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TURFGRASS

Conversion of Fairway-Height Perennial Ryegrass Turf to Kentucky Bluegrass without Nonselective Herbicides

Dep. of Hortic., Forestry, and Recreation Resour., Kansas State Univ., Manhattan, KS 66506

^* Corresponding author (skeeley{at}oznet.ksu.edu).

Received for publication July 8, 2003.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Many golf course superintendents wish to convert perennial ryegrass (Lolium perenne L.) fairways to other cool-season species such as Kentucky bluegrass (Poa pratensis L.) because of the former's high fungicide requirement. However, interseeding into a mature turfgrass stand often results in poor seedling survival because of competition from the existing stand. The objective of this research was to evaluate methods for enhancing Kentucky bluegrass (KB) establishment when interseeded into a mature perennial ryegrass fairway turf. Core aeration, scalping, plant growth regulators (PGRs), postseeding low mowing (PSLM), high seeding rates, and multiple seedings were used singly and in factorial combinations. Postseeding low mowing (0.6 cm twice weekly for 4 wk) and high seeding rates [196 kg pure live seed (PLS) ha^–1] were most effective in enhancing KB establishment. Plots receiving PSLM averaged 31.9% KB 21 months after seeding (MAS) compared with 18.1% in non-PSLM plots. Seeding KB at 196 kg PLS ha^–1, both fall and spring, resulted in an average of 29.8% KB 21 MAS while seeding at 98 kg PLS ha^–1, in the fall only, resulted in only 19.7% KB. A single preseeding core aeration or scalping treatment did not increase KB establishment, nor did preseeding applications of the PGR trinexapac-ethyl [4-(cyclopropyl--hydroxy-methylene)-3,5-dioxo-cyclohexane-carboxylic acid ethyl ester]. Mefluidide {–[2,4-dimethyl-5-[[(trifluoromethyl)-sulfonyl]amino]phenyl] acetamide} increased KB establishment at 6 MAS, but the effect was gone by 21 MAS. Where nonselective herbicides are not used, fastest conversion to KB will occur with a combination of PSLM and high seeding rates.

Abbreviations: KB, Kentucky bluegrass • MAS, months after interseeding • PGR, plant growth regulator • PLS, pure live seed • PSLM, postseeding low mowing

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
GRAY LEAF SPOT [Pyricularia grisea (Cooke) Sacc.], brown patch (Rhizoctonia solani Kühn), and Pythium blight (Pythium spp.) have become expensive problems on perennial ryegrass fairways in the transition zone. Golf course superintendents may spend $20000 or more annually to apply fungicides for the control of these diseases (N.A. Tisserat, personal communication, 2001). Gray leaf spot is potentially the most devastating of the three diseases, with the ability to kill large areas of turf in a matter of days (Dernoeden, 1996). Because perennial ryegrass has a bunch-type growth habit, it has little recuperative ability and must be overseeded annually to fill in bare spots caused by biotic and abiotic stresses. Annual overseeding is expensive and can lead to excessive density, which may lead to higher disease incidence (Brede, 1996).

Kentucky bluegrass was once used extensively on golf course fairways in the transition zone. Over the past 25 yr, mowing heights have decreased, and KB has been largely replaced by perennial ryegrass, primarily because of the latter's superior tolerance to mowing heights below 2.5 cm and its establishment vigor. Recently, KB breeders have focused on improving low-mowing-height tolerance and have developed a number of improved cultivars (Brede, 1996). Kentucky bluegrass offers the potential for reduced fungicide usage on fairways because it is resistant to the aforementioned diseases that have plagued perennial ryegrass fairways. Based on research conducted at Kansas State University, a relatively small group of KB cultivars appear well suited for fairway use in the upper transition zone. These cultivars develop a dense stand at mowing heights as low as 1.4 cm (Keeley, 1999). Consequently, many superintendents may be interested in converting perennial ryegrass fairways to KB or to a perennial ryegrass–KB mixture.

Unfortunately, establishing seedlings of an introduced species into an actively growing, mature turfgrass can be a difficult task, especially if the introduced species and the existing species have similar optimum growth temperatures. Perennial ryegrass, in particular, has been shown to be capable of outcompeting many other grasses for both moisture (Milthorpe, 1961) and nutrients (King, 1971). McTear (1984) suggested that the combination of shading and the limited capacity of the seedling root system to compete for finite nutrient and water resources forms a competition pressure giving the mature stand a major advantage over the interseeded grass. Other researchers have determined that a plant's ability to compete above ground is related to its height or position within the canopy (Cattani and Nowak, 2001). Therefore, it would seem that any method that slows or suppresses perennial ryegrass' growth should give KB seedlings a better chance of long-term survival.

A number of tools are available to the superintendent in the conversion process. Core aeration, verticutting, and rolling all help to achieve good seed-to-soil contact. Plant growth regulators and/or PSLM can slow the growth of the existing turfgrass or suppress its canopy height, giving seedlings a chance to successfully compete for light, water, and nutrients (Eggens et al., 1989). Seeding rates can be adjusted, or multiple seedings used, in an attempt to gain increased seedling survival. It is possible that a combination of two or more of these techniques will be most effective in enhancing survival of seedling KB in an existing perennial ryegrass fairway.

This study was developed to compare the effectiveness of core aeration, PGRs, PSLM, high seeding rates, and multiple seedings, singly and in factorial combinations, on establishment of KB interseeded into a mature perennial ryegrass fairway-height turf.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Study 1
We initiated the study during the fall of 1999 in an existing perennial ryegrass turf, maintained at 1.4 cm, at the Rocky Ford Turfgrass Research Center in Manhattan, KS. Soil was a Chase silty clay loam (fine, montmorillonitic, mesic Aquic Arquidolls) with pH = 7.2, P (Bray 1) = 83 mg kg^–1, and K = 488 mg kg^–1 at time of planting. Experimental design was a randomized complete block with three replications. Plots measured 1.5 by 1.8 m. Treatments included scalping, core aeration, and the PGR trinexapac-ethyl singly and in all possible factorial combinations. Thus, the combination treatments were scalping + core aeration, scalping + PGR, core aeration + PGR, and scalping + core aeration + PGR. Control treatments included an untreated control and a preseeding glyphosate [N-(phosphonomethyl)glycine]-treated control. The herbicide concentrate contained 41% glyphosate; we mixed a 1% (v/v) solution of the herbicide and applied it with a backpack sprayer.

A walk-behind reel mower, set to cut at 0.6 cm, was used for the scalping treatments. Core aeration was accomplished with a walk-behind lawn aerator equipped with 1.2-cm-diam. hollow tines on 5-cm spacing. The holes were approximately 2.5 cm deep. We performed the scalping and core aeration treatments 1 d before seeding.

Trinexapac-ethyl was applied at 0.24 kg a.i. ha^–1 4 d before seeding, using a CO₂–pressurized hand-held boom sprayer with a spray pressure of 0.21 MPa and a spray volume of 374 L ha^–1. All plots, including controls, were vertically mowed in two perpendicular directions immediately before seeding to facilitate seed–soil contact. The vertical mower was set to penetrate the soil to a 0.6-cm depth, and the blades were on 2-cm spacing. Debris was removed by raking before seeding. ‘Apollo’ KB was seeded on 14 Sept. 1999 at 98 kg PLS ha^–1 using a drop spreader.

During establishment, plots were typically irrigated at 0.7 cm d^-1 in three to four light applications to keep the seedbed moist. During the 2000 growing season, we irrigated with approximately 1.5 cm of water whenever visual wilt became apparent. Urea (46–0–0) was applied to all treatments at 49 kg N ha^–1 in October and November of 1999 and in May and September of 2000.

In December 1999, three 10-cm-diam. plugs were randomly removed from each plot, and the number of KB and perennial ryegrass tillers in each plug was counted. Percentage KB was determined by dividing KB tillers by total tillers. In October 2000, a grid transect was used to estimate the percentage of KB in the plots. This method allowed us to use the majority of the plot area to develop the estimate. The grid consisted of a 1.3- by 1.3-m plastic pipe frame with nylon fishing line running lengthwise and crosswise on 10-cm centers. Consequently, there were 110 intersection points covering the plot when the grid was laid over it. The number of KB plants under intersections was recorded, and this value was converted to percentage KB coverage by dividing by 110. Data were subjected to an analysis of variance, and treatment means were separated using Fisher's protected least significant difference test (Gomez and Gomez, 1984).

Study 2
Results from Study 1 indicated a more aggressive approach was necessary to establish KB in an existing stand of perennial ryegrass. In the fall of 2000, a new study was initiated in an identical stand of perennial ryegrass adjacent to Study 1. The experimental design was a strip-split plot with three replications. Whole plots measured 7.2 by 1.8 m and consisted of a PGR treatment (mefluidide vs. no PGR). Seeding rate/timing treatments (98 or 196 kg PLS ha^–1; fall only or fall + spring) were split across the whole plots. Postseeding low mowing was imposed as the strip plot. Thus, half of each individual subplot (consisting of the seeding rate/timing treatments) received PSLM, and the other half did not.

On the day of seeding, we mowed all plots at 0.6 cm, core-aerated and vertically mowed (as previously described) to enhance seed-to-soil contact. Debris was removed by raking before seeding. Eight days before seeding, the mefluidide treatment was applied at a rate of 0.18 kg a.i. ha^–1 with the same sprayer and sprayer setup as described for trinexapac-ethyl in Study 1. We also applied the preseeding glyphosate treatment at this time, as a control (same as described in Study 1).

‘Apollo’ KB seed was applied with a shaker bottle on 15 Sept. 2000. We added 130 g of composted sewage sludge (Milorganite, 6–2–0) to each bottle to facilitate uniform distribution of the seed. The Milorganite carrier supplied 24 kg N ha^–1. Following seeding, the study area was raked lightly with a garden rake and rolled with a Brinkman smooth power roller to assure good seed-to-soil contact. Establishment and postestablishment irrigation were similar to that described for Study 1.

The PSLM treatment consisted of mowing at 0.6 cm twice a week with a walk-behind greens mower for a period of 4 wk following seeding. By the end of this period, KB seedlings had just begun to grow taller than 0.6 cm. All other plots, with the exception of glyphosate-treated plots, were mowed three times a week at 1.4 cm. Glyphosate-treated plots were not mowed until shoots grew taller than 1.4 cm.

The entire study area was fertilized with urea (46–0–0) at 49 kg N ha^–1 in October and November of 2000; May, September, and November of 2001, and May of 2002 before the study was terminated at the end of July 2002.

Treatments that included a spring seeding were seeded on 21 Apr. 2001. All procedures were identical to those used in the previous fall, including PGR application and PSLM to appropriate plots.

Plots were rated weekly for visual quality during the PSLM period and for the subsequent 4 wk after PSLM ceased. After that, rating was done on a biweekly basis through the growing season, using a 1 to 9 scale, where 1 = lowest and 9 = highest quality. A rating of 6 was considered minimum acceptable visual quality.

The grid–transect method was used to estimate the percentage of KB in the plots in March 2001 and July 2002. The data were subjected to analysis of variance, and treatment means were separated using Fisher's protected least significant difference test (Gomez and Gomez, 1984).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Study 1
The methods used in this study were not aggressive enough to allow the slow-establishing KB seedlings to compete with the mature perennial ryegrass. In December 1999 (3 MAS), no treatment had more than 5% KB, and by October 2000 (13 MAS), no treatment had more than 3.6% KB (data not shown). Excluding the glyphosate-treated check plots, there were no significant differences among treatments at either date. By comparison, the glyphosate-treated check plots averaged 75 and 99% KB coverage at 3 and 13 MAS, respectively.

Reicher and Hardebeck (1997) also found trinexapac-ethyl to be ineffective in converting golf course fairways from perennial ryegrass to creeping bentgrass (Agrostis palustris Huds.). Rather than suppressing the perennial ryegrass, trinexapac-ethyl improved its visual quality, which may indicate that the introduced seedlings were in a highly competitive environment for nutrients and water.

Where a preseeding nonselective herbicide such as glyphosate is not used, a more aggressive approach is needed for successful conversion of perennial ryegrass fairway turf to KB.

Study 2
In Study 2 at 6 MAS, PGR, seeding rate/timing, and PSLM all had a significant effect on KB establishment (Table 1). There was also a significant interaction between seeding rate/timing and PSLM. The interaction occurred because KB establishment was minimal under the non-PSLM treatment regardless of seeding rate/timing (Table 2). At 21 MAS, seeding rate/timing and PSLM were the only factors that were significant (Table 1). The percentage of KB under the various treatment combinations ranged from 1.0 to 15.1% at 6 MAS and from 12.7 to 39.7% at 21 MAS (Table 2).

Visual quality ratings during the PSLM period were significantly different between the PSLM and non-PSLM treatments. Average visual quality for the PSLM plots during the 4-week period was 4 (on a 1–9 scale) compared with 6 for the non-PSLM plots. Based on our observations, PSLM fairways could remain in play because the surface remained smooth and would support a golf ball above the canopy remnant. After PSLM ceased, the plots recovered rapidly, and after 3 wk, there was no difference in visual quality between the PSLM and non-PSLM plots for the remainder of the growing season.

The effectiveness of PSLM likely occurred for several reasons. Postseeding low mowing removed most of the perennial ryegrass foliage, thereby reducing aboveground competition with the KB seedlings and allowing more sunlight to reach the KB seedlings. In a study involving seeded establishment of KB–perennial ryegrass mixtures, Brede and Duich (1984) found that the percentage of KB in the stand after two months was higher when mowing commenced sooner after seeding (i.e., 2 vs. 3 wk) and where a lower mowing height was used (i.e., 1.3 vs. 3.8 cm). The reduction in leaf area caused by PSLM would also reduce root competition from the perennial ryegrass because root growth depends on carbohydrates from the shoots (Beard, 1973). Finally, it is possible that PSLM enhanced KB seed germination by allowing more light to reach the seeds; KB is one of several species reported to show enhanced germination with exposure to light (Beard, 1973).

In addition to PSLM, high seeding rates also enhanced KB establishment. By 6 MAS, mean percentage KB across all plots seeded in the fall at 98 kg PLS ha^–1 was 2.8%, whereas plots seeded with 196 kg PLS ha^–1 averaged 7.3%. By 21 MAS, plots seeded in the fall with 98 kg PLS ha^–1 averaged 19.7% KB, whereas plots seeded in the fall and spring with 196 kg PLS ha^–1 (i.e., 392 kg PLS total) averaged 29.8%. It should also be noted that, all other factors being equal, the percentage of KB obtained with a single fall seeding at 196 kg PLS ha^–1 was never significantly different from that obtained by seeding both fall and spring at 196 kg PLS ha^–1 (Table 2).

The normal seeding rate for KB is 49 to 74 kg PLS ha^–1 (Christians, 1998). Because of KB's rhizomatous growth habit and tillering capacity, such rates can be successful when there is little competition for resources. However, our research indicates that a higher rate is advantageous when interseeding because of the competition from the established turf. Mefluidide was of little value in increasing KB establishment in perennial ryegrass. By 21 MAS, plots treated with mefluidide did not have significantly more KB than untreated plots when the other factors were similar (Table 2).

Kentucky bluegrass coverage increased under all treatments between 6 and 21 MAS (Table 2). Therefore, if time is not a concern, it appears that gradual conversion towards KB will occur even if lower seeding rates are used and PSLM is not implemented. This is probably a result of the rhizomatous growth habit of the KB.

In most cases, however, a faster conversion would be desirable to the golf course superintendent. The results of this study show that fastest conversion can be obtained, without closing fairways, by using a seeding rate of 196 kg PLS ha^–1 in September, combined with PSLM for 4 wk following seeding.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Contribution no. 04-014-J of the Kansas Agric. Exp. Stn. Grant support for this research came from the Kansas Golf Course Superintendents Association and the Golf Course Superintendents Association of America.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

• Beard, J.B. 1973. Turfgrass: Science and culture. Prentice-Hall, Englewood Cliffs, NJ.
• Brede, A.D., and J.M. Duich. 1984. Initial mowing of Kentucky bluegrass–perennial ryegrass seedling turf mixtures. Agron. J. 76:711–714.[Abstract/Free Full Text]
• Brede, D. 1996. Big changes in Kentucky bluegrass. Golf Course Manage. 64(6):61–66.
• Cattani, D.J., and J.N. Nowak. 2001. Interseeding in creeping bentgrass: A viable option or wishful thinking? Golf Course Manage. 69(8):49–54.
• Christians, N. 1998. Fundamentals of turfgrass management. Ann Arbor Press, Chelsea, MI.
• Dernoeden, P.H. 1996. Perennial ryegrass and gray leaf spot. Golf Course Manage. 64(1):49–52.
• Eggens, J.L., C.P.M. Wright, and K. Carey. 1989. Use of mefluidide for turf overseeding. HortScience 24:300–302.
• Gomez, K.A., and A.A. Gomez. 1984. Statistical procedures for agricultural research. 1st ed. John Wiley & Sons, New York.
• Keeley, S. 1999. High-maintenance Kentucky bluegrass cultivar evaluation. Agric. Exp. Stn. Rep. Prog. 836. Kansas State Univ., Manhattan, KS.
• King, J. 1971. Competition between established and newly sown grass species. J. Br. Grassl. Soc. 26:221–229.
• McTear, A. 1984. Rootgrowth of Poa annua L. alone and in competition with Poa pratensis L. M.S. thesis. Univ. of Guelph, Guelph, ON, Canada.
• Milthorpe, F.L. 1961. The nature and analysis of competition between plants of different species. p. 330–355. In Mechanisms in biological competition. Proc. Symp. of the Soc. for Exp. Biol., 15th, Univ. of Southampton, Southampton, UK. September 1960. Academic Press, New York.
• Reicher, Z., and G. Hardebeck. 1997. Conversion of golf course fairways without using nonselective herbicides [Online]. Available at http://www.agry.purdue.edu/turf/report/1997/fairway.htm (verified 18 Nov. 2003). Purdue Univ., West Lafayette, IN.

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