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a Dep. of Agronomy and Soils, Auburn Univ., AL, 36849
b The Landings Club, 75 Green Island Rd., Savannah, GA 31411
* Corresponding author (eguertal{at}acesag.auburn.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Within ten years patches of "off-type" bermudagrass began to appear in Tifdwarf putting greens (Moncrief, 1975). Phenotypically different from the original sprigged Tifdwarf, some of the off-types were superior; having higher shoot densities, shorter internodes, and the ability to be mowed at lower heights (Foy, 1997). Additional research has shown that there are genotypic differences between Tifdwarf and the off-types as well (Caetano-Anolles et al., 1995; Ho et al., 1997; Zhang et al., 1999).
Selected off-types have been released as cultivars. These cultivars are commonly lumped together under the term ultradwarfs, and in the past decade they have emerged as a favorite for newly constructed or renovated bermudagrass putting greens. Common commercial cultivars are TifEagle, Mississippi (MS) Supreme, Floradwarf, and Champion. Champion is a vegetative selection from Tifdwarf, with DNA testing to demonstrate that it is genetically different from Tifdwarf (Beard, 1996). Floradwarf is also genetically different from Tifdwarf and is a vegetative selection from Tifdwarf (Dudeck and Murdoch, 1998). MS Supreme is an ecotype selection from Tifgreen and is genetically different from both Tifgreen and Tifdwarf (Krans et al., 1999). TifEagle was produced by 
irradiation of ‘Tifway II’ and is genetically different from Tifgreen, Tifdwarf, and the ultradwarfs previously mentioned (Hanna and Elsner, 1999). In general, the released ultradwarf bermudagrasses have shorter internodes, higher shoot densities, better turf quality, and the ability to withstand lower mowing heights than Tifdwarf (Gray and White, 1999).
Although the ultradwarf bermudagrasses may offer improved turf quality, they do present new management issues. There is limited research which shows that the ultradwarfs may produce excess thatch (Hollingsworth et al., 2000), and little is known about their cultivation needs or if those needs differ from Tifdwarf (White et al., 2001). Additionally, the N requirement of the grasses is largely unstudied (Taverner and McCrimmon, 1999; Cisar and Snyder, 2001). Also, because of their high shoot density the ultradwarfs require different overseeding strategies (Sifers and Beard, 1999), another management area with limited research.
Because the general area of ultradwarf management is largely unstudied, we chose to evaluate selected ultradwarf cultivars and Tifdwarf via two different management plans. One plan was the typical southern management system for Tifdwarf bermudagrass, and the second was an intensive plan designed to manage the thatch-producing ultradwarfs. These plans differed in N use, cultivation type, and frequency and aggressiveness of thatch control. In general, the objective of this research was to evaluate two cultural management programs and two N sources and determine their effect on thatch, quality and color, and overseed establishment of five ultradwarf bermudagrasses and Tifdwarf.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The experiment was arranged as a split-strip-strip design with three replications. Cultivars were main blocks (1.8 by 12.2 m), with management program as the first split (1.8 by 6.1 m), and N source as the second split (1.8 by 3.0 m). Bermudagrass cultivars were Champion, TifEagle, Floradwarf, MS Supreme, Mobile 9, and Tifdwarf. Mobile 9 was a vegetative off-type selected from Tifgreen, currently under evaluation by the Alabama Agricultural Experiment Station. Management programs were (i) standard, with two vertical mowings at 2.5 cm deep per year plus sand topdressing, one core aerification, and monthly summer spiking, or (ii) high, with monthly summer vertical mowing at 1.3 cm deep plus topdressing, monthly summer core aerification, and an additional light topdressing each month (Table 1).
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For the cultural treatments the following equipment was used. Vertical mowing was applied with a Graden vertical mower (Graden Turf Machinery, 29 Scammel Street, Campbellfield, Victoria, Australia) with 1-mm wide cutting blades set 2.6 cm apart. Standard management plots were vertical mowed at a depth of 2.5 cm, while high management plots were mowed at a depth of 1.3 cm. In both years of the study high management plots were vertical mowed the first week of June, July, August, and September, and the standard management plots were vertical mowed the third week of June and August (Table 1). Once vertical mowing was completed debris was raked from the plots and discarded. All plots were immediately topdressed after any vertical mowing, with sufficient sand applied and brushed into the slits left by vertical mowing. High management plots were topdressed at a rate of 2.2 x 10–4 m3 sand m–2 and standard management plots at a rate of 4.4 x 10–4 m3 sand m–2. The higher rate was needed in the standard management plots to adequately fill the slits left by the deeper vertical mowing.
Core aerification and spiking were applied with the same piece of equipment (Ryan GA 30, Cushman, Inc., Lincoln, NE), with different tines. Core aerification treatments were applied with hollow tines that were 10-cm long and 0.6 cm in diam., with holes spaced 5 cm apart. Spike treatments were applied with solid tines that were 8-cm long, 0.5 cm in diam., and spaced 5 cm apart. Cores removed during hollow tine aerification were ground and brushed back into the plots. Holes remaining after solid-tine cultivation were left open.
Data collection included twice monthly color and quality ratings using a 1-to-9 scale. Plots with a score of 1 were completely brown or dead, and plots with a score of 9 were dark green and lush. A score of 5 indicated a putting green with acceptable color or putting quality. Color and quality ratings were collected in weeks 2 and 4 of each month, before any cultivation treatment for that week. Thatch depth was also collected throughout each year of the study. Each month three core samples (2.2-cm diam.) were removed randomly from each plot and compressed thatch depth measured (Callahan et al., 1997).
All cultural maintenance treatments were stopped 1 mo before overseeding to prevent seed germination in slits or holes left from cultivation. Plots were overseeded on 15 Oct. 1999 and 2000 by broadcast seeding P. trivialis (cv. Sabre) at a rate of 488 kg–1 ha. Topdressing was applied at 2.2 x 10–4 m3 sand m–2 immediately after seeding to help aid germination.
Collected data included shoot density counts of P. trivialis and bermudagrass. At each counting, six 2.0-cm diam. cores were removed from each plot. P. trivialis and bermudagrass shoots were separated in each core and hand counted. P. trivialis and bermudagrass shoot counts were completed in December 1999, March 2000, May 2000, July 2000, November 2000, March 2001, and April 2001.
Background turfgrass management included mowing the green at heights of 3.2 to 4.8 mm six times per week. The higher mow height was used during overseeding, with the mow height reduced during summer. Irrigation was applied as needed (typically 3–4 times per week), with a minimum of 2.5 cm of water applied each week. Background soil fertility (P, K, and lime) was applied as needed and according to soil test. Lime and P were not needed during the 2-yr experiment period, and K was applied monthly at 2 g K m–2 (K2SO4, 0–0–50).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TifEagle was most affected by the level of management, with quality reduced in the high management treatment when compared to standard management at 11 of 14 rating dates (Fig. 1). Floradwarf was least affected, with quality reduced in the high management plots at 8 rating dates. This was not because Floradwarf was increasingly damaged by high levels of management, but because its quality in any treatment was already lower than that of other cultivars. Floradwarf had an average quality score of 4.4 (from Fig. 1) under standard management, while Champion, TifEagle, MS Supreme, Tifdwarf, and Mobile 9 had average scores of 5.7, 5.3, 5.7, 5.0, and 5.6, respectively.
The only other significant interactions for turf quality were management x N source, which occurred on 12 June, 27 July, 14 Aug., and 27 Aug. 2000. In each case the interaction occurred because plots receiving high management had higher quality scores when soluble fertilizers were applied (data not shown). At those ratings the average quality score for plots receiving soluble N was 5.0, compared to 4.4 in plots receiving slow-release N. Plots receiving standard management were unaffected by N source. Plots receiving standard management were less likely to show a response to soluble N, as their overall quality was higher, regardless of N source.
Quality of the overseeded bermudagrass was affected only by cultivar and not management or N source. From November 1999 until March 2000, Mobile 9, Champion, and MS Supreme had significantly higher quality ratings than TifEagle, Tifdwarf, or Floradwarf (data not shown). During that period, in 6 of 10 biweekly ratings, Floradwarf had a poorer quality overseed rating than any other cultivar. In 2001 Mobile 9 had higher (average of 6.5) quality overseed scores than any other cultivar in 4 of 7 ratings (January through April).
Color
There was a significant cultivar x management interaction for the periods 13 Aug. to 6 Oct. 1999 and 12 June to 13 Oct. 2000 for relative turf color. For example, on 13 Aug. 1999 the color of Champion and TifEagle was darker green in plots receiving standard management (average = 6.1 in standard; 4.5 in high; P = 0.05), while the color of MS Supreme and Mobile 9 was darker in the high management plots (average = 5.2 in standard; 6.2 in high; P = 0.05). The color of Floradwarf and Tifdwarf was unaffected by management (average = 4.2 in standard; 4.3 in high; P = 0.68). While such variability occurred throughout the 14 ratings in which there was a significant cultivar x management interaction, in general, plots receiving the high management treatments had lower color scores than those receiving standard management. On the 14 rating dates there were 7, 7, 5, 3, 3, and 3 dates in which Champion, TifEagle, MS Supreme, Floradwarf, Tifdwarf and Mobile 9, respectively, had darker color in the standard management plots. For any one cultivar there were, at most, only two rating dates where plots receiving high management had darker color. In those cases the ratings were always taken just before the next vertical mowing, after three weeks of turf healing.
The main effect of N source affected bermudagrass color from August 1999 until May 2000 (Fig. 2) , and there was a significant N source x cultivar interaction for bermudagrass color from June to October 2000 (data not shown). From August to January 2000, plots receiving soluble N had a darker color than those receiving slow-release N (Fig. 2). In the winter and early spring (February–May 2000) this effect reversed, and plots receiving slow-release N generally had a darker green color. The slow-release N source for this study was a urea–formaldehyde, of which 70% was controlled-release N. In other research, application of urea–formaldehyde fertilizers that contained water-insoluble N produced color responses that were slower than those observed with soluble N sources (Landschoot and Waddington, 1987). However, there were slight residual growth and color responses after 2.5 yr of urea–formaldehyde application (Landschoot and Waddington, 1987). Additionally, in our study soluble N applied during this same time period (February–April) may have been lost from the root zone due to leaching from typical winter rains and/or denitrification.
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P. trivialis overseed color was affected by the main effects of N source, management, and cultivar from November 1999 until February 2000 and from January until May 2001. There were no significant interactions on the color of the P. trivialis overseed. The P. trivialis overseed was often greener when soluble N was applied, as compared to plots that received slow-release N. This occurred on 12 of 21 ratings. At 3 ratings there was no difference in color due to N source, and on 6 dates plots receiving slow-release N had a darker green color than those receiving soluble N. The slow-release N plots were darker green in February, March, and early April 2000 when plots receiving slow-release N had a darker color than those to which soluble N had been applied (Fig. 2). Reasons for this color difference due to N source during these two months is not known; the months did not have excessive rainfall or heat, nor were there any temperature extremes.
Thatch Depth
Eighteen measurements of thatch depth were made during this study, from July 1999 until May 2001. During this period there were two significant three-way cultivar x N source x management interactions (December 1999 and June 2000), and one significant two-way interaction (management x cultivar, April 2001). Thatch depth was affected by cultivar at every sampling (Table 2), by management at 4 of 18 samplings (data not shown), and was unaffected by N source. Management affected thatch depth in August, September, and October 1999 and in June 2000, and in all these cases thatch depth was greater in the high maintenance plots than standard (average of 19.8 mm in high management, 18.4 mm in standard).
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In a Penncross bentgrass (A. palustris Huds.) putting green the best treatments for thatch control were vertical mowing at 4 or 8 times per year, and vertical mowing plus core aerification, each conducted four times per year (Callahan et al., 1998). Adding any topdressing (3 or 6 times per year) decreased thatch, as compared to plots that did not receive topdressing. The largest amount of topdressing (6 times per year) decreased thatch the most (Callahan et al., 1998).
Others have found no difference in thatch accumulation due to N sources or some cultural practices (White and Dickens, 1984). Cultural maintenance included twice yearly or monthly core aerification, once yearly or 4 times yearly topdressing, and twice yearly or biweekly vertical mowing. Of these cultural practices, only topdressing significantly affected thatch + mat depth, with more thatch + mat in the plots that were topdressed 4 times a year (White and Dickens, 1984). There were no differences in thatch depth due to N source (sewage sludge or NH4NO3). Conversely, other research suggested that slow-release N sources sometimes reduced thatch, as compared to soluble N sources (Meinhold et al., 1973; Sartain, 1985). In our study N source had no effect on thatch depth.
Cultivar affected thatch depth at every sampling. Agreeing with common perception, the ultradwarf cultivars, with the exception of MS Supreme, almost always produced more thatch, when compared to Tifdwarf (Table 2). MS Supreme often had thatch depths equal to that of Tifdwarf, and in 13 of 18 samplings the thatch depth of MS Supreme was either equal to that measured in Tifdwarf, or lower. Given that MS Supreme is an ecotype selection from Tifgreen (Krans et al., 1999), and not Tifdwarf, this lower tendency to thatch is not surprising. Of the remaining cultivars (Champion, TifEagle, Floradwarf, and Mobile 9), no one cultivar was more prone to thatch production than another. Differences in thatch depth between those cultivars were few and not consistent (Table 2).
Bermudagrass and P. trivialis Shoot Densities
Main effects of cultivar, N source, and maintenance were significant for both bermudagrass and P. trivialis shoot densities throughout the length of the experiment (Tables 3 and 4). There were not any significant two- or three-way interactions for shoot density. As with previous work (Gray and White, 1999) the ultradwarf bermudagrasses evaluated in this study often had higher shoot densities than Tifdwarf (Table 3). At 4 of 7 samplings Tifdwarf had significantly fewer bermudagrass shoots than any other cultivar. Champion consistently had higher shoot densities, and in 6 of 7 samplings it either had the highest shoot density or was in the top group of cultivars with high shoot densities. In general, over two years of sampling, shoot densities of the bermudagrasses followed the following trend: Champion and Mobile 9 with highest shoot densities; TifEagle, Floradwarf, and MS Supreme with shoot densities as a middle grouping; and Tifdwarf with the lowest shoot densities (Table 3).
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The main effect of management affected both bermudagrass and P. trivialis shoot densities (data not shown), but only at 2 (bermudagrass) or 3 (P. trivialis) of 7 samplings. In July 2000, plots receiving standard management had a greater bermudagrass shoot density (11.1 shoots cm–2) than those receiving high management (10.0 shoots cm–2). This was reversed in March 2001, when there was a greater bermudagrass shoot density in the high management plots. There was no significant difference in bermudagrass shoot density due to management at any other sampling. The July difference was likely a result of the frequent cultivation practices that occurred on the high management plots during the summer.
Density of the P. trivialis overseeding was greater in the high management plots at the March 2000 sampling and greater in the standard management plots in March and April 2001 (data not shown). There were no other differences in P. trivialis overseed density due to management at any other sampling. In the case of the P. trivialis overseeding, focusing on the dates after overseed establishment provides the most information about the effect of management on overseeding. In those cases (December 1999 and December 2000) there was no significant difference in overseed density due to management, an indication that summer and fall cultivation practices did not affect overseed establishment.
The effect of N source on shoot densities of bermudagrass and P. trivialis was variable, and no clear trend between months or the two grass species was evident (data not shown). At 3 of the 7 samplings, applications of slow-release N increased bermudagrass shoot density (average of 3.2 shoots cm–2) as compared to bermudagrass receiving soluble N (2.5 shoots cm–2). This occurred in early spring of each year (March 2000; March and April 2001). When the bermudagrass was dormant (December), N source did not affect bermudagrass (average of 8.4 shoots cm–2) or P. trivialis (22.5 shoots cm–2) shoot densities. When the bermudagrass was actively growing (July) shoot density was higher in plots receiving applications of soluble N (average of 10.9 shoots cm–2). This was matched by a decrease in P. trivialis shoot density (average of 2.2 shoots cm–2), possibly because the actively growing bermudagrass was effectively utilizing soluble N for growth, crowding the less-adapted cool-season P. trivialis out at the July sampling.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Shoot density of bermudagrass and P. trivialis had no consistent response to either management or N source. Both thatch depth and shoot density were affected by cultivar, with Tifdwarf and the Tifgreen ecotype MS Supreme having lower thatch depths throughout much of the study. Although not always significantly different, shoot densities of the bermudagrasses could be grouped with Champion and Mobile 9 having the greatest density; TifEagle, Floradwarf, and MS Supreme with middle-ranked shoot density; and Tifdwarf with lowest shoot density.
From the results of this 2-yr study, it appears that the ultradwarf bermudagrass cultivars currently being sprigged on putting greens may not need frequent (>2 times per year), deep (2.5 cm) vertical mowing for thatch control. Color and quality of these grasses were often best when soluble N sources were applied, especially when turf needed to heal after cultivation. Quality and density of P. trivialis overseeding was largely unaffected by cultivar or management, indicating that quality stands of P. trivialis can be established in the ultradwarf cultivars.
Received for publication June 23, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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= 0.05. Downward and upward facing arrows in the top two figures indicate when vertical mowing was applied for standard and high management treatments, respectively. Break in time on the x axis indicates the winter period when cultural management treatments were not imposed on overseeded bermudagrass.