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thesis entitled

EFFECT OF THREE PLANT GROWTH REGULATORS
AND TWO NITROGEN REGIMES ON GROWTH AND PERFORMANCE
OF CREEPING BENTGRASS (Agrostis palustris Huds.)

 

presented by

Ronald Nigel Calhoun

has been accepted towards fulfillment
of the requirements for

Master's Crop and Soil Science

degree in

 

 

 

 

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EFFECT OF THREE PLANT GROWTH REGULATORS AND TWO NITROGEN
REGIMES ON GROWTH AND PERFORMANCE OF CREEPING BENTGRASS
(A grostis palustris Huds.)

By

Ronald Nigel Calhoun

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Crop and Soil Sciences

1996

ABSTRACT
EFFECT OF THREE PLANT GROWTH REGULATORS AND TWO NITROGEN
REGIMES ON GROWTH AND PERFORMANCE OF CREEPING BENTGRASS
(A grostis palustris Huds.)
By

Ronald Nigel Calhoun

Research was initiated in 1994 to study the interaction of nitrogen fertilization and PGRs
on fairway height creeping bentgrass (Agrostis palustris H.) performance, clipping
production, divot recovery, and wear tolerance. Two nitrogen regimes and three PGRs at
three rates were applied in a randomized complete block design. The PGRs chosen were
flurprimidol { a-( l -methylethyl)-0t- [4-(trifluoro-methoxy)phenyl]-S-pyrimidine-ethanol },
paclobutrazol {(i)-(R*.R*)-B-[(4-chlorophenyl)methyl]-0t-(l ,1 -dimethylethyl)-1H-
1,2,4-triazole-l-ethanol}. and trinexapac-ethyl {4(cyclopropyl-(x-hydroxy-methylene)-
3,S-dioxocyclohexanecarboxylic acid ethyl ester}. Three montly PGR applications were
made in 1994 and four were made in 1995. The high nitrogen regime doubled clipping
production, increased turf quality. and enhanced divot recovery and wear tolerance.
Clipping reduction >60°/o was observed after the first PGR application but not after the
second application. The high nitrogen regime accelerated divot closure by 10 days
compared to the low nitrogen regime. There was little effect of PGRs on divot closure
rate. PGRs generally increased visual turfgrass quality. Flurprimidol treatment decreased

wear tolerance on the low nitrogen regime.

To My Family

Kristi and Emily
your love. support. and understanding

ACKNOWLEDGMENTS

I would like to express my gratitude to my advisor, Dr. Bruce Branham for his guidance
and friendship during my studies. I am also appreciative of the unique opportunities he
offered me through working as his technician. In this I gained invaluable experience and
exposure to a much broader range of research topics than a typical masters program
would offer. Also a sincere thank you to Dr. Donald Penner for facilitating the
conclusion of my thesis work. I would like to recognize Dr. Paul Rieke for his willingness
to listen, advise, and let me form my own conclusions (as frustrating as that can be). Dr.
James Flore for his suggestions in the design of the field experiment and an elegant means
to predict and compare divot closure rates. I would like to express my appreciation to
the staff at the HTRC including Mark Collins, who encouraged me to work for Bruce as

an undergraduate, and Allaire Goetschy, who was a great help in my field research.

1‘

TABLE OF CONTENTS

LIST OF TABLES ........................................................................................................... vi
LIST OF FIGURES ......................................................................................................... vii
INTRODUCTION .......................................................................................................... 1
MATERIALS AND METHODS ........................................................................... ‘ ........ 8
RESULTS AND DISCUSSION ...................................................................................... 16
CLIPPING PRODUCTION ................................................................................ 16
TURFGRASS QUALITY ................................................................................... 31
DIVOT CLOSURE .............................................................................................. 33
WEAR TOLERANCE ......................................................................................... 36
DISEASE ACTIVITY .......................................................................................... 38
APPENDIX ..................................................................................................................... 40

LIST OF REFERENCES ................................................................................................. 55

LIST OF TABLES

Table nge
1 Analysis of variance table for a 3-factor RCBD ......................................... 15
2 The influence of nitrogen regime, PGR type, and PGR rate on

cumulative growth reduction of a creeping bentgrass fairway.

1994, 1995 ................................................................................................... 28
3 The effect of three plant growth regulators on quality ratings for a

creeping bentgrass fairway. 1994, 1995 ...................................................... 32
4 The effect of the interaction of nitrogen regime, plant growth regulator,

and rate on divot closure rate of a creeping bentgrass fairway.

1994, 1995 ................................................................................................... 34
5 The effect of plant growth regulator treatment and nitrogen regime on

wear tolerance of a creeping bentgrass fairway. 1995 ................................. 37
6 The effect of plant growth regulator treatment and nitrogen regime on

number of centers of dollar spot infection on a creeping bentgrass
fairway. 1995 .............................................................................................. 39

Fi ure

3a

3b

4a

4b

5a

5b

6a

6b

LIST OF FIGURES

Page
Radial Sod Extraction Device (Divot Cutter) .............................................. 10
Linear Wear Induction Mechanism (Wear Sled) .......................................... 13
Clipping production of a creeping bentgrass fairway as affected
by two nitrogen regimes of 125 and 250 kg ha'1 year“1 and daily
mean temperature. 1994 .............................................................................. 17
Clipping production of a creeping bentgrass fairway as affected
by two nitrogen regimes of 125 and 250 kg ha‘1 year'1 and daily
mean temperature. 1995 .............................................................................. 18

Clipping production of a creeping bentgrass fairway as affected by the
main effect of nitrogen. 1994 ....................................................................... l9

Clipping production of a creeping bentgrass fairway as affected by the
main effect of nitrogen. 1995 ....................................................................... 20

Clipping production of a creeping bentgrass fairway as affected by the
main effect of plant growth regulator. 1994 ................................................ 22

Clipping production of a creeping bentgrass fairway as affected by the
main effect of plant growth regulator. 1995 ................................................ 23

Clipping production of a creeping bentgrass fairway as affected by the
main effect of rate averaged over PGR and nitrogen. 1994 ......................... 25

Clipping production of a creeping bentgrass fairway as affected by the
main effect of rate averaged over PGR and nitrogen. 1995 ......................... 26

vii

Appendix

Clipping production as affected by flurprimidol at three rates where
nitrogen regime is 125 kg ha" year". 1994 .................................................. 40

Clipping production as affected by paclobutrazol at three rates where
nitrogen regime is 125 kg ha" year". 1994 .................................................. 41

Clipping production as affected by trinexapac-ethyl at three rates where
nitrogen regime is 125 kg ha'1 year". 1994 .................................................. 42

Clipping production as affected by flurprimidol at three rates where
nitrogen regime is 250 kg ha" year". 1994 .................................................. 43

Clipping production as affected by paclobutrazol at three rates where
nitrogen regime is 250 kg ha" year". 1994 .................................................. 44

Clipping production as affected by trinexapac-ethyl at three rates where
nitrogen regime is 250 kg ha" year". 1994 .................................................. 45

Clipping production as affected by flurprimidol at three rates where
nitrogen regime is 125 kg ha" year". 1995 .................................................. 46

Clipping production as affected by paclobutrazol at three rates where
nitrogen regime is 125 kg ha" year". 1995 .................................................. 47

Clipping production as affected by trinexapac-ethyl at three rates where
nitrogen regime is 125 kg ha" year". 1995 .................................................. 48

Clipping production as affected by flurprimidol at three rates where
nitrogen regime is 250 kg ha" year". 1995 .................................................. 49

Clipping production as affected by paclobutrazol at three rates where
nitrogen regime is 250 kg ha" year". 1995 .................................................. 50

Clipping production as affected by trinexapac-ethyl at three rates where
nitrogen regime is 250 kg ha" year". 1995 .................................................. 51

viii

Cumulative observations of quality indices of a creeping bentgrass
fairway as affected by plant growth regulator treatment and nitrogen
regime. 1994 ................................................................................................ 52

Cumulative observations of quality indices of a creeping bentgrass
fairway as affected by plant growth regulator treatment and nitrogen
regime. 1995 ................................................................................................ 53

Casual observations of scalping severity of a creeping bentgrass
fairway as affected by plant growth regulator treatment and nitrogen
regime. 1995 ................................................................................................ 54

INTRODUCTION

Plant Growth Regulators

Plant growth regulators are any number of naturally occurring or synthetic
compounds that regulate plant growth and development. Plant growth regulators Can
have endogenous or exogenous sources. Plants produce several compounds such as
abscisic acid, auxins, cytokinins, ethylene, gibberellins, and polyamines that all regulate
plant growth processes. Although these naturally occurring plant growth regulators
control plant functions, most synthetic plant growth regulators are applied to turfgrass to
reduce seedhead or clipping production. Kaufmann (1986) and Watschke (1985),
classified plant growth regulators (PGRS) used in turf as Type I and Type II.

Type I PGRS can temporarily reduce inflorescence appearance and vegetative
growth of turfgrass by inhibiting cell division and cell elongation through the disruption of
cytokinin biosynthesis in the plant. Mefluidide, introduced in the early 1970’s, is an
example of a Type I PGR. The injury to turf that often accompanied mefluidide
applications was unacceptable for golf course turf and limited its use.

Type II PGRS suppress cell” elongation through inhibition of gibberellin
biosynthesis. Type II growth regulators tend to extend the period of growth reduction

and have less injury to turf than Type I compounds but have little effect on seedhead

production (Watschke, 1981). Two Type II PGRS, flurprimidol and paclobutrazol, were
labeled for turf use in the 1980’s. These products were primarily used for clipping
reduction and conversion of mixed species stand to a monoculture. The 1993 labeling of
trinexapac-ethyl as a PGR for turfgrass rekindled research in this area. Most Type II
PGRS, such as flurprimidol and paclobutrazol, are absorbed by the crown and root
(Beard, 1985, Batten, 1983 and Barrett & Bartuska, 1982), inhibit gibberellin biosynthesis
through interference with cytochrome P-450-dependent reactions which blocks the
oxidation of ent—Kaurene to ent—Kaurenoic acid (Jennings et al., 1993). Trinexapac-ethyl

is foliar absorbed and affects a different site of action by blocking the 3B-hydroxylation of

GAzo to GAI in gibberellin biosynthesis. Due to the differences of these Type II PGRS,
Watschke and DiPaola (1995) reclassified PGRS used on turf into three groups. The
Type II PGRS were broken into two groups. They listed trinexapac-ethyl as a Class A
PGR because of its activity at the final step in gibberellin biosynthesis and fewer reported
Side-effects. Flurprimidol and paclobutrazol were labeled as Class B PGRS due to a
different Site of action, uptake, and perceived increase in turfgrass injury that occurred
from their application. Class C PGRS are the mitotic inhibitors that are used for turfgrass
seedhead inhibition and included all Type I PGRS.

Most of the recent PGR research on turf has included the Type II PGRS and their
effects on clipping production as measured by mowing frequency (Johnson, 1992, 1993,

1994) and secondary plant processes such as tillering, carbohydrate partitioning, and root

growth (Hanson & Branham, 1987, Menn et al., 1991, Hull, 1992, Lee & Diesburg, 1993,
Spak et al., 1993, Christians, 1994).

Golf course managers use Type II PGRS to reduce clipping production which
results in lower labor and equipment maintenance costs. Clipping production is also a
concern as clipping disposal becomes cost prohibitive or unavailable. PGRS may reduce
shoot growth so that bagging is not necessary and clippings can be returned to the turf
without interference with turf usage. With increased use in turfgrass, questions arise
regarding the effect of PGRS on clipping production, recovery from injury, and tolerance
to stresses.

The reduction in shoot growth in response to PGRS is commonly accepted as well
as the PGR influence, positive or negative, on turfgrass quality. The reduction in shoot
elongation from PGRS can be dramatic. Johnson (1992, 1993, 1994) has characterized the
responses of many warm and cool season turfgrasses to PGRS. He observed a reduction
in mowing frequency needed to maintain roadside and landscape turfgrass. Overall
reduction has been measured as high as 83% without loss to turfgrass quality (Razmjoo et
al., 1994). However, PGR application is commonly associated with a loss of turfgrass
quality.

Turfgrass quality is defined by Several characteristics, including color, texture,
density, and uniformity. Turfgrass color has a major impact on overall quality. Turfgrass
quality losses from PGRS, usually foliar burn, are typically observed when the PGR
chemicals are applied at rates required to achieve desired growth reduction (Elkins et al.,

1974). DiPaola et a1. (1985) reported that turfgrass discoloration from PGRS can be

increased by moisture Stress. However, turfgrass quality losses that occur after PGR
applications are usually temporary and are often followed by improved turfgrass quality
as compared to untreated plots. Johnson and Faulkner (1985) reported that
paclobutrazol treated plots were initially discolored and then observed darker green
foliage 3 to 4 weeks after application. PGRS applied to warm season turfgrass increased
turfgrass density (Menn et al., 1991). McElroy (1984) observed darker green foliage after
the PGR effects wore off and that while the dark green turfgrass color was a desirable
attribute of PGR usage, foliar appearance of a sward may not necessarily reflect its
overall quality.

The optimum time for PGR application is in the spring when PGRS will not delay
the greening of the turfgrass and greatest growth reduction is realized. Watschke et a1.
(1992) listed several researchers who observed rapid growth flushes following foliar
suppression that may negate the savings from reduced mowing experienced earlier in the
season. Most of the published research on PGR use is based on single, not multiple PGR
applications. It seems useful to consider the benefit of multiple PGR applications
through the season to reduce the growth rebound and extend the growth reduction period.

Research examining the divot recovery of PGR treated turfgrass has been limited.
This research has almost exclusively focused on warm season turfgrass species. The
effect of PGRS on the tillering and rhizome growth and total nonstructural carbohydrate
levels of turfgrass has been reported for flurprimidol, paclobutrazol (Hanson and
Branham, 1987), and trinexapac-ethyl (Han and Fermanian, 1995). This information may

indicate what effect PGR applications will have on divot closure. Flurprimidol has been

shown to increase density of treated stands of Kentucky bluegrass (Demoeden, 1984).
Tillering of some turfgrasses was enhanced but lateral growth was greatly reduced.
Shearing and Batch (1982) reported that the stems and stolons of paclobutrazol treated
turfgrass became thicker and more prostrate and rosette-like in growth habit. Hanson and
Branham (1987) measured increase photosynthate partitioning into tillers of Kentucky
bluegrass at 2 to 4 weeks following application of flurprimidol and paclobutrazol. In
most studies tillering and turfgrass density were increased by PGR application. This was
attributed to a reduction in intemode length. Increased turfgrass density does not
necessarily indicate accelerated divot closure. As shoot growth is inhibited it would
follow that stolon growth would also be reduced. PGR treatment would most likely
result in delayed divot closure for turfgrass.

Brueninger and Watschke (1982) found that regardless of the PGR type used,
treated golf course roughs did not tolerate traffic as well as nontreated areas. They
concluded that the recuperative potential of PGR treated turfgrass is reduced, if wear
damage occurs, due to the decreased shoot growth. This conclusion did not consider
whether the wear tolerance. resistance to damage. of turf was altered by PGR application.
Testing wear tolerance, not recuperative potential. would measure any change in the
amount of wear required to induce damage to a PGR treated turfgrass. Recuperative
potential measures turf recovery after damage occurs. Traffic on untreated turfgrass
caused visible wear damage but had less obvious effects on shoot growth and root growth
(Cuddeback and Petrovic, 1985). Although winter hardiness and thatch accumulation

were reduced by traffic they found little or no effect on root growth. Traffic includes

compaction and wear components. Any experiment that examines wear should take care
to separate wear from traffic. Wear damage should be evaluated 1 to 2 days after the
damage has been imposed (Carrow and Petrovic, 1992).
Nitrogen

The beneficial effects of nitrogen on turfgrass are often assumed. A wealth of
published information is available on the specific influences of nitrogen on turfgrass
systems. Nitrogen fertilization is one of the main tools used by golf course
superintendents to maintain healthy, playable turfgrass surfaces on golf courses. The
aesthetics of fine turfgrass are enhanced with nitrogen leading to greater uniformity and
darker green color. While nitrogen increases color there is also a decrease in the total
nonstructural carbohydrate levels in rhizomes (Goatley et al., 1994). Snyder and Schmidt
(1972) showed that nitrogen enhanced turfgrass appearance and shoot growth but
severely reduced root growth. Finally. light frequent applications of nitrogen have been
shown to provide the overall best turfgrass quality but were not correlated closely with
root growth (Carrow. 1989).

Many types of turfgrass injury are remedied by application of nitrogen fertilizers.
Divots and wear damaged turfgrass recover more rapidly when nitrogen is readily
available to the turfgrass. Divot recovery was more rapid if clippings were returned and
increased nitrogen was provided (Shearman. 1989). Under traffic conditions turfgrass
quality increased with increased nitrogen nutrition (Shearman, 1989). Eggens and Carey
(1988) evaluated wear from riding mowers on creeping bentgrass fairways. Moderate

nitrogen fertility can enhance wear tolerance by increasing total cell wall content on a dry

weight basis and recuperative potential of turfgrass (Beard, 1973). Very high nitrogen
levels can have deleterious effects on turfgrass wear tolerance due to hydration of the
plant cell and thinning of the cell wall (Beard, 1973). Unfortunately, the nitrogen rates
necessary to maintain recuperative potential of turfgrass also inhibit root production and
increase the shoot growth creating greater clipping production. It would be desirable if a
balance existed where the turfgrass community retained the benefits realized from nitrogen
without excess top growth.

If a decrease in top growth is associated with the enhancement of other plant
processes then perhaps plant growth regulators could be used to force secondary growth
during the non-optimal conditions that occur in the summer. Initially, concerns were
raised as to the effectiveness of PGRS on high maintenance turfgrass. However,
tn'nexapac-ethyl and flurprimidol remain effective at reducing tissue production in high
and low nitrogen regimes (Christians, 1994).

The objectives of this research were a) to determine the effects of three plant
growth regulators on the production of clippings and on the quality of a creeping
bentgrass fairway throughout the season maintained under a high and low nitrogen
fertility regime, b) to evaluate the effect of three plant growth regulators on divot closure
and wear tolerance of a creeping bentgrass fairway, and c) to evaluate the benefit of Single

vs. multiple applications of PGRS on clipping production through the season.

MATERIALS AND METHODS

This research was initiated during 1994 at the Hancock Turfgrass Research Center
in East Lansing, MI. The turfgrass used in this experiment was a 3-year-old 'Penncross'
creeping bentgrass (Agrostis palustris Huds.) maintained at 1.6 cm. Soil type was a
Marlette sandy loam soil (fine-loamy, mixed, mesic Glossoboric Hapludalfs). Soil tests
determined that phosphorus and potassium were not limiting. The plots were irrigated to
prevent drought stress and received at least 2.5 cm irrigation or rain per week. Three
PGRS each at three rates were applied on two nitrogen regimes. In addition, a control
plot was included on each nitrogen regime. The treatments were arranged in a randomized
complete block design with three blocks for a total of 60 plots. These plots were
established inside a 334 m2 irrigation block. Individual plots measured 1.22 by 4.27 m.
The plots were divided into three areas for evaluation purposes. Clipping production and
turfgrass quality were measured on a 1.22 by 2.44 m area of each plot. Divot recovery
was measured on a 1.22 by 0.91 m area of each plot. Wear tolerance was evaluated on the
remaining 1.22 by 0.91 m area of each plot. Wear tolerance was only evaluated in 1995.

The PGRS included in this experiment were flurprimidol, paclobutrazol, and

trinexapac-ethyl. Each PGR was applied at 0.25, 0.50, and 1X, where X equals the

current label rate for creeping bentgrass maintained at fairway height. Those rates are

0.56, 0.28, and 0.2 kg ha" for flurprimidol, paclobutrazol, and trinexapac-ethyl,
respectively. PGR treatments were made on June 7, July 5, and August 2 of 1994.
Application dates for 1995 were May 1, May 29, June 27, and July 24. PGR appliCation
was made using a COz-pressurized four-nozzle backpack Sprayer that was calibrated to
deliver 518 L ha" at 276 kPa.

Two fertility regimes were established with nitrogen levels of 125 and 250 kg ha"
year". Monthly applications of urea (46-0-0) were made from May 15 to November 15.
For the 250 kg N ha" year" regime, 50 kg N ha" were applied in May, June, September,
and November and 25 kg N ha'l were applied in July and August. Rates were reduced by
1/2 for the 125 kg N ha" year" regime. The urea was applied using a COz-pressurized
four-nozzle walk-behind sprayer. The sprayer was calibrated to 320 L ha" at 215 kPa.

Clippings were collected with a John Deere 20 SR7, Moline, IL., walk behind reel
mower. One mower pass was made down the center of the plots collecting clippings
from a l m2 area. Clippings were taken once per week after three days growth. Clippings
were oven dried for 48 h at 50° C and weighed. The entire plot area was mowed two
more times per week with a riding fairway mower. The clippings were removed but not
weighed from these two additional mowings.

Divots were made with a divot tool. The divot tool was developed to ensure
consistent divot size. The divot tool was constructed with a cutting blade attached to a
wooden handle that pivoted on an axle above the ground (Figure 1). The machine cut an
arc that was 1.25 cm deep by 9 cm long by 5 cm wide. One divot was made in each plot

on a weekly basis for four weeks after the first PGR application. To evaluate

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recovery, a template was made to the size of the original divot. The recovery template
consisted of a grid of 72 sections. Each grid section measured 50 by 75 mm. Percent
recovery was determined by dividing the observed number of filled sections by 72 and
multiplying by 100. Divot recovery was rated weekly until the entire grid filled. Divot
recovery data were analyzed using linear regression to estimate days to 90 percent
closure.

Visual ratings of the plots were done to determine the effects of fertility regime
and growth regulator treatment on overall turfgrass quality. These ratings were based on a
1-9 scale where l=dead, 5=acceptable, and 9=excellent.

Minor modifications in the experiment were made in 1995. An additional PGR
application was made in May of 1995. Weekly nitrogen treatments were substituted for
monthly fertilizer applications in an attempt to minimize the growth surge associated
with monthly fertilizer applications. Weekly applications of urea were made from April
18 to September 26, 1995. For the 250 kg N ha" year" regime 62.5 kg N ha" were
applied in May, 50 kg N ha" were applied in June and September, and 30 kg N ha" were
applied in April, July. and August. Rates were reduced by 1/2 for the 125 kg N ha" year'
' regime. Clippings were collected twice per week in 1995, and at no time were the plots
mowed with a riding unit, hence all clippings were collected. One divot was made in each
plot for four weeks after the first and second PGR applications for a total of eight divots
per plot. The second set of divots correspond by date to the June divots made in 1994.
Divot recovery was measured twice per week in 1995 to more precisely predict the rate

of recovery.

12

Wear tolerance of creeping bentgrass as affected by PGR treatment and nitrogen
regime was evaluated in 1995. A drag sled was developed to provide wear (Figure 2) to
the turfgrass. The drag sled was made of a 17.5 mm thick plywood that measure 1.22 by
2.13 m. The entire contact surface of the sled was covered with a 6.4 mm thick
polypropylene sheet. Structural support for the sled and the tongue were made of lineal
lumber. The gross weight of the drag sled was 90 kg and applied 0.003 kg cm2" of
compaction. In comparison a person walking applies about 0.365 kg cm2" of
compaction, so compaction was considered nil.

The sled was effective in applying wear damage to the turf. However, the number
of passes needed to induce damage led to extreme injury of turf in the tire tracks. The
wear sled was pulled across the plots 15 times per day, six days per week for three
weeks. The smooth polyprop) lene sheet did not provide enough resistance to efficiently
apply wear. Covering the bottom of the sled with a canvas tarp (or other abrasive
material) would decreaw the number o1 passes required for damage to occur. Although
this was not done in this stud}

The turfgrass disease dollar spot (Sr‘lr'rntinia homoeocarpa), was very active in
1995. Dollar Spot is otten exhibited as 2 to 3 em circles of necrotic turf. If left untreated
these necrotic circles exentualh coalesce This disease was controlled with a contact
fungicide in 1995. Before tungtetde treatment the number of dollar spot centers of
infection were recorded '1 \w nhxcnttllnns were made during the summer in 1995 when

clear differences hem een PGR treatments and nitrogen fertility were observed.

13

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Clipping production data and quality ratings were transformed to percent of
control for analysis. Data were transformed by dividing by the corresponding low or high
nitrogen control for each block. Analysis of variance was performed (Table 1) on all data

and means were separated using Fisher’s protected LSD procedure at the 0.05 level of

probability.

15

Table 1: Analysis of variance table for a 3-factor RCBD.

 

 

K Value Source Degrees of Freedom
1 Replication 2
2 Nitrogen Regime 1
4 PGR Type 2
6 N x PGR 2
8 PGR Rate 2
10 N x Rate 2
12 PGR x Rate 4
14 N x PGR x Rate 4
-15 Error 34
Total 53

RESULTS AND DISCUSSION

Clipping Production

The clipping production from a creeping bentgrass fairway, not treated with PGR,
as affected by nitrogen regimes of 125 and 250 kg ha" year" and daily mean temperature
is Shown in Figures 3a and 3b for 1994 and 1995, respectively. Maximum turfgrass
growth occurred during the month of June in both years. Clipping production decreased
markedly at the end of June and rebounded in mid-July only to decline in August. An
additional growth peak occurred in 1995 in response to high temperatures in August.

Nitrogen applied at 250 kg ha" year"

increased clipping yield. Clipping production for
the seasons were 121 and 228 gm m'2 in 1994 and 125 and 220 gm m'2 in 1995 for the low
and high nitrogen regimes, respectively.

The influence of nitrogen fertility level and three plant growth regulators each applied at
three rates on clipping production was analyzed as a multifactorial design. Main effects
are reported in Figures 4 to 6 as less than 20 percent of the observation dates had
significant interactions. Clipping production of a creeping bentgrass fairway as affected
by nitrogen regime, averaged over PGR and PGR rate, is shown in Figures 4a and 4b for

1994 and 1995 for the 125 and 250 kg N ha" year" regimes, respectively. On a

percentage basis clipping production was reduced similarly by the three PGRS at both

16

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nitrogen regimes. However, for the entire 1994 season, a greater percentage reduction was
observed for the 250 kg N ha'1 year‘l regime than the 125 kg N ha'‘ year " regime. The
short period of growth reduction (<28 days) in 1994 was attributed to the monthly
application of nitrogen that was applied 14 days after the PGR treatments. Nitrogen
applications were made two weeks after each PGR treatment and coincided with the
observed growth surge in the PGR treated plots.

In 1995, PGRS provided more growth reduction for the 125 kg N ha"l year”l
regime than the 250 kg N ha'1 year'l regime. Growth reduction from PGR treatment was
again followed by a growth surge, although nitrogen applications were made weekly in
1995.

PGRS were effective at reducing clipping production on both low and high
nitrogen regimes. The effect of PGR, averaged over nitrogen regime and PGR rate, on
clipping production of a creeping bentgrass fairway is shown in Figures 5a and 5b for
1994 and 1995, respectively. In 1994, all three PGRS provided similar levels of growth
reduction. There were periods of growth reduction observed after the first and third PGR
application date. However, there was no reduction from the second PGR application.
Periods of growth reduction were followed by a growth surge when the clipping
production of PGR treated turfgrass plots exceeded the production of the control plots.

Creeping bentgrass treated with PGRS in 1995 followed the same trend of growth
reduction followed by growth surge. The maximum growth reduction was observed after
the first PGR application and lasted 4 to 6 weeks. No additional reduction was realized

from the second PGR application and by the third application date the clipping

22

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24

production from all plots treated with PGRS exceeded the production of control plots. A
short period of growth reduction followed the third PGR application but the growth again
rebounded and exceeded that of the controls. PGR treatments from the fourth application
caused the same pattern of growth reduction and growth surge.

The effect of PGR rate, averaged over PGR type and nitrogen regime, on clipping
production of a creeping bentgrass fairway is shown in Figures 6a and 6b for 1994 and
1995, respectively. The highest (1X) rate caused the greatest growth reduction in 1994.
The PGRS at the lowest two rates did not reduce the amount of clippings produced as
compared to the control in 1994. The PGRS at the highest rate were also most effective
at reducing clipping production in 1995, but the reduction of clipping production was lost
to an increase in the rebound amplitude for the plots receiving the high PGR rate. The
PGR treatments causing the greatest reduction of clipping weight also exhibited the largest
rebound peaks during the growth surges.

Following the first PGR application in 1995, differences in duration of the growth
reduction period were observed. The PGRS applied at the 0.5 and 1X rates reduced
clipping weight >50 percent for six weeks where the PGRS applied 0.25X rate reduced
clipping weight >50 percent for four weeks. Greater separation of clipping production
between PGR rates existed in 1994 than in 1995. PGRS at the 1X rate provided more
growth reduction than the other rates. Differences in growth reduction from PGR rate
were only observed after the first PGR application. No differences in growth reduction

were evident from PGR rate after the first PGR application in either year.

25

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27

It is possible that the growth surge could be a side effect of PGR treatment and
not solely attributable to nitrogen fertilization. One explanation is while PGRS inhibit
specific sites of gibberellin biosynthesis, the formation of GA] precursors most likely
continues. These precursors, if not broken-down or translocated, may accumulate in the
GA biosynthesis pathway. When the PGRs no longer inhibit gibberellin biosynthesis,
abnormally high concentrations of GA could result. GA] is usually found in very small
amounts in the plant. PGR treatment is not considered to affect carbohydrate production
in the plant. With an adequate carbohydrate source, higher than normal levels of GAI
could lead to greater cell elongation and increased clipping production. The consistent
presence and increasing amplitude of this growth surge or rebound may indicate that the
production of carbohydrates and gibberellin biosynthesis precursors continues during
foliar growth reduction. These products may not necessarily be redistributed through the
plant during foliar growth reduction. On the contrary, it appears that sugars and GA
precursors are accmnulating in the plant during the growth reduction period and then
cause a growth surge when the inhibition wears off. Clipping production for a given
treatment period is then redistributed in time. counteracting some of the reduction effects
with subsequent growth surges.

When analyzing the data on a growth reduction basis (percent of control), growth
reduction was greater on the high nitrogen plots in 1994, but in 1995 the opposite was
true (Table 2). There was no statistical difference between the effect of PGRS on clipping
production in 1994, but in 1995 turf treated with flurprimidol and paclobutrazol

displayed greater growth reduction than turf treated with trinexapac-ethyl. Finally, the

28

Table 2: The influence of nitrogen regime, PGR type, and PGR rate on cumulative
growth reduction of a creeping bentgrass fairway.

 

Main Effect 1994 1995
—% of control—
125 kg N/ha' 101.6 b4 83.2 a
250 kg N/ha 83.5 a 89 b
Flurprimidolz 91.6 a 82.6 a
Paclobutrazol 90.7 a 82.9 a
Trinexapac-ethyl 95.4 a 92.7 b
Low Rate3 98.5 b 88.2 a
Medium Rate 99.6 b 83.7 a
High Rate 79.6 a 86.3 a

1 Low and high nitrogen effect averaged over PGR type and PGR rate.

2 PGR type effect averaged over nitrogen regime and PGR rate.

3 PGR rate effect averaged over nitrogen regime and PGR type.

4 Means followed by the same letter are not different according to Fisher’s
Protected LSD at the 0.05 significance level. Do not compare means across main
effects.

29

growth reduction response to PGR rate showed that in 1994 the high rate was needed for
maximum seasonal growth reduction whereas in 1995 all treatments elicited the same
response. This illustrates the effect of the growth rebound since the high PGR rate
reduced growth more than the other treatments but also had a greater growth surge.
Creeping bentgrass treated with plant growth regulators can provide growth
reduction for 4 to 6 weeks after the initial application on either low or high nitrogen
regimes. The growth reduction benefits of repeat applications of PGRS on 28 day
spacing are minimal. These additional applications of PGRS reduce clippings of a
creeping bentgrass fairway for only a short time followed by a growth surge where the
clipping production of PGR treated turfgrass exceeds the clipping production of turfgrass
in the control plots. On a seasonal basis the growth surge that occurs from PGR
applications counteracts some of the growth reduction that was earlier realized. This
rebound in growth was initially attributed to nitrogen applications but when nitrogen was
applied weekly instead of monthly the rebound in growth was still evident and
determined to be an effect of PGR application. Creeping bentgrass treated with PGRS
applied at the full label rate displayed the greatest growth reduction. However, the
turfgrass in these plots also experienced the largest growth rebound peaks. Future studies
should examine the rebound growth of creeping bentgrass from application of Type II
PGRS. These studies could incorporate shorter PGR application intervals (<28 days)
using full and sub-label rates. The current understanding PGRS suggests that while cell
elongation is reduced from PGR application, secondary plant growth processes are

enhanced. This may be true, but the data from this study indicate that sugars and GA.

30

precursors may be accumulating during the inhibition period and are not being used
elsewhere in the plant or breaking down. When the PGR no longer functions to stop
gibberellin biosynthesis, the large store of precursors in the pathway generate a greater
growth rate than before inhibition occurred.

The clipping production of a PGR treated turf for the season, is then very similar
to the clipping production of turfgrass that did not receive PGR treatment. This is
illustrated by the >50 percent growth reduction for four weeks after the first PGR
application in 1995. However, growth reduction for the season is not’dramatic. Although
the clipping weights for the season are not reduced, there could still be benefits to be
realized from PGR treatment.

Depending on the objective, PGRS may or may not be a cost effective
management tool for creeping bentgrass fairways. If clipping reduction is the main
objective then maximum growth suppression is desired. Reduced clipping production
would lower the labor and handling costs. Reduced clipping production would also
decrease the time required to mow fairways. The operator would not have to empty the
buckets as often and could mow more area before needing to dispose of the collected
clippings. The maximum turf growth occurs in the spring. PGRS appear to be very
effective when applied at this time. However, growth surges that occur during the
summer appear to negate clipping reduction from earlier in the season. A growth surge
may not necessarily be detrimental to the objective. Turf growth is less rapid in the
summer. A more vigorous growth rate in the summer when the controls are growing

slowly, will increase turfgrass tolerance to disease and wear.

31

Maximum growth reduction, for the purpose of reduced mowing frequency or
reduced clipping disposal, may be difficult to consistently attain with multiple
applications of PGRS. Reducing growth in order to return clippings to the turf is perhaps
a more attainable objective. The amount of growth reduction necessary to work clippings
into the turf would be less than that required to reduce mowing frequency. Daily turf
growth may be reduced by PGRS so that the amount of clippings returned to the turf
does not interfere with playability. Growth surges that follow growth suppression
would not necessarily violate this objective. Growth at 200 percent of control would be
irrelevant if the clipping production remained below the tolerance threshold. If the
clippings produced did not interfere with playability, returning clippings to the turf
would equate to significant labor savings. The operator could remain on the mower during
the time that was previously spent emptying mower buckets. Using PGRS in order to
return clippings to the turf may be a more achievable goal.

Whether using PGRs for clipping disposal reduction or to allow re-incorporation
of clippings to the turf, well defined objectives will help to determine the success of the
PGR program.

Turfgrass Quality

The effects of three plant growth regulators, averaged over nitrogen regime and
PGR rate, on quality ratings of a creeping bentgrass fairway are given in Table 3 for 1994
and 1995. The effects of increased nitrogen on turfgrass quality were always evident and
are not reported. The enhancement of turfgrass quality from PGR applications that has

been reported by other researchers was observed in this study. However, the injury

32

Table 3: The effect of three plant growth regulators on a creeping bentgrass

 

fairway.
PGR 1994 1995
—% of control—
Flurprimidol 104.9 ab‘ 113.7 a
Paclobutrazol 100.1 b 111.5 a
Trinexapac-ethyl 108.3 a 113.1 a

1 Means followed by the same letter are not different according to Fisher’s
Protected LSD at the 0.05 significance level.

33

associated with PGR treatments was subtle. Turfgrass treated with tn'nexapac-ethyl had
significantly higher quality for 1994 than the turfgrass in the controls or the paclobutrazol
treated plots. All PGRs positively enhanced turfgrass quality in 1995. Quality ratings
were regressed against clipping production to check for a correlation between low quality
and growth reduction. There was not a strong relationship between these two sets of
observed data (r=0.48, l994)(r=0.43, 1995).
Divot Closure

The effect of the interaction of nitrogen regime, plant growth regulator, and rate on
divot closure rate of a creeping bentgrass fairway is shown in Table 4 for 1994 and 1995.
Nitrogen at the 250 kg ha'1 year’l regime accelerated divot closure rate of the control plots
by an average of ten days in both years on all divots observed. The effects of PGRs was
generally more dramatic on the divot closure rate of the turfgrass receiving 125 kg N ha’1
year". Although inhibition of divot closure from PGR was hypothesized, this was not
observed in 1994. Divot closure rate was similar to the control for all treatment
combinations in 1994. Large differences among PGR treatments on divot closure rate
were not evident. Creeping bentgrass treated with the 0.25 and 0.5X rates of flurprimidol
and the 0.5X rate of paclobutrazol displayed the fastest divot closure rate for 1994. Two
sets of divots were made in 1995, one set of four divots after the first and second PGR
applications. Turfgrass treated with PGRS displayed some inhibition of divot closure
rate in 1995. Divots made on turfgrass treated with trinexapac-ethyl provided the most

consistent divot recovery results in 1995. The divot' closure rate of trinexapac-ethyl

treated turf was no different (1St set) or slightly faster (2nd set) than the divot closure rate

34

Table 4: The effect of the interaction of nitrogen regime, PGR type, and PGR rate
on divot closure rate of a creeping bentgrass fairway.

PGR Rate 1994 1995 1st set 1995 2nd set

 

kg hatl Low N High N Low N High N Low N High N
——days to 90% divot closure

 

Control 34 23 34 25 42 30
Flurprimidol 0.28 27 24 38 27 45 32
F lurprimidol 0.42 29 24 46 28 45 33
Flurprimidol 0.56 32 25 43 34 45 38
Paclobutrazol 0.07 31 23 34 27 36 29 ~
Paclobutrazol 0.14 28 25 38 32 45 33
Paclobutrazol 0.28 34 25 53 34 46 34
Trinexapac-ethyl 0.05 33 22 40 26 41 26
Trinexapac-ethyl 0.1 33 19 34 24 38 28
Trinexapac-ethyl 0.2 32 22 36 27 36 32

LSD (p=0.05) 5 6 6

 

35

of the controls. The divot closure rate of turfgrass treated with the 0.5 and 1X rate of
flurprimidol and paclobutrazol was slower for the first set than the second set of divots in
1995. The divot closure rate of turfgrass in the control plots for the second set of divots
was slower than the divot closure rate of turfgrass in the control plots for the first set of
divots by 5 to 10 days. The inhibitory effect of flurprimidol on divot closure rate that
was observed in the first set of divots was only present at the 1X rate in the 250 kg N ha-
1 year-l regime. No inhibition of divot closure rate was seen on turfgrass treated with
paclobutrazol on the second set of divots in 1995.

PGRs did not inhibit divot closure rate as much as had been expected. Although
divot closure rate of turfgrass treated with flurprimidol and paclobutrazol displayed some
delay in divot closure in 1995, divot recovery rate of turfgrass treated with both products
was enhanced in 1994. Divot closure rate of the creeping bentgrass, in general, was not
inhibited by PGR treatment. It was initially thought that divots made during growth
reduction would be obvious for a longer period of time. This would allow a longer
window for weed encroachment and a disruption to the playability of the turfgrass. The
creeping bentgrass turf treated with PGRS did not effect the divot closure rate for 1994.
The 1X rate of flurprimidol and paclobutrazol delayed divot closure in 1995 on the first
set of divots. Trinexapac-ethyl did not change divot closure rate from the control plots
for either set of divots in 1995. Statistical differences in divot closure rate do not
necessarily indicate biological significance. The additional number of days required to

allow weed encroachment may be greater than that observed in this study. Inhibition of

36

divot closure from Type II PGRs on creeping bentgrass may not be a great concern to the
turfgrass practitioner.
Wear Tolerance

The effect of plant growth regulator treatment and nitrogen regime on wear
tolerance of a creeping bentgrass fairway is shown in Table 5 for 1995. Wear can affect
turf quality by reducing density and vigor. Quality ratings were used to evaluate wear
damage. In this scale a l=dead, 5=acceptable, and 9=excellent. Quality ratings for
creeping bentgrass that did not receive PGR application under wear conditions were
increased to acceptable levels under the 250 kg N ha" year'1 regime. Trinexapac-ethyl
was the only PGR treatment that resulted in acceptable turfgrass quality ratings on the
low nitrogen regime. The turfgrass quality values under the low nitrogen regime were
similar to those under the high nitrogen regime for those plots treated with trinexapac-
ethyl. Creeping bentgrass treated with the 0.5X rate of flurprimidol and paclobutrazol
and all rates of trinexapac-ethyl displayed acceptable turfgrass quality in the high fertility
regime. Turfgrass quality under the high nitrogen regime was not statistically different
from the high nitrogen control. Wear tolerance of creeping bentgrass was not enhanced by
the application of flurprimidol or paclobutrazol. Both products decreased turfgrass
quality observed on the low nitrogen regime although it was not always statistically
different from the control plots. All .plots treated with trinexapac-ethyl displayed
acceptable turfgrass quality. F lurprimidol treated plots exhibited higher quality ratings at
the 250 kg N ha‘I year‘l regime. Nitrogen fertilization did not affect the quality

trinexapac-ethyl treated creeping bentgrass.

37

Table 5: The effect of plant growth regulator treatment and nitrogen regime
on wear tolerance of a creeping bentgrass fairway.l995

 

 

PGR Rate Low N High N
kg ha'I —Quality Indexl

Control 4.3 5.0
Flurprimidol 0.28 3.1 4.2
Flurprimidol 0.42 1.4 5.1
Flurprimidol 0.56 1.8 4.0
Paclobutrazol 0.07 3.3 4.7
Paclobutrazol 0.14 4.4 5.3
Paclobutrazol 0.28 3.1 4.7
Trinexapac-ethyl 0.05 5. 1 5.7
Trinexapac-ethyl 0.1 6.3 5.7
Trinexapac-ethyl 0.2 5.5 5.5
LSD (p=0.05) 1.5

 

' Quality rating taken on 1-9 scale where l=dead, 9=excellent, and 5=acceptable.

38

Disease Activity

The fungal disease Dollar Spot (Sclerotinia homoeocarpa) was very active in
1995. Casual observations were made of dollar spot incidence. As would be expected,
the number of centers of infection of dollar spot were dramatically reduced for plots in
the high nitrogen regime (Table 6). However, PGR treatment also reduced the severity of
dollar spot infestation as compared to the control plots. The greatest reduction of
number of dollar spot lesions was observed on turfgrass treated with paclobutrazol. The
fungicidal activity of flurprimidol and paclobutrazol can be related to their chemical
structures. There are several different chemistries that yield sterol biosynthesis
inhibition. Examples of sterol inhibiting compounds are pyridines, pyrimidines,
imidazoles, and 1,2,4-triazoles. Flurprimidol and fenarimol, a commercial fungicide, are
pyrimidine compounds. The 1.2.4-triazole chemistry, which includes paclobutrazol,
includes commercial fungicides triadimefon and propiconazole. The relationship between
these PGRs and sterol inhibiting fungicides is clearly shown in their chemical structures

and fungicidal activity.

39

Table 6: The effect of plant growth regulator treatment and nitrogen regime
on number of centers of dollar spot infection on a creeping bentgrass

 

 

 

fairway. 1995
Season Average
PGR Rate Low N High N
kg ha'I number per plot
Control 75 24
Flurprimidol 0.28 53 20
Flurprimidol 0.42 37 20
Flurprimidol 0.56 29 13
Paclobutrazol 0.07 48 27
Paclobutrazol 0. 14 41 21
Paclobutrazol 0.28 13 8
Trinexapac-ethyl 0.05 50 35
Trinexapac-ethyl 0. l 55 28
Trinexapac-ethyl 0.2 5 1 23

LSD (p=0.05) 17

 

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LIST OF REFERENCES

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