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a Department of Natural Resources and Environmental Sciences, W-417 Turner Hall, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801
b Crop Science Dep., 100 Dereiwx St., North Carolina State Univ., Raleigh, NC 27695-7620
Corresponding author (bhorgan{at}uiuc.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: WAIT, weeks after initial treatment • WP, wettable powder
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Although overseeding provides benefits, the spring transition from perennial ryegrass to bermudagrass can be troublesome and inconsistent due to heat-tolerant perennial ryegrass varieties and variable weather conditions in spring and early summer. Perennial ryegrass may survive longer into spring than is desired due to cool and wet conditions, and delay the transition of bermudagrass out of physiological dormancy.
High temperatures can result in perennial ryegrass transitioning out before bermudagrass can fill in. This may lead to poor playability and poor usability of the turf. Ideally, bermudagrass greens up and begins actively growing as perennial ryegrass slowly transitions. In order to achieve uniform turf color and texture, perennial ryegrass should be eradicated by full bermudagrass green–up. This ideal transition is becoming increasingly difficult due to development of heat, disease, and drought tolerant varieties of perennial ryegrass. These new varieties may result in better playing surfaces during winter months, but may also lead to further problems with spring transition (Mazur and Wagner, 1987). While these variety characteristics may not be ideal for overseeding, more suitable varieties may no longer be commercially available.
Dormant bermudagrass survives on carbohydrates and other photosynthates accumulated during the previous growing season. Shading of the bermudagrass canopy by actively growing perennial ryegrass along with root competition can extend dormancy of bermudagrass into the spring. Dunn et al. (1980) observed less severe winter kill where bermudagrass utilized stored carbohydrates and greened early. If stored carbohydrates are depleted before the plants green up, this may result in winter kill (Knoop, 1987).
Cultural practices are often implemented by turfgrass managers in spring and early summer to enhance transition. A reduction in mowing height, vertical mowing and coring, and reducing soil water inputs has had varying results to expedite spring transition on overseeded bermudagrass putting greens (Bruneau et al., 1985; Meyers and Horn, 1970; Palmertree, 1975). Vertical mowing and topdressing have not been shown to be effective in promoting bermudagrass transition in the spring. In most cases, vertical mowing delayed bermudagrass transition and overall turfgrass quality was reduced (Mazur and Wagner, 1987). Close mowing when night temperatures approach 15.6°C adversely affected overseeded turf, reduced competition, and warmed the soil surface to enhance bermudagrass recovery (Bruneau et al., 1985). Carrow et al. (1987) found that cultivation can open the soil to more direct sunlight, thus warming the soil and increasing green–up of bermudagrass on putting greens. Vertical mowing, scalping, and core cultivation will disrupt the playability of a golf course fairway, but the impact of these practices on spring transition for fairways has not been documented.
Burt et al. (1970) observed that pronamide completely eliminated, or severely injured, most cool-season grasses when growing in warm-season grasses. Since most warm-season grasses are tolerant to pronamide and cool-season grasses are susceptible to pronamide, this herbicide has proven to be a standard to transition perennial ryegrass from overseeded bermudagrass (Johnson, 1976).
There were four objectives of this research project. One was to determine if cultural and chemical treatments, commonly applied to golf course fairways and athletic fields in spring, would expedite transition from overseeded perennial ryegrass to bermudagrass. The second objective was to determine if the two selected varieties (Derby Supreme and Gator) of perennial ryegrass which are commonly used for overseeding purposes had any effect on spring transition due to differences in heat tolerance. The third objective was to determine if there was a significant association between weather data and natural ryegrass disappearance from nontreated control plots. The fourth objective was to determine the effect overseeding had on the natural bermudagrass transition from physiological dormancy. For this purpose, two control treatments were used, one overseeded and the other not overseeded. It was hypothesized that nonoverseeded bermudagrass will transition out of physiological dormancy earlier than overseeded areas, thus producing a more uniform and dense bermudagrass stand.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Plots were overseeded on 12 Oct. 1995 and 20 Oct. 1996. Prior to overseeding, plots were vertically mowed with a Ryan Mat-Away (Textron Turf Care and Speciality Products, Racine, WI) in two directions and cleared of resulting debris. Derby Supreme and Gator perennial ryegrasses were seeded at 488 kg ha-1 using a Scotts Drop Spreader (The Scotts Company, Marysville, OH). Plots were seeded in two directions to ensure uniform distribution of seed. Nonoverseeded areas were included in the experiment for bermudagrass shoot density comparisons.
Nine treatments, including an overseeded nontreated control and a nonoverseeded nontreated control, were applied to plots each year. Vertical mowing, scalping, vertical mowing plus scalping, and core cultivation commenced each year as bermudagrass began to break dormancy on 25 Apr. 1996, and again on 11 Apr. 1997. Vertical mowing was performed with a Jacobson Aeroking 1321 Seeder-Verticutter (Textron Turf Care and Speciality Products, Racine, WI) with vertical blades set to penetrate 5 mm into the soil. Scalping was performed with a John Deere 22R mower (Deere & Company, Moline, IL), set at a mowing height of 1.3 cm. For the vertical mowing/scalping treatment, plots were vertically mowed and then scalped. This allowed debris from the vertical mowing to be harvested by the reel mower. Aerification was performed using a Toro Greens-Air (The Toro Company, Bloomington, IL) with 1.9-cm diam., 7.6-cm-deep hollow tines on a 5.1- by 5.1-cm pattern. Cores from aerification and debris from vertical mowing were removed after each respective treatment was applied. Each cultural treatment was applied every two weeks until perennial ryegrass disappearance was complete.
Chemical removal using the herbicide pronamide was included as a standard comparison in the trial. Pronamide treatments were applied with a CO2 backpack sprayer equipped with TeeJet 8002XR flat fan nozzles (Spraying Systems Co., Wheaton, IL). The sprayer applied 304 L ha-1 spray volume at 
193 kPa pressure. Pronamide 50 WP was applied at 1.12 kg ai ha-1 on 25 Apr. 1996 and 11 Apr. 1997.
Granular NH4NO3 treatments were applied with a hand-held shaker jar at a rate of 287 kg fertilizer ha-1. The total product weight was divided in half and applied in two directions to ensure uniform distribution. Early applications of NH4NO3 were applied on 25 Apr. 1996 and 11 Apr. 1997, while late applications of NH4NO3 were applied on 11 June 1996 and 23 May 1997. To increase the burn potential of the fertilizer, 0.5 liters of water per 4.5 m2 was sprayed uniformly on each plot prior to application of the granular product.
The statistical design was a split plot with four replications. The two varieties of perennial ryegrass were main plots and nine treatments were subplots. Subplot sizes were 1.5 m by 3.1 m.
Visual ratings of ryegrass coverage were made 5 d after initial treatments and continued biweekly until complete perennial ryegrass disappearance. Perennial ryegrass coverage ratings were based on a 100–to–0 scale (100 = completely covered with perennial ryegrass, 0 = no perennial ryegrass). Visual ratings of turfgrass quality were made on a scale of 1 to 9 (1 = complete turf desiccation, 5 = minimally acceptable, 9 = ideal turf). Bermudagrass shoot density was measured weekly from 5 wk after initial treatment (WAIT) through 11 WAIT in 1996 and weekly from 5 WAIT through 15 WAIT in 1997, using randomly placed 39-cm2 counting frames and extrapolating for a 1-m2 area. Counting frame dimensions were 2.5 cm by 15.2 cm and constructed using wooden dow rods and cotton string.
Weather data were collected daily at the Turfgrass Field Center for both years of study. Air temperature and relative humidity were collected with a Campbell Scientific CS500 Vaisala- 50Y (Campbell Scientific, Inc., Logan, UT). Rainfall was measured with a Sierro Misco Tipping Bucket Rain Gauge (Jarek Manufacturing Limited, Sooke, B.C., Canada) and soil temperature was measured with a CSI 107B Water–Soil Temperature Probe (LI-COR, Inc., Lincoln, NE).
Data from each year were evaluated separately for each treatment using analysis of variance for a split plot design (SAS Institute, 1989). When the analysis of variance indicated a significant treatment effect (P < 0.05), treatment effect and nonsignificant treatment by variety interaction means were separated by the least significant difference (LSD) procedure.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Complete transition of perennial ryegrass to bermudagrass occurred the same time as the overseeded nontreated control during both years of study. Analysis of variance indicated significant year-by-treatment interaction due to perennial ryegrass transition timing. For this reason, growing seasons (1996 and 1997) were analyzed and will be discussed separately. Even though there were significant differences among treatments, none of the treatments differed greatly and were determined to have limited biological significance. Therefore, for simplicity of presentation, all treatments were compared to the nontreated.
During the first year of study, scalp, scalp/vertical mowing, and pronamide treatments increased the rate of perennial ryegrass disappearance through 7 WAIT (Fig. 1 and 2)
. An increase in average daily temperatures from 10.5°C in mid-May to 27.0°C in mid-June caused bermudagrass to rapidly green up, resulting in an overall decrease in ryegrass coverage from 5 to 7 WAIT (Fig. 1 and 2). However, perennial ryegrass reappeared in the scalp and scalp/vertical mowing treated plots 8 WAIT (Fig. 1 and 2). This was probably because of a drop in average daily temperatures from 27°C to 22°C, and a rain event measuring 1.12 cm, but it is unclear why this pattern did not appear in other treatments.
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With the exception of core cultivated plots 3 WAIT, there were no additional differences between treated and nontreated turf, with respect to perennial ryegrass disappearance in the first season (Fig. 2). Core cultivated plots had lower bermudagrass shoot density 8, 9, and 11 WAIT than nontreated plots (Fig. 2). Routine cultural practices, such as cultivation, should only be used as needed to correct soil compaction and associated problems such as localized dry spots (Beard, 1973). Biweekly core cultivation can stress turfgrass in hot summer months. Even bermudagrass could not sustain normal growth with the selected core cultivation schedule. High summer temperatures along with the potential added stress of drier soils resulted in lower bermudagrass turf quality ratings and reduced shoot density. Shoot density may have also been affected by the physical removal of turfgrass shoots with each coring treatment.
Compared with overseeded plots, nonoverseeded plots had an average of 38% higher bermudagrass shoot density when averaged over the spring transition period (Fig. 3) . Without perennial ryegrass competition for light, nutrients, and moisture, nonoverseeded bermudagrass can transition out of dormancy earlier, producing a more uniform bermudagrass stand earlier in summer than overseeded plots.
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Early applied NH4NO3 (Fig. 4) stimulated ryegrass growth in 1997 at 7 and 9 WAIT, resulting in a dense perennial ryegrass sward. Competition in these plots from vigorously growing ryegrass slowed bermudagrass growth. Perennial ryegrass shoot density decreased rapidly from 9 WAIT until 13 WAIT. Bermudagrass shoot density was correspondingly higher during the 11 and 13 WAIT time frame (Fig. 4). Late applied NH4NO3 applied at 7 WAIT stimulated perennial ryegrass growth 9 WAIT, but this effect did not persist beyond this point (Fig. 5). This early flush of ryegrass growth was followed by a rapid decline in ryegrass coverage, allowing for increased bermudagrass growth (Fig. 5).
Nonoverseeded plots had higher bermudagrass shoot density throughout most of the rating period in 1997, with 15 WAIT (Fig. 3) as the only exception. Perennial ryegrass had fully transitioned out by 15 WAIT and bermudagrass shoot density in overseeded plots was equal to that for nonoverseeded plots at this time. The natural transition period was 15 wk in 1997, and 11 wk in 1996 (Fig. 3). The additional 4 wk of growth allowed shoot density in overseeded nontreated plots to equal nonoverseeded plots in 1997. In 1996, with a shorter transition period, shoot densities at the end of the rating period were significantly different when comparing nonoverseeded and overseeded nontreated plots.
There was a large difference in the end of the transition period bermudagrass shoot counts for nontreated plots between 1996 and 1997 (Fig. 3). On 7 Jul. 1996, the mean shoot density in overseeded nontreated plots was 7362 shoots m-2 with 1.5% ryegrass remaining. On 7 Jul. 1997, the mean shoot density in overseeded nontreated plots was 6393 shoots m-2 with 50% ryegrass remaining. As the summer progressed and temperatures continued to increase beyond July 1997, perennial ryegrass continued to naturally transition out and bermudagrass shoot density continued to rise. By the end of the rating season in 1997, mean shoot density in overseeded nontreated plots was 19 537 shoots m-2 (Fig. 3), over twice as high as end of season bermudagrass shoot density in 1996.
Turf Quality
Quality ratings reflected injury to turfgrass. Although biweekly ratings of turfgrass quality were taken (data not shown), results were variable and inconsistent. The injury observed by individual treatments was often deficient within days of the original rating and would become evident again following the next cultural treatment.
Treatments that did cause turfgrass quality differences were core cultivation, early and late applied NH4NO3, and the 1997 pronamide treatment. Core cultivation was the only treatment to significantly reduce turf quality throughout the rating seasons. Such results were not observed by Mazur and Wagner (1987) because core cultivation of overseeded plots occurred only once in their study. Early and late applied NH4NO3 enhanced turfgrass quality in both years of this study. Supplemental N caused a darker green color and resulted in increased bermudagrass shoot density with early applied NH4NO3 at the end of the transition period in both 1996 and 1997 (Fig. 1 and 4). Pronamide, while reducing perennial ryegrass coverage in both years, only reduced turf quality from 7 WAIT through 11 WAIT in 1997. During this time period, perennial ryegrass coverage (Fig. 5) decreased from 52 to 21%. Johnson (1976)(and 1991) found similar results as turfgrass quality was severely reduced 4 to 8 wk after treatment when pronamide was applied too early in the spring. Turf quality in pronamide treated plots subsequently increased from 11 WAIT through the end of the transition period, at which point bermudagrass cover had reached 100%. Therefore, pronamide should not be applied too early as this may result in ryegrass disappearance before environmental conditions are conducive for bermudagrass growth.
Weather Data
Average daily temperatures and relative humidity readings were taken to compare with natural ryegrass disappearance and bermudagrass shoot emergence in nontreated and nonoverseeded plots. A regression coefficient (R2 = 0.5038 with, P = 0.0005) was calculated between relative humidity and natural perennial ryegrass disappearance (Fig. 6)
. The calculated regression coefficient (R2 = 0.5079, with P = 0.0038) for air temperature and perennial ryegrass disappearance was also significant (Fig. 6). Bermudagrass shoot density was not related to either temperature or relative humidity. Results imply that both increased relative humidity and temperature accelerate natural ryegrass disappearance, while having negligible impact on bermudagrass vigor. Without any additional factors, such as the cultural treatments used in this study, sustained increased temperatures and relative humidity appear to be more important parameters in dictating the time at which full transition from perennial ryegrass to bermudagrass takes place.
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ACKNOWLEDGMENTS |
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Received for publication May 5, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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= 0.05 between treated and nontreated plots
