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Crop Science 40:493-497 (2000)
© 2000 Crop Science Society of America

TURFGRASS SCIENCE

Tissue Production and Quality of `Tifway' Bermudagrass as Affected by Seasonal Application Patterns of Trinexapac-Ethyl

M.J. Fagernessa and F.H. Yelvertona

a 100 Derieux St. Box 7620, North Carolina State University, Raleigh, NC 27695-7620 USA

mfagerness{at}aol.com


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Research on the gibberellic acid (GA)–inhibiting plant-growth regulator (PGR) trinexapac-ethyl (TE) regarding seasonal effects of single vs. multiple applications of this product on turfgrass quality and tissue production has not been reported. Research was conducted at North Carolina State University on `Tifway' bermudagrass [Cynodon dactylon (L.) Pers. x Cynodon transvaalensis (Burtt-Davy)] to compare effects of different seasonal application patterns of TE. Applications of TE were made to fully established Tifway bermudagrass at either 0.107 or 0.071 kg a.i. ha-1. Seasonal application patterns included an initial application, followed by zero, one, or two sequential applications at 4-wk intervals. Plots were rated weekly for turfgrass quality and clippings were collected weekly for evaluation of tissue production. Results demonstrated that one application of TE provided temporary growth inhibition of Tifway bermudagrass. This effect, as expected, disappeared 4 wk after initial treatment (WAIT). Post-inhibition growth enhancement (PIGE) was observed from 6 to 10 WAIT in areas treated with a single application of TE at 0.071 kg a.i. ha-1. However, total seasonal tissue production following a single application of TE was equal to untreated areas, suggesting limited overall effects of PIGE. Conversely, one or two sequential applications of TE provided prolonged growth inhibition; total seasonal tissue production was reduced by up to 40%. Overall quality of Tifway bermudagrass was enhanced by multiple TE applications and perceived fall dormancy was delayed. Results support the use of sequential applications of TE in Tifway bermudagrass for consistent growth management throughout the growing season, avoidance of PIGE, and improvement in visual turfgrass quality.

Abbreviations: GA, gibberellic acid • PGR, plant growth regulator • PIGE, post-inhibition growth enhancement • TE, trinexapac-ethyl • WAIT, weeks after initial treatment


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
PLANT-GROWTH REGULATORS are effective agents in controlling the growth of numerous turfgrass species (Fagerness and Penner, 1998a, 1998b; Hoffman and Ilnicki, 1989; Johnson, 1990a, 1990b; Nelson et al., 1977). Desirable attributes include growth inhibition for a determinate period and seedhead suppression, without reductions in turfgrass quality. The combination of growth suppression and acceptable quality has been shown to be most feasible with PGRs that inhibit the biosynthesis of GA. Examples of such PGRs are paclobutrazol, flurprimidol, and TE (Watschke and DiPaola, 1995). Comparative efficacy between single and multiple applications of flurprimidol and paclobutrazol has been assessed (Johnson, 1990a, 1992a, 1994), but similar comparisons are lacking for TE.

Research highlighting the effects of fertility and growth-regulating compounds has been conducted previously for Tifway bermudagrass. Growth inhibition of Tifway bermudagrass was reported after applications of all three commercial GA-inhibiting PGRs (Johnson, 1992a, 1992b, 1997; Wiecko, 1997). Results indicated maximum growth inhibition at 2 to 5 wk after PGR applications, along with improvements in turfgrass quality. Existing concerns in the golf course industry with PGR applications in bermudagrass include turfgrass discoloration 1 to 2 wk after application and PIGE. Discoloration has been observed with applications of TE but may be counteracted by tank mixing TE with an Fe source (Johnson, 1997; Wiecko, 1997). The literature has not reported on either PIGE or how it may be affected by the seasonal impact of multiple PGR applications.

Known warm-season turfgrass growth responses to decreasing temperatures and reduced light levels in the fall include cessation of new growth and net loss of chlorophyll from leaf tissues. Research has been conducted to investigate how these effects may be delayed or counteracted, presumably to artificially extend the perceived growth period of these species. Exogenous additions of growth-regulating substances such as either the fungicide carboxin or the hormone GA have aided in the delay of bermudagrass dormancy symptoms (DiPaola et al., 1981; Dudeck and Peacock, 1985; Karnok and Beard, 1983; Sachs et al., 1971). Observed responses to these applications under chilling conditions included maintenance of chlorophyll levels and enhancements of shoot growth. Therefore, we hypothesized that bermudagrass treated with a GA-inhibiting PGR, such as TE, in late summer may show similar responses if (i) the effects of the PGR diminish during the fall period when temperatures induce dormancy and (ii) diminishing effects of GA-inhibiting PGRs include a release of GA.

The purpose of these experiments was to compare different seasonal application patterns of TE applied to Tifway hybrid bermudagrass with respect to patterns of shoot growth and turfgrass quality. Results will help address questions concerning (i) behavior of this species in response to TE during periods of both optimum and suboptimum growth potential and (ii) rudimentary characterization of the phenomenon known as "rebound effect". Results will help generate sound management recommendations for seasonal use patterns of TE on warm-season turfgrass species.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Experiments were conducted in the summer of 1996 and 1997 at the North Carolina State University Turfgrass Field Laboratory in Raleigh, NC. Soil was a mixed, thermic Typic Kanhapludult (Cecil series) sandy loam (75% sand, 15% silt, and 10% clay) with 29 g kg-1 organic matter. Soil pH was 5.6 and cation-exchange capacity was 3.1 cmol kg-1. Treatments were applied to a stand of Tifway bermudagrass established by sprigging in May 1994. The stand was fertilized with 49 kg N ha-1 mo-1 from May through September each year and was irrigated as needed. The stand was maintained throughout the growing season at a cutting height of 1.6 cm.

Spray applications were made at three intervals in each of the two seasons of the experiment. The initial application was made 28 June 1996 (22°C air temperature, 100% relative humidity) or 2 July 1997 (31°C air temperature, 53% relative humidity), when growth of Tifway bermudagrass had become fully active. Sequential applications at 4-wk intervals were made to selected treatments on 26 July 1996 (31°C air temperature, 33% relative humidity) or 31 July 1997 (28°C air temperature, 40% relative humidity) and again on 23 Aug. 1996 (35°C air temperature, 40% relative humidity) or 28 Aug. 1997 (35°C air temperature, 36% relative humidity). Wind speed was negligible for all applications. Applications were made using the 1E formulation of TE. Spray pressure and carrier volume were 179 kPa and 562 L ha-1, respectively, with CO2 as the propellant and water as the carrier.

Plots measured 1.53 by 3.05 m and were arranged in a randomized complete block design with four replications. The experiment featured seven treatments: untreated, TE at either 0.107 kg a.i. ha-1 or 0.071 kg a.i. ha-1 applied initially, initially with one sequential application 4 wk later, or initially with two sequential applications 4 and 8 wk after the initial treatment (4 and 8 WAIT).

Beginning 1 WAIT, Tifway bermudagrass was evaluated weekly for turfgrass quality (1–9 scale; 1 = dead or fully dormant turf, 9 = ideal turf, 5 = minimally acceptable turf; ratings were assigned in increments of 0.5). Quality ratings were made on a weekly basis each year until Tifway bermudagrass had achieved full dormancy. Evaluation of tissue production was based on clipping harvest. All plots were mowed at a height of 1.6 cm with a John Deere 22R greens mower 3 d prior to collection of clippings. Strips at the front and back of each plot were mowed in advance of clipping collection to ensure an equal-sized harvest area (0.56 by 2.49 m). Harvested clippings were oven-dried at 70°C for 72 h and analyzed on a dry weight percentage of untreated basis.

Tissue production and turfgrass quality data were analyzed using the Statistical Analysis System (SAS Version 6.12, SAS Inst., Cary, NC). Analysis of variance (ANOVA) procedures were used to test for treatment effects, and Fisher's LSD test was used for separation of treatment means when F tests showed significance at a particular rating date. Orthogonal contrasts were performed on both quality and tissue production data so that inferences about differential rates or sequential application patterns of TE could be made.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Tifway Bermudagrass Quality
Assessments of turfgrass quality in both 1996 and 1997 revealed that detectable treatment differences were most consistently attributable to contrasts between untreated and treated turf (Table 1) . A significant interaction between treatment effects and years prevented the combination of quality data from 1996 and 1997. Orthogonal contrasts performed for each rating date in each of two years illustrated the following patterns:


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  		Table 1 Significance of orthogonal contrast F tests for Tifway bermudagrass quality ratings under different trinexpac-ethyl (TE) treatments and a control during the 1996 and 1997 growing seasons

 
1. Rate of application of TE, averaged across seasonal application regime, had more of an impact on turfgrass quality in 1997 than in 1996.

2. Turf treated with one sequential application of TE differed in quality from turf treated with only an initial application 9 and 10 WAIT in 1996 and from 6 to 19 WAIT in 1997.

3. Quality was additionally affected by the second sequential application at or beyond 10 WAIT (Table 1).

Quality data presented will focus on seasonal application patterns of TE at the label recommended 0.107 kg a.i. ha-1 rate, but, since differences between the two rates were often insignificant, the rationale for using reduced rates of TE may be justified.

For 3 wk following initial treatment in 1996, reductions in quality were observed in TE-treated turf, as compared with the untreated control (Table 2) . Discoloration was characterized by leaf chlorosis. Recovery from discoloration was complete by 4 WAIT. Beyond 8 WAIT (the time at which the second repeat application was made to appropriate plots), quality ratings were consistently higher in those plots that had received an initial application of TE and both sequential applications at 4 and 8 WAIT (Table 2). Tifway bermudagrass treated with three seasonal applications of TE exhibited significantly higher quality from 17 to 19 WAIT than that for untreated turf or for turf that received fewer applications of TE, suggesting that continuous summer use of TE may delay the onset of fall dormancy (Table 2). Similar effects have been reported in turf treated with growth-stimulating compounds during the fall (DiPaola et al., 1981; Dudeck and Peacock, 1985; Karnok and Beard, 1983; Sachs et al., 1971).


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  		Table 2 Tifway bermudagrass quality ratings, as affected by the number of seasonal applications of trinexapac-ethyl (TE) at 0.107 kg a.i. ha-1 during the 1996 growing season.{dagger}

 
Quality ratings taken during the 1997 growing season showed that TE did not cause any early reductions in quality, as was the case in 1996, suggesting that TE-induced discoloration is inconsistent in Tifway bermudagrass and may be dependent on other environmental factors such as temperature or soil moisture (Table 3) . The absence of reductions in quality resulted in no detectable treatment effects on quality during this early period (Table 3). Enhancement of turfgrass quality, as compared with the untreated control, occurred in treated turf as early as 3 WAIT and, in the case of plots that received multiple applications of TE, persisted through 19 WAIT (Table 3). Enhancement of turfgrass quality was characterized by darker green color and perceived increases in shoot density. Turf that received a single application of TE had enhanced turfgrass quality from 3 through 7 WAIT but was equivalent to untreated turf beyond this point (Table 3). Due to mild temperatures in the fall of 1997, quality ratings were consistently higher 17 through 19 WAIT than they were for the same period after treatment in 1996. Trinexapac-ethyl had a positive effect on late-season turfgrass quality in 1997 similar to that observed in 1996, again supporting the hypothesis that multiple seasonal applications of TE may delay the onset of bermudagrass fall dormancy (Table 3).


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  		Table 3 Tifway bermudagrass quality ratings, as affected by the number of seasonal applications of trinexapac-ethyl (TE) at 0.107 kg a.i. ha-1 during the 1997 growing season.{dagger}

 
Tifway Bermudagrass Growth Reduction
Trinexapac-ethyl effectively reduced tissue production of Tifway bermudagrass during the 1996 and 1997 growing seasons (Table 4) . Inclement weather prevented the measurement of tissue production 1 WAIT in both 1996 and 1997. Patterns of growth inhibition between 2 yr were comparable, and the data presented are combined across 1996 and 1997. Pooled treatment effects were only evident 2 to 6 WAIT (Table 4), suggesting diminishing effects of some TE treatments later in the growing season. Because data are presented as a percentage of the untreated, TE treatment data were analyzed independent of the untreated control data, such that pairwise comparisons among TE treatments might not be influenced by fixed values for untreated plots. Orthogonal contrasts revealed that application rate of TE, averaged across seasonal application regimes, had no significant effect on tissue production at any time during the experiment, suggesting the possibility of using reduced rates of TE without sacrificing any inhibition of growth (Table 4).


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  		Table 4 Significance of orthogonal contrast F tests for Tifway bermudagrass tissue production during the 1996 and 1997 growing seasons

 
A single application of TE at 0.107 kg a.i. ha-1 resulted in inhibition of growth from 2 through 4 WAIT, while a single application of TE at 0.071 kg a.i. ha-1 resulted in inhibition only through 2 WAIT (Table 5) . One sequential treatment of TE 4 WAIT resulted in extension of the growth-inhibition period through 7 WAIT for the 0.107 kg a.i. ha-1 rate and through 6 WAIT for the 0.071 kg a.i. ha-1 rate. (Table 5). A second sequential application of TE 8 WAIT resulted in further extension of the period of growth inhibition to 10 WAIT (Table 5). Neither extent nor duration of growth inhibition was dependent on the rate of application of TE when two sequential applications were made (Table 5).


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  		Table 5 Tifway bermudagrass tissue dry weight, as affected by the number of seasonal applications of trinexapac-ethyl (TE)

 
Extension of the period of growth inhibition for Tifway bermudagrass seems highly dependent on continual use of TE throughout the growing season. Significant PIGE occurred from 6 through 10 WAIT in turf treated with an initial application of TE at 0.071 kg a.i. ha-1 (Table 5). Such an effect was not present when the initial application of TE was made at 0.107 kg a.i. ha-1, suggesting that TE, applied at label rates, may be less prone to cause PIGE in Tifway bermudagrass. Post-inhibition growth enhancement was not evident in turf treated with TE initially and once sequentially at either rate (Table 5). However, tissue production in these areas was higher than in those treated with three applications of TE 10 and 12 WAIT (Table 5).

Duration of growth inhibition by TE at three applications of 0.071 kg a.i. ha-1 extended through 10 WAIT, while TE at two applications of 0.107 kg a.i. ha-1 was only efficacious through 7 WAIT, even though the total quantity of chemical used in each case was equal (Table 5). Therefore, a better management strategy for growth inhibition in Tifway bermudagrass could be three applications at 0.071 kg a.i. ha-1 instead of two applications at 0.107 kg a.i. ha-1. Growth inhibition, as affected by three seasonal applications of TE, was far less persistent than was enhancement of turfgrass quality. However, it is important to consider that Tifway bermudagrass ceases to actively produce new shoot tissue beyond early October in North Carolina (12–14 WAIT in the experimental time frame). Therefore, the effects of TE on tissue production become irrelevant beyond 12 to 14 WAIT, while quality differences are still detectable, important to turfgrass managers, and perhaps of ecological or physiological significance.

Comparisons of total seasonal biomass production for Tifway bermudagrass showed that no rate or application patterns of TE increased cumulative tissue production in comparison with untreated turf (Table 6) . Trinexapac-ethyl at either rate of application applied three times seasonally reduced cumulative tissue production in Tifway bermudagrass plots by at least 40% (Table 6) and resulted in a perceived delay in fall dormancy. While no TE treatment significantly affected cumulative tissue production as compared with the control, continuous treatment with TE resulted in reduced tissue production, as compared with areas that received fewer seasonal applications (Table 6).


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  		Table 6 Cumulative seasonal tissue production, as affected by the number of seasonal applications of trinexapac-ethyl (TE)

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Results from this experiment demonstrate the benefits of sequential applications of TE for prolonged inhibition of Tifway bermudagrass growth. However, sequential applications of TE to other turfgrass species may not additionally benefit growth management to the extent that was seen in Tifway bermudagrass. The occurrence of TE-induced discoloration in 1996 suggested the need for a way to mask such an effect. It has been shown that supplemental Fe may enhance turfgrass quality and may offset early discoloration without stimulating growth beyond the capacity of the PGR to inhibit growth (Johnson, 1997).

Comparisons of TE effects on turfgrass quality and biomass production revealed that reduced rates of TE have some promise, from an efficacy standpoint, in Tifway bermudagrass growth management. However, duration of growth inhibition may be compromised when only one or two applications of TE are made at 0.071 kg a.i. ha-1 (Table 5). Orthogonal contrasts comparing 0.107 and 0.071 kg a.i. ha-1 rates of TE on Tifway bermudagrass tissue production showed no consistent separation and, therefore, no clear advantage at any given point during the growing season, to using the higher rate (Table 4). Patterns of Tifway bermudagrass tissue production in 1996 and 1997 demonstrated a clear advantage to using three seasonal applications of TE at 0.071 kg a.i. ha-1 compared with two applications at 0.107 kg a.i. ha-1, even though the quantity of TE used in each instance is equal.

Avoidance of PIGE in Tifway bermudagrass may require the use of sequential applications of TE. Post-inhibition growth enhancement was evident in Tifway bermudagrass following a single initial application at 0.071 kg a.i. ha-1 but became less pronounced when sequential applications were made (Table 5). It can therefore be hypothesized that the duration and magnitude of PIGE is, at least in part, based on temporal growth potential for a given species. Tifway bermudagrass is a much more aggressive species during summer months so TE use patterns that allow growth recovery during the summer may also increase the occurrence of PIGE. Thus, further work is warranted to quantify and characterize PIGE in both warm-season and cool-season turfgrasses, so as to better understand the long-term effects of TE on species for which it is labeled for use.

Received for publication March 30, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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