using a tractor-mounted roller filled with water, was done immediately after dazomet had been applied to the plots receiving tillage. Clear plastic tarps were applied by hand, and tarp borders were sealed with soil. Tarps were removed after one week. Plots were undisturbed for six weeks after initial treatment, at which time routine mowing commenced.

Data collection

Plots were evaluated visually for bermudagrass cover estimated on a scale of 0% (no bermudagrass cover) to 100% (complete bermudagrass cover) at eight, 15 and 46 weeks after initial treatment in North Carolina, and five, 10, 16 and 42 weeks after initial treatment in Texas.

Results and discussion

In North Carolina, dazomet alone resulted in unacceptable bermudagrass control (>10% cover) from eight to 46 weeks after initial treatment (45% to 85% cover; Figure 1). Fluazifop + glyphosate applied one week before dazomet applications reduced bermudagrass cover to < 10% from eight to 46 weeks after initial treatment. Within these treatments, one irrigation-incorporated dazomet application of 262 pounds/acre resulted in 1% to 6% bermudagrass cover, which was comparable to the 525 pounds/acre rate (0% to 1% cover) and a sequential application of 262 pounds/acre (0% to 3% cover). This suggests applying fluazifop + glyphosate before a dazomet application can reduce the total amount of dazomet needed for acceptable bermudagrass control. In addition, the current maximum single-application allowance for dazomet soil-incorporation by water is 262 pounds/acre, and these results support its utility for bermudagrass control (1).

When pretreated with fluazifop + glyphosate in North Carolina, an irrigation-incorporated dazomet application of 525 pounds/acre resulted in similar bermudagrass cover from eight to 46 weeks after initial treatment (0% to 1%) compared with tillage incorporation with and without tarping (0% to 10%; Figure 1). This information supports incorporating dazomet into the soil via irrigation, as it is a less costly, less intensive practice than tillage incorporation. All non-dazomet treatments (that is, fluazifop + glyphosate-alone treatments and nontreated-tillage) in North Carolina resulted in comparable bermudagrass cover at eight weeks after initial treatment (0% to 18%), while at least two fluazifop + glyphosate applications were required to maintain bermudagrass cover < 5% cover at 46 weeks after initial treatment.

Figure 3. Fluazifop + glyphosate (FLU + GLY) treatment regimens at 10 weeks after initial treatment in College Station, Texas, and 20 weeks after initial treatment in Raleigh, N.C.
Photos 3a-3c by Casey Reynolds; Photos 3d-3f by Travis Gannon

Compared with the treatments in North Carolina, the Texas treatments generally provided superior bermudagrass control in the summer immediately following application; however, inferior control was observed the following spring. Excluding irrigation-incorporated dazomet (262 pounds/acre) and nontreated tillage, all treatments resulted in < 2% cover through five weeks after initial treatment in Texas (Figure 2). Additionally, all dazomet treatments including fluazifop + glyphosate resulted in 0% cover at this time. However, a pronounced increase in bermudagrass cover was observed in many Texas plots from five to 42 weeks after initial treatment. At 42 weeks after initial treatment, treatment regimens using only dazomet applications did not provide acceptable bermudagrass control (65% to 99% cover), suggesting turfgrass managers should not rely solely on this fumigant for control. In Texas, and similarly in North Carolina, applying fluazifop + glyphosate before dazomet increased bermudagrass control. At 42 weeks after initial treatment, the only dazomet treatments providing acceptable control included fluazifop + glyphosate pretreatment coupled with dazomet application at the high rate, and tillage incorporation of the dazomet (4% cover). In addition, tarping the area following this treatment did not improve bermudagrass control, suggesting that tarping may be excluded during renovations. Only treatment regimens that included three applications of fluazifop + glyphosate provided comparable long-term control across locations (<5% cover). However, two applications of fluazifop + glyphosate resulted in acceptable control in North Carolina (5% cover), but not in Texas (39% cover) (Figure 3).

Differing control between locations may be due in part to more favorable bermudagrass growing conditions in North Carolina shortly after dazomet application. In contrast, Texas conditions were generally superior over the entire 10- to 11-month research period. In the short term, precipitation from June through September in North Carolina (21 inches water [53 cm]) totaled 118% of the 30-year average, whereas precipitation in Texas (7 inches water [18 cm]) totaled 53% of the 30-year average (data not shown). Because plots were not routinely irrigated following dazomet application, this may have created more favorable growth conditions for common bermudagrass in North Carolina than in Texas, which resulted in reduced efficacy in North Carolina. In the long term, warmer winter climatic conditions in Texas may have created more favorable conditions for common bermudagrass recovery. Bermudagrass dormancy commences when soil temperatures at a 4-inch (10-cm) depth fall below 50 F (10 C) (3). Average North Carolina soil temperatures were 43 F (6 C) in January and 45 F (7 C) in February, and bermudagrass dormancy was observed from December through February. Average Texas soil temperatures were 52 F (11 C) in January and 55 F (13 C) in February, and bermudagrass dormancy was not observed. An extended growing period in Texas may have created more favorable conditions for common bermudagrass regrowth.

Figure 4. Fluazifop + glyphosate (FLU + GLY) followed by tillage-incorporated dazomet (DAZ) application at 525 pounds a.i./acre (583 kilograms a.i./hectare) at 10 weeks after initial treatment in College Station, Texas, and 20 weeks after initial treatment in Raleigh, N.C.
Photos 4a-4b by Casey Reynolds; Photos 4c-4d by Travis Gannon

Lastly, it should be noted that the research presented here does not align with in situ management approaches, as plots were not re-established with a new turfgrass in the time frame within which a renovation project would typically be conducted. This could have created more favorable conditions for common bermudagrass regrowth by excluding plant competition from the subsequent turfgrass stand and ultimately reducing long-term dazomet efficacy. Future research is needed to determine optimal regrassing timing following dazomet application.

Conclusions

In our research locations in North Carolina and Texas, applying fluazifop + glyphosate before dazomet was required for acceptable bermudagrass control. At both locations, this pretreatment followed by tillage-incorporated dazomet at 525 pounds/acre — the maximum labeled rate for soil-incorporation on golf course fairways — resulted in < 10% bermudagrass cover at the final evaluation date (Figure 4). Tarping after tillage did not improve efficacy and, consequently, is not recommended for common bermudagrass renovations in order to save management resources and reduce potential worker exposure to dazomet. Three fluazifop + glyphosate applications resulted in < 5% cover at the final evaluation date and should be considered for renovations where effective dazomet treatment regimens are not feasible.

Overall, results from this research suggest golf course superintendents have a difficult yet fairly straightforward decision to make when choosing a bermudagrass control approach. Control is possible with three relatively cheap fluazifop + glyphosate applications (≈ $100 US/acre [≈ $247/hectare]), but the treatment duration is 42 to 56 days, and up to an additional 30 days are required before beginning to plant the new turfgrass (depending on planting method and species). Control is also possible at a higher price tag (≈ $3,500/acre [≈ $8,650/hectare]), but in a much shorter treatment duration of eight days, with one application of fluazifop + glyphosate made before the dazomet is applied. Even with recommended plant-back dates ranging from eight to 15 days (depending on the soil temperature) after dazomet application, substantial time savings are possible. This is an important consideration for golf courses, as revenue losses during the renovation period may be reduced given that the establishment period for the newly planted turfgrass can be substantially decreased. Ultimately, established common bermudagrass can be controlled cheaply or quickly, but not both.

Acknowledgments

The information in this paper was originally published as “Herbicide applications and incorporation methods affect dazomet efficacy on bermudagrass” by Matthew D. Jeffries, Travis W. Gannon, Casey W. Reynolds, Fred H. Yelverton and Charles A. Silcox in the December 2016 issue of HortTechnology.

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Matthew D. Jeffries is a graduate research technician, Travis W. Gannon is an assistant professor, and Fred H. Yelverton is a professor and extension specialist at North Carolina State University in Raleigh, N.C. W. Casey Reynolds is an assistant professor and extension specialist at Texas A&M Agrilife Extension Service in College Station, Texas.

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