Annual bluegrass control on greens

By and |  December 4, 2015 0 Comments

Annual bluegrass (Poa annua L.) is probably the most troublesome weed on golf courses worldwide.

Annual bluegrass control at greens height is difficult because of few labelled products, the risk of damage and no golfer tolerance of disturbed surfaces.

Annual bluegrass control at greens height is difficult because of few labelled products, the risk of damage and no golfer tolerance of disturbed surfaces.

In fairway-height turf, the growth regulators paclobutrazol (Trimmit), TGR (Paclobutrazol) or flurprimidol (Cutless) can be effective in shifting the competitive balance from annual bluegrass to creeping bentgrass. Selective postemergence herbicides such as bispyribac-sodium (Velocity), ethofumesate (Prograss) or mesotrione (Tenacity) can be effective for annual bluegrass control, depending on the desired species.

However, controlling annual bluegrass on greens is more difficult than on fairways because there are fewer labeled products, as well as potential injury to creeping bentgrass and the need to minimize or eliminate surface disruption for golfers. The growth regulators paclobutrazol or flurprimidol are most commonly used to reduce annual bluegrass in greens. Most of the previous research with growth regulators on greens was done in the southern U.S. and with applications made on four- to six-week intervals.

Only one research study evaluated paclobutrazol or flurprimidol in the northern half of the U.S. (Idaho and Washington) with modern application intervals (every two weeks). They found paclobutrazol or flurprimidol applied at two-week intervals increased creeping bentgrass cover up to 39 percent after two years in creeping bentgrass/annual bluegrass putting greens in Idaho and Washington. In this same study, season-long applications of the commercially available combination of flurprimidol plus trinexapac-ethyl provided marginal to no control of annual bluegrass, depending on rate and location. There currently are no postemergent herbicides labeled for annual bluegrass control on greens, but bispyribac-sodium has reduced annual bluegrass in greens-height turf in research trials, albeit with varying levels of discoloration or injury.

Researchers often used transects to accurately measure cover of annual bluegrass in this study.

Researchers often used transects to accurately measure cover of annual bluegrass in this study.

Annual bluegrass control studies traditionally are on one or two experimental sites and often only last for one or two years. However, given the variability of this species from site to site and its ability to adapt to environments, it’s important to understand long-term control methods and how these control methods vary over different sites and environmental conditions. Therefore, our objective was to evaluate six season-long regimes of herbicide or growth regulators for annual bluegrass control in creeping bentgrass greens over three or four years in three states in the northcentral U.S.
 

How we did it


TABLE 1
Location description and management of sites evaluating season-long programs for annual bluegrass control on greens-height turf
Nebraska Michigan Indiana
Age of stand at initiation of study 20 years 11 years 10 years
Creeping bentgrass cultivar Penncross Penncross Penncross
Source of annual bluegrass Natural infestation Natural infestation Natural infestation
Soil Type USGA sand/peat 93% sand, 6.5% silt, 0.5% clay Native soil (silt loam)
Mowing 0.105 inch 7X/week 0.125 inch 5x/week 0.156 inch 6X/week
Fertility 3.0 lbs. N/1000 ft2/year 3.0 lbs. N/1000 ft2/year 3.0 lbs. N/1000 ft2/year
Irrigation As needed to prevent drought stress As needed to preven tdrought stress As needed to prevent drought stress
Verticut 4-5 times during growing season 2-3 times during growing season None during this experiment
Aerification Solid tine (April & August) None during this experiment None during this experiment
Topdressing Light USGA sand every 2 weeks Light USGA sand every week Light USGA sand every 4 weeks

 
 
We conducted identical studies on creeping bentgrass/annual bluegrass turf maintained as greens at the William H. Daniel Turf Center at Purdue University in West Lafayette, Ind.; the Robert W. Hancock Turfgrass Research Center at Michigan State University in East Lansing, Mich; and Firethorn Golf Club in Lincoln, Neb. Site and management details are listed in Table 1. All experiments were arranged in a randomized complete block design with three replications. Individual plots measured 5 feet by 5 feet.

We applied herbicides and growth regulators season-long to the same plots according to recommendations from the label, company representatives or superintendents. We evaluated bispyribac-sodium (Velocity, Valent), paclobutrazol (Trimmit 2SC, Syngenta), flurprimidol (Cutless MEC, SePro Corp.), flurprimidol plus trinexapac-ethyl (Legacy, SePro Corp.), or trinexapac-ethyl (Primo Maxx, Syngenta). We included bispyribac-sodium because it had pending label changes when this experiment was initiated that would have allowed use on greens, but those changes currently are discontinued. We applied products up to 12 times per year at two-week intervals. Application rates and frequencies are listed in Table 2. Applications began in April of 2009 in Indiana and Michigan and in April 2010 in Nebraska, and continued through fall 2012 at all locations. We made applications with CO2-powered backpack sprayers in 88 gal/acre water at 30 psi, with a three-nozzle (8002VS flat fan, TeeJet Spraying Systems, Wheaton Ill.) spray boom.

Data collected included transect counts of annual bluegrass taken during peak seedhead production in spring prior to the first application and in the spring of 2013 following the last year of applications. We took estimates of annual bluegrass using a modification of the vertical-point quadrat method, where we laid a 4-foot by 4-foot or larger frame over the plots with an interval filament grid of 49 or 64 intersections, depending on location. We recorded the total number of times annual bluegrass was present under each intersection for each plot and calculated percentage cover. We also took visual ratings of annual bluegrass cover, turfgrass phytotoxicity and quality throughout each growing season. We performed analysis of variance using PROC ANOVA in SAS. Significant treatment by location interactions occurred, so we analyzed locations separately. We determined mean separation using Fisher’s least significant difference (LSD) at P≤ 0.05.
 

Wide variation among rating dates as well as state to state

TABLE 2
Treatments evaluated to control annual bluegrass in identical experiments in three states over 2009-2013
Active ingredient Mode of action Brand name and formulation Rate oz./acre Rate lbs. ai/acre Application Frequency Application Dates Total applications per year
Bispyribac-sodium B(2) Acetolactate Synthase (ALS) Inhibitor Velocity
17.6 SG
1 0.011 2 wks May-Sep 8
Bispyribac-sodium Velocity
17.6 SG
2 0.022 2 wks Aug-Sep 4
Paclobutrazol Gibberellic acid biosynthesis
inhibitor, Class B
Trimmit
2SC
8
16
0.125
0.25
2 wks
2 wks
Apr-May
Aug-Sep
June-July
8
4
Flurprimidol Gibberellic acid biosynthesis
inhibitor, Class B
Cutless
MEC
8
16
0.081
0.163
2 wks
2 wks
Apr-May
May-Aug
5
7
Flurprimidol + trinexapac-ethyl Gibberellic acid biosynthesis
inhibitor, Class B + A
Legacy 10 0.118 2 wks Apr-Sep 12
Trinexapac-ethyl Gibberellic acid biosynthesis
inhibitor, Class A
Primo
Maxx
11 0.086 2 wks Apr-Sep 12
Untreated
,For each herbicide, the letter and number is a descriptive code for the mode of action that follows. This classification system (codes) was developed by the Herbicide Resistance Action Committee (capitalized letter) and the Weed Science Society of America (superscript number).
,Class A plant growth regulators (PGRs) interfere with the production of gibberellins late in the biosynthetic pathway. Class B PGRs interfere with the production of gibberellins early in the biosynthetic pathway

Depending on the rating date and location, treatment response varied widely. Even the best performing treatments were different visually than the untreated control on only 30 to 60 percent of the rating dates. There could be dual practical implications of this, in that the golfers might not notice that changes are taking place, which is positive for minimizing play disruption. Conversely, if the superintendent sold the annual bluegrass control project promising visual changes, the golfers may be looking for noticeable improvement.

In Indiana, annual bluegrass cover decreased across the entire study in 2010 during an abnormally warm and wet summer. The annual bluegrass didn’t recover in the last two summers of the study as 2012 was also abnormally warm, and even the untreated control showed a 50-percent decrease in annual bluegrass cover from initiation to the final rating date (Table 3). This underscores the importance of long-term studies over multiple locations because weather had a huge effect on annual bluegrass cover in Indiana. Paclobutrazol reduced annual bluegrass cover to 2 percent by May 2013, which was a 95-percent reduction over the course of the study, while flurprimidol also reduced cover to 11 percent by the end of the study, a 75-percent decrease from initiation.

In spite of up to 48 applications over four years, no treatments in Michigan reduced annual bluegrass cover between the start and end of the study (Table 4). The best performing treatment was paclobutrazol, which resulted in a 5-percent increase in annual bluegrass over the course of the study. Flurprimidol resulted in a 19-percent increase in annual bluegrass cover over the course of the study, but all other treatments produced increases of less than 73 percent, while annual bluegrass cover in the untreated check increased by 81 percent over the course of the study.

TABLE 3
Cover of annual bluegrass (ABG), % change from initiation, and number of rating dates where treatment effects were different than the untreated control during four years of season-long applications of herbicides or growth regulators to creeping bentgrass/annual bluegrass maintained at putting green height in West Lafayette, Ind.
% ABG cover
Treatment Applications
/year and rate
Initial
(May 2009)
Final
(May 2013)
% change § Number of rating dates/total where ABG < than in untreated control
Bispyribac-sodium 8 @ 1 oz./A 43 26 a# –37 a 3/33 (9%)
Bispyribac-sodium 4 @ 2 oz./A 45 19 ab –58 abc 6/33 (18%)
Paclobutrazol 8 @ 8 oz./A
4 @ 16 oz./A
39 2 d –95 d 11/33 (33%)
Flurprimidol 5 @ 8 oz./A
7 @ 16 oz./A
41 11 c –75 cd 10/33 (30%)
Flurprimidol + trinexapac-ethyl 12 @ 10 oz./A 34 13 bc –61 bc 7/33 (21%)
Trinexapac-ethyl 12 @ 10 oz./A 43 19 ab –56 abc 1/33 (3%)
Untreated 49 22 a –50 ab
Annual bluegrass cover was estimated with vertical-point quadrat method where a 4-foot by 4-foot frame was laid over the plots with an interval filament grid of 49 intersections. The total number of times ABG was present under each intersection was recorded for each plot and percentage cover was calculated.
§ Percent change from the initial ABG cover, calculated as [((ABG cover in final spring rating – ABG cover in initial spring rating)/ABG cover in initial spring rating) *100].
Means of three replications
# Means within a column followed by the same letter are not significantly different according to Fisher’s least significant difference at P ≤ 0.05.

Paclobutrazol or flurprimidol reduced annual bluegrass compared to the untreated control on about or more than 64 percent of the rating dates in the study, while no other treatment affected annual bluegrass on more than a third of the rating dates. In Nebraska, treatments with bispyribac-sodium or paclobutrazol were the best performers, reducing annual bluegrass cover by 24 percent to 47 percent from the start of the study. Bispyribac-sodium applied four times at 2 oz./acre reduced annual bluegrass cover compared to the untreated check on 55 percent of the rating dates.

The highest cover and poorest control of annual bluegrass among the three states occurred in Michigan, and may be due to a number of factors. More moderate summer weather compared to Nebraska and Indiana likely favored annual bluegrass competitiveness and also may have reduced its susceptibility to bispyribac-sodium or flurprimidol. Furthermore, a stand farther north is more likely to be perennial annual bluegrass (Poa annua spp. reptans) than a more southerly stand, and recent thesis data suggest perennial subspecies are more tolerant than the annual subspecies to bispyribac-sodium. Previous research also indicates differential susceptibility among subspecies to other annual bluegrass controls, including rimsulfuron or fenarimol.

Though the treatments we used caused phytotoxicity in creeping bentgrass/annual bluegrass stands in other studies, we had few instances of damage, with none long-lasting. Most of the decreased turf quality we experienced was due to treatments reducing annual bluegrass cover and thus causing overall decreased density. If superintendents choose aggressive annual bluegrass control programs like we used, they should plan for — and communicate about — decreased density and/or phytotoxicity.

Turf specialists rarely recommend application strategies like the one used in this study — with usually only one primary active ingredient applied multiple times over many years — for fear of developing annual bluegrass resistance to a particular product. Though annual bluegrass resistance to growth regulators has not been reported, 27 cases of annual bluegrass resistance to herbicides, including bispyribac-sodium, have been reported as of this writing. There were no obvious indications of the development of resistant annual bluegrass in any location in this study.

However, early indications from a companion genetic study suggest that many of these herbicide or growth regulator treatments affected structure and diversity of annual bluegrass populations when sampled two or three years after study initiation. In other words, our treatments caused changes in annual bluegrass populations almost without regard to their effect on actual annual bluegrass cover. However, we cannot conclude from the available data that our treatments shifted populations to more growth regulator- or herbicide-tolerant populations. That being said, it’s critical for superintendents to employ all available tools to reduce annual bluegrass and alternate products with different modes of action.
 

TABLE 4
Cover of annual bluegrass (ABG), % change from initiation, and number of rating dates where treatment effects were different than the untreated control during four years of season-long applications of herbicides or growth regulators to creeping bentgrass/annual bluegrass maintained at putting-green height in East Lansing, Mich.
% ABG cover
Treatment Applications
/year and rate
Initial
(May 2009)
Final
(May 2013)
% change § Number of rating dates/total where ABG < than in untreated control
Bispyribac-sodium @ 1 oz./A 8 @ 1 oz./A 27 50 a# 93 a 8/25 (32%)
Bispyribac-sodium @ 2 oz./A 4 @ 2 oz./A 25 46 a 88 ab 8/25 (32%)
Paclobutrazol 8 @ 8 oz./A
4 @ 16 oz./A
27 28 c 5 c 17/25 (68%)
Flurprimidol 5 @ 8 oz./A
7 @ 16 oz./A
30 33 bc 19 c 16/25 (64%)
Flurprimidol + trinexapac-ethyl 12 @ 10 oz./A 25 46 a 92 a 6/25 (24%)
Trinexapac-ethyl 12 @ 10 oz./A 30 49 a 74 ab 0/25 (0%)
Untreated 28 49 a 81 ab
Annual bluegrass cover was estimated with vertical-point quadrat method where a 5-foot by 5-foot frame laid over the plots with an internal filament grid of 64 intersections. The total number of times ABG was present under each intersection was recorded for each plot and percentage cover was calculated.
§ Percent change from the initial ABG cover, calculated as [((ABG cover in final spring rating – ABG cover in initial spring rating)/ABG cover in initial spring rating) *100].
.
Means of three replications
# Means within a column followed by the same letter are not significantly different according to Fisher’s least significant difference at P ≤ 0.05.
TABLE 5
Cover of annual bluegrass (ABG), % change from initiation, and number of rating dates where treatment effects were different than the untreated control during three years of season-long applications of herbicides or growth regulators to creeping bentgrass/annual bluegrass maintained on a practice putting green in Lincoln, Neb.
% ABG cover
Treatment Applications
/year and rate
Initial
(May 2009)
Final
(May 2013)
% change § Number of rating dates/total where ABG < than in untreated control
Bispyribac-sodium @ 1 oz./A 8 @ 1 oz./A 31 23 ab# –24 bc 4/33 (12%)
Bispyribac-sodium @ 2 oz./A 4 @ 2 oz./A 32 17 b –47 c 18/33 (55%)
Paclobutrazol 8 @ 8 oz./A
4 @ 16 oz./A
32 18 b –43 c 5/33 (15%)
Flurprimidol 5 @ 8 oz./A
7 @ 16 oz./A
37 30 a –16 abc 1/33 (3%)
Flurprimidol + trinexapac-ethyl 12 @ 10 oz./A 32 32 a 2 ab 2/33 (6%)
Trinexapac-ethyl 12 @ 10 oz./A 23 24 ab 1 ab 2/33 (6%)
Untreated 29 32 a 13 a
Annual bluegrass cover was estimated with vertical-point quadrant method where a 5-foot by 5-foot frame laid over the plots with an internal filament grid of 64 intersections. The total number of times ABG was present under each intersection was recorded for each plot and percentage cover was calculated.
§ Percent change from the initial ABG cover, calculated as [((ABG cover in final spring rating – ABG cover in initial spring rating)/ABG cover in initial spring rating) *100].
Means of three replications
# Means within a column followed by the same letter are not significantly different according to Fisher’s least significant difference at P ≤ 0.05.

 

Recommendations for annual bluegrass control in greens

Our data show that treatment effectiveness varies widely depending on location and from year to year within location. Superintendents may have to experiment to find the most effective treatments for the population of annual bluegrass on their golf course.

Furthermore, many of our treatments reduced annual bluegrass only marginally, and so maximizing cultural practices to control annual bluegrass is critical prior to initiating aggressive use of growth regulators or herbicides. Our data suggest that the first growth regulator superintendents should use as part of a control strategy is paclobutrazol, followed by flurprimidol. Flurprimidol plus trinexapac-ethyl would be less likely to reduce annual bluegrass on greens-height turf than paclobutrazol or flurprimidol, and superintendents should not consider trinexapac-ethyl for annual bluegrass control on greens.

Though bispyribac-sodium is not labeled for greens, anecdotal reports indicate superintendents are experimenting with it. If that is the case, four applications of bispyribac-sodium at 2 oz. /acre every two weeks in August and September likely would be more effective than 1 oz./acre bispyribac-sodium applied every two weeks from May through September. To limit chances of annual bluegrass resistance to these treatments, superintendents should alternate products in long-term control programs.
 
Acknowledgments

This research was made possible by funding and support from the United States Golf Association, the Michigan Turfgrass Association, the Midwest Regional Turf Foundation and the Nebraska Turfgrass Association. We also thank Scott Wilke, superintendent of Firethorn Golf Club, Lincoln, Neb.
 
 
 
 
References

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Photos by: Zac Reicher



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