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NOTES

Trinexapac-ethyl effects on kentucky bluegrass evapotranspiration

^a Dep. of Horticulture, 1-87 Agriculture Building, Univ. of Missouri, Columbia, MO 65211
^b Dep. of Horticulture and Landscape Architecture, Colorado State Univ., Fort Collins, CO 80523

Corresponding author (ErvinE{at}missouri.edu)

	ABSTRACT

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Many researchers have reported that trinexapac-ethyl [4-(cyclopropyl-hydroxy-methylene)-3,5-dioxo-cyclohexane-carboxylic acid ethylester] effectively reduces turfgrass leaf elongation and subsequent clipping production. Our hypothesis is that trinexapac-ethyl (TE) induced canopy growth and conductance reductions may also reduce turfgrass evapotranspiration (ET) rates. This study was conducted to determine if application of TE to mature, field-grown Kentucky bluegrass (Poa pratensis L. `NuStar') would result in reduced weekly ET rates as measured by weighing bucket lysimeters. A randomized complete block design was used to compare the weekly ET rate (mm week^-1) of Kentucky bluegrass (KBG) treated or not with TE (0.27 kg ha^-1). Weighing bucket lysimeters containing KBG were treated with TE three times per year at 6-wk intervals in 1995, 1996, and 1997. Trinexapac-ethyl reduced KBG weekly ET in 5 wk out of a total of 34 wk sampled over 3 yr. Higher ET rates were not correlated with weekly clipping production for either untreated or TE-treated KBG. Possible ET reductions, coupled with reduced clipping production, indicate TE is an effective tool for managing numerous turfgrass systems.

Abbreviations: ET, evapotranspiration • ET_o, grass reference evapotranspiration • KBG, Kentucky bluegrass • LAI, leaf area index • PGR, plant growth regulator • PVC, polyvinyl chloride • r_c, canopy resistance • TE, trinexapac-ethyl • WAT, weeks after treatment

	INTRODUCTION

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TRINEXAPAC-ETHYL suppresses vegetative growth by interfering with gibberellin biosynthesis, reducing laminar cell elongation (Adams et al., 1992). Subsequent reductions in leaf elongation rate and clipping production have been demonstrated for various cool-season grasses after TE application (Ervin and Koski, 1998, 2001; Burpee et al., 1996; Daniels and Sugden, 1996; Johnson, 1993).

Another possible beneficial aspect of TE use may be reduced turf ET rates, potentially resulting in lower irrigation requirements. Trinexapac-ethyl treated turfgrass stands may have lower ET rates because (i) reduced leaf elongation between mowings could result in less leaf area for transpirational losses; and (ii) the maintenance of a lower canopy height between mowing events might also reduce canopy conductance and subsequent transpirational losses.

Rosenberg et al. (1983), stated that "the evidence seems conclusive that transpiration in most mesophytic crop plants and other mesophytic vegetation well supplied with water increases with leaf area to a leaf area index (LAI) of about three." Mature cool-season turfs have LAIs of three to six (Brede and Duich, 1986, 1984). In well-watered turf situations, it seems unlikely that small differences in LAI due to TE-treatment would have a significant effect on turfgrass ET. We are not aware of any research reporting on possible differences in turfgrass LAI due to plant growth regulator treatment.

A second possibility is that reduced canopy height between mowings due to TE-treatment may function to reduce ET. Shearman and Beard (1973) report that Penncross creeping bentgrass (Agrostis palustris Hudson) used 50% more water when maintained at 2.5 cm vs. 0.7 cm. Feldhake et al. (1983) report a 15% increase in water use by Kentucky bluegrass maintained at 5 cm compared with 2 cm.

From a biophysical perspective, Allen et al. (1989) developed procedures for adjusting Penman-Monteith estimates of ET_o for different leaf area indices and canopy heights. Allen (1991) incorporated these empirically derived relationships into a Reference-ET computer program. As an example this program predicts that grass maintained at an average height of 12 cm has a canopy resistance (r_c) of 70 s m^-1, while grass at 7 cm has a r_c of 119 s m^-1. When weather station data from three seasons in Loveland, CO, were used in this program, and the canopy height variable was changed from the default value of 12 cm to 7 cm, a 12% reduction in predicted ET_o was found (Mecham, 1996). These estimates indicate that PGR-induced reductions in canopy height between mowing events may reduce ET.

Research examining TE effects on turfgrass ET is limited. Greenhouse-grown `Kentucky 31' tall fescue (Festuca arundinacea Schreb.) treated with a TE rate of 370 mg L<sup>-1</sup> showed significantly reduced measured ET (11%) over a 6-wk period (Marcum and Jiang, 1997). In another greenhouse experiment, a mixed turf of `Bronco' Kentucky bluegrass and `Mustang' tall fescue treated with a TE rate of 0.64 kg ha^-1 used approximately 20% less water than the control from 0 to 4 wk after treatment (King et al., 1997).

While not conclusive, limited previous research suggests that PGR-induced effects on turfgrass may function to reduce ET in the field. The objective of this study was to determine if the weekly ET rate of field-grown KBG is reduced by TE.

	Materials and Methods

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A field experiment to test the effect of TE on KBG ET was initiated 20 April 1995 on a 3-yr-old stand of NuStar KBG at the Colorado State University Horticulture Field Research Center. The research center is located 10 km northeast of Fort Collins, CO; the site elevation is 1525 m, 40.6°N and 105°W. The climate is semiarid with average annual maximum temperature of 16.8°C average minimum of 1.2°C, and average annual precipitation of 365 mm. The soil is a Nunn clay loam (fine montmorillonitic mesic aridic argiustoll); the soil has a pH of 7.8 and an organic matter content of 30 g kg^-1.

The experiment consisted of one factor with two levels, TE and control, arranged in four randomized complete blocks. Each block contained two experimental units that were 2.7 by 0.9 m. Weighing bucket lysimeters were installed during June of 1995 in the middle of each of eight plots. The polyvinyl chloride (PVC) lysimeters were 30.5 cm in diameter and 80 cm in length. The native Nunn clay loam soil occupied the top 61 cm; the soil was separated by a 1.3-cm-thick PVC drainage plate covered by filter fabric. Below the drainage plate was a 6 cm deep air space area (drainage chamber) connected to a 1-cm-diam PVC air inlet tube glued to the outside of the lysimeter and open to the air at the top of the lysimeter. The drainage chamber was closed at the bottom by a solid 1.3-cm-thick PVC plate. This bottom plate had a ball valve attached to the drainage chamber. The soil for each lysimeter was dug in 7.6-cm increments and placed in separate buckets. The lysimeters were filled with each appropriate 7.6-cm layer of soil, packed to an approximate bulk density of 1.30 g cm^-3, and placed in the ground at a height uniform with the surrounding turf. A sleeve of sheet-metal was also placed between the lysimeter and the surrounding soil. Lysimeters were left in the field all year round.

The eight experimental units which contained lysimeters were also part of a larger three-factor experiment (Ervin and Koski, 2001). Each lysimeter was in the middle of plots that received 196 kg N ha^-1 yr^-1 and no traffic. Lysimeter plots were mowed weekly to a height of 5.7 cm with a rotary mower (clippings returned). Weekly mowing, with lysimeters in place, occurred the day before lysimeter weighing.

Trinexapac-ethyl (0.27 kg ha^-1) was applied to half of the study plots three times each year at 6-wk intervals. Application dates were 27 June, 14 August, and 26 September 1995; 22 May, 5 July, and 16 August 1996; and 20 May, 2 July, and 07 August 1997. Trinexapac-ethyl was applied with a CO₂ sprayer at a pressure of 242 kPa and a spray volume of 0.10 L m^-2. Control plots, with lysimeters, were maintained in identical fashion to the TE plots. An electronic load cell (Revere Transducers, Tustin, CA) connected to a calibrated Campbell Scientific 21X (Campbell Scientific, Logan, UT) data logger was used to weigh the lysimeters each week. The load cell had a resolution equivalent to 0.25 mm of water. Weekly lysimeter mass, taken from 0800 to 0900 h each Thursday, was the average of two readings per lysimeter. Weekly ET was determined from 21 Aug. to 17 Oct. 1995, from 27 May to 01 July and 15 Aug. to 19 Sept. 1996, and from 27 May to 16 Sept. 1997. Evapotranspiration was calculated by the mass-balance method:

where M is the drained mass of the lysimeter at the start of the week, P is the amount of precipitation and/or irrigation received during the week, DP is deep percolation or drainage collected at the end of the week, and Md is the drained mass of the lysimeter at the end of the week (Jensen et al., 1990). All measured units were in millimeters. Mass losses due to irrigation or precipitation runoff were not accounted for in this study. While irrigation was scheduled so as to avoid significant runoff from the lysimeters, runoff due to rain intensities that exceeded the infiltration rate of the lysimeter soil columns could not be controlled.

Precipitation was measured by a tipping bucket precipitation gauge connected to a weather station (Campbell Scientific) managed by the Northern Colorado Water Conservancy District. This weather station was approximately 10 m from the south edge of the experimental area over a well-irrigated sward of KBG mowed twice weekly to 6.4 cm.

The weather data collected were used to estimate daily reference grass ET_o with the Penman-Monteith combination equation. The American Society of Civil Engineers manual on evapotranspiration and irrigation requirements (Jensen et al., 1990) conclude that the Penman-Monteith equation provides the best reference ET estimate because it accounts for elevation, geographical location, and differences in canopy resistance due to crop height. Irrigation was applied every 3 d to replace 100% of ET_o. The amount of irrigation applied to individual lysimeters was determined as the average amount of irrigation water collected in three graduated catch cones placed around each lysimeter at each irrigation event. Uniformity of irrigation applied at each lysimeter was very high (95%).

The data were analyzed as a split plot in time by the general linear model procedure in Statistical Analysis System (SAS Institute, 1990). The main plot contained the effects of replication and TE. The subplots were regarded as repeated measures and contained the effects of time (each week sampled in each of 3 yr) and TE x time. Mean differences were ascertained using Fisher's protected LSD for a split plot design as outlined by Steel and Torrie (1980). Correlation analysis comparing weekly ET with clipping dry weight was also performed.

	Results

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Trinexapac-ethyl treated KBG had lower measured weekly ET rates on the first two sample dates of the experiment in 1995, followed by no differences for the rest of the sampled weeks during the year (Fig. 1) . In 1996, TE treated KBG had lower weekly ET rates at three and 5 wk after treatment (WAT) during the first treatment cycle; no differences were measured during the third TE treatment cycle (Fig. 2) . Throughout most of the 1997 sample period, TE and control weekly ET rates were very similar, the only exception being a higher control ET at 5 WAT during the first treatment cycle (Fig. 3) .

View larger version (19K): [in this window] [in a new window]   Fig. 1. Evapotranspiration (mm week-1) of Kentucky bluegrass as affected by trinexapac-ethyl treatment from 21 August to 17 October 1995. Fisher's protected LSD (P = 0.05) for testing the effect of trinexapac-ethyl at each sample week = 4.9. * indicates significance at P = 0.05

View larger version (17K): [in this window] [in a new window]   Fig. 2. Evapotranspiration (mm week-1) of Kentucky bluegrass as affected by trinexapac-ethyl treatment from 27 May to 01 July and 15 August to 19 September 1996. Fisher's protected LSD (P = 0.05) for testing the effect of trinexapac-ethyl at each sample week = 4.9. * indicates significance at P = 0.05

View larger version (22K): [in this window] [in a new window]   Fig. 3. Evapotranspiration (mm week-1) of Kentucky bluegrass as affected by trinexapac-ethyl treatment from 27 May to 16 September 1997. Fisher's protected LSD (P = 0.05) for testing the effect of trinexapac-ethyl at each sample week = 4.9. * indicates significance at P = 0.05

Correlation analysis did not reveal any association between clipping dry weight and ET (data not presented).

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion REFERENCES

 
Weekly ET rate of TE-treated KBG was consistently similar to that of the untreated KBG in 1995, 1996, and 1997; however, there were certain periods, especially from three to five WAT, when small ET reductions due to TE were measured (Fig. 1–3). As reported elsewhere, clipping production on these plots was maximally suppressed by TE from about three to five WAT (Ervin and Koski, 2001). Correlation analysis between weekly clipping production and ET did not, however, indicate an association at any sampling date. As previous research has demonstrated, turfs maintained at a lower height may use significantly less water than those maintained at higher heights most likely because of reduced canopy conductance (Allen et al., 1989; Feldhake et al., 1983; Shearman and Beard, 1973). It is not clear whether reductions in canopy height due to TE between weekly mowing events in this study were large enough to reduce ET. Clipping production data indicate that TE effects can vary considerably depending on time of year and weeks after treatment (Ervin and Koski, 2001; Fagerness and Penner, 1998). Daily monitoring of canopy height, ET, and canopy conductance before, during, and after TE-induced suppression, are required to adequately characterize any cause and effect relationship between TE and turfgrass ET.

On the basis of the results presented here, the possibility of reducing turfgrass irrigation requirements with TE should not be the main reason for its use, but this potential benefit, in conjunction with reduced mowing frequency, indicate that it is an effective tool for managing numerous turfgrass systems.

	NOTES

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Contribution from the Colorado Agricultural Experiment Station (Research Project 157801).

Received for publication April 19, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion REFERENCES

 

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• Allen, R.G., M.E. Jensen, J.L. Wright, and R.D. Burman. 1989. Operational estimates of reference evapotranspiration. Agron. J. 81: 650–662.[Abstract/Free Full Text]
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