Bentgrass Cultivar and Annual Nitrogen Regime Affects Seasonal Shoot Density

Adam C. Moeller, Cale A. Bigelow, Jared R. Nemitz, and Glenn Hardebeck, Department of Agronomy, Purdue University, West Lafayette, IN 47907

Corresponding author: Adam C. Moeller. acmoelle@purdue.edu

Abstract

Creeping bentgrass (Agrostis stolonifera var. palustris Huds. Farw.) is the preferred turfgrass species for golf greens. During summer months, however, shoot density (SD) often declines, resulting in poor stand quality. Turf managers utilize several management practices to maintain turf vigor. One practice is light, frequent N fertilization. However, annual N rates vary widely. Recently, high shoot density (HSD) bentgrasses bred to provide superior appearance and stress tolerance compared to the industry standard, ‘Penncross,’ have been widely planted. The effect of variable N rates on these cultivars is unclear. This field study measured seasonal SD changes of three contrasting cultivars maintained with low and high N regimes, 112 and 196 kg of N per ha/year, respectively. Significant seasonal SD differences were measured and cultivars ranked ‘A-4’ (1400 to 2160 shoots/dm²) > ‘L-93’ (1230 to 1780 shoots/dm²) > ‘Penncross’ (760 to 1470 shoots/dm²). Generally, increasing N resulted in only slight differences in SD. In August, however, when N regime was averaged across cultivars, higher N significantly increased SD in both study years. For the densest, most persistent turf during summer months, a HSD cultivar should be planted and moderate N should be applied to sustain appearance and ensure adequate recovery should turf damage occur.

Introduction

Creeping bentgrass (Agrostis stolonifera var. palustris Huds. Farw.) is the most widely planted turfgrass species for golf course putting greens in the United States (2). Creeping bentgrass is a cool-season grass that forms an extremely dense, fine-textured persistent turf which tolerates close (< 3 mm), frequent mowing. The most commonly planted cultivar for putting greens since the 1950s has been ‘Penncross’ (1). In recent decades new cultivars such as ‘A-4’ and ‘L-93’ have been developed and commercialized from existing cultivars, predominately ‘Penncross,’ to improve playing conditions and meet golfer demands for more closely-mowed putting greens. These cultivars possess a high shoot density (HSD) and generally have better overall turfgrass quality (TQ) compared to ‘Penncross.’ Some of the characteristics associated with the superior quality are: a more upright growth habit, higher shoot density (SD), finer leaf texture, increased rooting depth, greater resistance to certain turf pathogens, better resistance to annual bluegrass (Poa annua L.) invasion, and improved heat and drought tolerance (1,5,10,17,18,23,24).

Since bentgrass is a cool-season turf, a decline in density during the summer months is common throughout the lower cool-humid and transition zone regions (6). Various cultural practices like mowing height, fertilization regime, and grooming can have an influence on SD, although little scientific data has reported the effects of these practices. The effect of mowing height (3.2 vs 4 mm) on SD for twenty cultivars was evaluated in North Carolina but no correlation between mowing height and SD was observed (5). Research conducted in Texas found that five bentgrass cultivars irrigated every four days versus every one or two days increased SD (11).

Contemporary golf course managers frequently apply N at between 100 and 195 kg/ha/year (24). In recent years there has been a trend toward applying minimal N amounts (≤ 100 kg/ha/year) in an effort to produce consistently long ball-roll distances. This practice of applying low annual N amounts could result in substantial thinning because plants lack sufficient vigor needed to produce new shoots, especially during heat stress. In addition to regular mowing and fertilization, sand topdressing is commonly applied to golf greens for thatch management and to firm and smooth the surface. Sand particles are very abrasive to plant leaves (6,8) and could severely reduce SD if intensive topdressing practices are conducted during summer stress. Anecdotal reports suggest that HSD cultivars require more intensive topdressing (10) and thus more potential for leaf abrasion and thinning turf in golf greens with these cultivars, especially when grown with low N. Seasonal data regarding the changes in bentgrass SD when subjected to regular frequent topdressing does not exist. Additionally, the changes in putting green surface functional characteristics [i.e., surface hardness (SH), ball roll distance] as a result of seasonal SD declines are unclear but could be influenced by N fertilization practices. Therefore, the objective of this field study was to measure seasonal SD changes and assess the effects of bentgrass cultivar and annual N regime on visual appearance, and key functional characteristics, SH and ball roll.

Comparing Bentgrass Cultivar Performance to an Annual Nitrogen Regime

This field study was conducted at the W.H. Daniel Turfgrass Research and Diagnostic Center at Purdue University, West Lafayette, IN, from April 2006 through November 2007 on a sand-based (80 sand: 20 peat mixture) research green built to United States Golf Association (USGA) construction specifications with > 90% of the sand particles between 0.1 to 1.0 mm size (25). Three creeping bentgrass cultivars, ‘A-4,’ ‘L-93,’ and ‘Penncross,’ were seeded on 5 August 2003 in 1.5 × 1.5-m plots. The study site was located in full sun with no surrounding obstructions, which was conducive to rapid drying of the canopy in the early morning hours. Overhead irrigation was used to supplement rainfall and promote plant growth during the growing months (April-November). In the absence of a significant (≥ 13 mm) rainfall event, irrigation was applied at approximately 5 mm nightly to achieve approximately 35 mm/week. Plots were mowed (3.6 mm) six days per week using a triplex reel mower (Toro Greensmaster 3100, The Toro Company, Bloomington, MN) with clippings removed. Fungicides (chlorothanonil, propiconazole, thiophanate-methyl, and flutolonil) were applied curatively in 2006 and preventatively in 2007 during periods of active disease pressure, primarily for dollar spot (Sclerotinia homoeocarpa F.T. Bennett) and brown patch (Rhizoctonia solani Kuhn.) control.

Two fertility regimes designated as "low" and "high" (112 vs 196 kg of N per ha/year, respectfully) were used to assess the varying range of N applied to golf course putting greens and selected to represent N regimes commonly used by golf course managers in the region. Nitrogen was applied either as a liquid or granular formulation depending on application rates and dates with roughly half of the total N applied as a liquid during the summer (June-September). Granular applications (greens grade mythlene-urea based product 40-0-0) for the "low" treatment were applied with a broadcast rotary spreader. "High" treatment plots receiving additional granular N applications were applied evenly over individual plots using a hand shaker with a pre-weighed amount of fertilizer. Liquid fertilizer (12% urea N, 18% methylene urea N) was applied using a 2-m wide hand-held boom sprayer attached to a 11.4-liter hand-pump (242 kPa) back-pack container (820 liter/ha spray volume) for "low" treatments. Additional liquid N applications for "high" treatments were applied using a pressurized (242 kPa) CO₂ back-pack sprayer equipped with an 8010E TeeJet XR nozzle and a 820-liter/ha spray volume.

A sub-angular topdressing sand that matched the rootzone particle size analysis was applied (0.3 m³/100 m²) every 7 to 10 days during the growing season using a 1-m wide drop spreader. Topdressing was incorporated into the turf canopy using a push-broom with moderate pressure in opposite directions completely brushing the area twice.

Seasonal SD was measured by removing two intact cores (1.9 cm diameter, 2.84 cm²) per plot with a soil probe and counting individual shoots which were then averaged and used for data analysis. A shoot was defined as a fully expanded leaf. Four (June, July, August, and October) SD measurements were taken in 2006 and six times (May-October) in 2007. Measurements of SH was performed using a Clegg Impact Soil Tester (0.5 kg model, Lafayette Instrument Co., Lafayette, IN). The Clegg device is a commonly accepted method of measuring SH (14). Measurements were taken within a short time (± 2 days) period of SD and measured throughout each study year. The hammer was dropped three times from a height of 0.46 m and units were recorded in Clegg Impact Values (CIVs). These values were converted to g_max (peak deceleration) (4) using the following equation [g_max = 10(CIV)]. Two measurements were taken at different locations within the central portion of each plot and averaged to one value to account for spatial variability. Plots were visually assessed every 7 to 10 days throughout the study for TQ on a 0 to 10 scale with 0 = brown dead turf; 10 = optimum greenness, density, and uniformity; and ratings ≥ 7 = acceptable putting green turf.

Statistical analysis. The study design was a 3 × 2 (cultivar × N regime) factorial with each treatment replicated four times and arranged in randomized complete block design. Due to the large number of TQ ratings, data were pooled into two-month periods and then used for statistical analysis. All data was subjected to analysis of variance using the SAS system (SAS Institute Inc., Cary, NC) general linear models procedure and treatment means separated using Fisher’s protected least significant difference (LSD) test at the P ≤ 0.05 level.

Seasonal Variations in Bentgrass Shoot Density

Seasonal SD counts ranged from 760 to 2160 shoots/dm² throughout the study (Table 1 and Table 2). The temporal changes in SD followed the cool-season growth pattern with the highest values measured during spring and autumn and a decline during the summer months. Spring density was determined using the time period from May-June, summer was from July-August, and autumn was from September-October. Cultivar had a significant effect on SD in both study years. Among cultivars, ‘A-4’ generally possessed the highest SD (1400 to 2160 shoots/dm²) compared to ‘Penncross’ (760 and 1470 shoots/dm²) which had the least, while ‘L-93’ was intermediate (1230 and 1780 shoots/dm²). While each cultivar experienced reduced SD during the summer, the magnitude of the decline varied with cultivar and, to a lesser extent, N regime. When comparing the cultivars on a percentage basis, the spring density of ‘Penncross’ in both years was on average 37% less than ‘A-4’ at 196 kg of N per ha/year. By comparison, summer density of ‘Penncross’ averaged over both years was 44% less than ‘A-4’ under 196 kg of N per ha/year. The higher SD of ‘L-93’ and ‘A-4’ is consistent with reports that have demonstrated the enhanced ability of HSD cultivars to provide superior turf conditions during the summer when compared to ‘Penncross’ and many other earlier bentgrass generations (13).

The SD values for HSD cultivars in this study were generally similar to some previously reported values (5,9) but lower than others (1,11,21). Sweeney et al. (22) reported a similar number of shoots/dm² for ‘A-4’ compared to our data. They did not, however, report significant density reductions and in some instances measured an increase in SD from spring through summer. Some possible reasons for our lower values may be the slightly higher cutting height and the more intensive light frequent sand topdressing program we employed to reflect contemporary management practices.

In general, annual N regime did not significantly affect SD (Table 1 and Table 2). In August of each year, however, when overall SD was lowest, the high N regime resulted in significantly more shoots than low N plots when averaged across cultivars, 1170 vs 1330 and 1220 vs 1310 for the low and high N regimes in the 2006 and 2007 study years, respectively. For example, SD reductions for ‘A-4,’ ‘L-93,’ and ‘Penncross’ from June to August of each year averaged 23, 23, and 37% at the N rate of 112 kg/ha/year, respectively. By comparison, at the N rate of 196 kg/ha/year, reductions were 17, 15, and 23%, respectively. Among cultivars, ‘L-93’ SD was least affected by N regime. These cultivar results are also consistent with previous research (18). Higher annual N fertility has also been associated with greater root length and density, and this may help avoid summer decline and allow the plant to explore a greater soil volume for nutrients (12,19). Based on these data and the recommendations of other agronomists (2,8), it is suggested that golf course managers should continue to fertilize greens moderately during the summer months. When cool-season turfgrasses experience stand loss, it is imperative to have a sufficient nutrient supply to promote rapid recovery when favorable growing conditions return to promote shoot growth This is especially important for recreational areas to contest some of the stresses associated with heavy use and moderate-intensive contemporary topdressing programs.

Cultivar and N Regime Affect Appearance

Both cultivar and N regime had strong effects TQ (Table 3). The TQ values ranged from 7.4 to 8.7 and increased for HSD cultivars and N regime within each cultivar. Among cultivars, ‘Penncross’ at the 112 kg/ha/year N rate provided the poorest TQ, while ‘A-4’ and ‘L-93’ both had superior TQ compared to ‘Penncross’ during both study years. These results were expected and consistent with previous research (5,18,23). When averaged across both study years, the highest mean TQ was associated with ‘L-93’ and ‘A-4’ at the 196 kg/ha/year N rate. When evaluating each individual study year, the mean annual TQ for ‘L-93’ was superior to ‘A-4’ at both N regimes in 2006. The poorer TQ of ‘A-4’ in 2006 was due to significant dollar spot outbreaks, particularly early during the growing season, which negatively affected TQ when a curative fungicide program was implemented [data not presented, but available in (16)]. The high susceptibility of ‘A-4’ to dollar spot is consistent with previous reports (3,18,23). In 2007, however, when a preventative fungicide program was used, ‘A-4’ TQ was equivalent to that of ‘L-93’ at the 112 kg/ha/year N regime and superior to ‘L-93’ TQ at the 196 kg/ha/year N regime. Ball roll distance was not significantly influenced by cultivar.

During the onset of summer heat stress, both ‘A-4’ and ‘L-93’ at either N regime maintained an acceptable quality putting green while ‘Penncross’ under the low N regime resulted in unacceptable TQ, with values < 7.0, TQ. As expected, increasing N from 112 to 196 kg/ha/year, significantly increased TQ values by 0.4 to 1.1 points, when averaged across cultivars. Lower annual N however, did slightly increase ball roll distances from 145 to 158 cm using a modified stimpmeter (38 cm) across all cultivars (Table 4).

In general, the superior TQ of HSD bentgrasses is often attributed to their increased SD relative to earlier bentgrass generations which has been demonstrated in this study. Although N-regime effects on SD were minor, N regime significantly affected cultivar visual characteristics such as leaf texture, architecture and an overall greener color [data not presented, but available in (16)].

Bentgrass Shoot Density Affects Surface Firmness

One of the characteristics most important to golf course managers is SH, which can be affected by soil organic matter content (7), SD (20), and rootzone water content (15). Values for SH ranged from 125 to 167 g_max, and were variable depending upon measurement date (Table 4). Since both ‘A-4’ and ‘L-93’ had higher SD values than ‘Penncross’ these cultivars generally yielded lower g_max values and softer surfaces, especially during the spring. Additionally, both ‘A-4’ and ‘L-93’ maintained at 196 kg of N per ha/year yielded the softest surfaces on the majority of the measurement dates. In July, as SD declined for all cultivars, ‘Penncross’ at both annual N regimes yielded the firmest surfaces. Averaged across all cultivars, plots fertilized with 112 kg of N per ha/year resulted in firmer surfaces in May and June, however, as SD declined and slowly recovered, N regime was less influential on SH. In July of 2007, however, SD was negatively correlated with SH (Fig. 1) (R² = 0.6162) demonstrating that as bentgrass SD declines during the summer a turf manager can naturally expect an overall increase in SH. Therefore, aggressive management practices to promote SH on older bentgrass cultivars may be unnecessary and in fact negatively affect overall putting green health.

Fig. 1. Surface hardness as affected by shoot density 23 July 2007 (Data are mean values averaged across all cultivars and N regimes. Eight data points within each replication are averaged per value).

Summary and Recommendations

As golf course managers continue to strive to produce smooth, firm, consistent, putting green conditions, it is clear that several important factors affect bentgrass appearance and overall health. Of utmost importance is cultivar selection and N-fertility level, particularly with respect to summer performance. This field study validates what has been frequently reported: HSD cultivars like ‘A-4’ and ‘L-93’ possess and maintain a substantially higher SD throughout the year and are, therefore, more reliable than ‘Penncross,’ especially when subjected to contemporary putting green management practices. The HSD cultivars are more reliable than ‘Penncross’ because they maintain a dense turf canopy even with SD losses during the summer. Although the SD of all cultivars declined from spring to late-summer, ‘Penncross’ was most affected, often losing nearly 40% of it’s SD, compared to ‘A-4’ and ‘L-93’ which lost 25 to 30%. Additionally, when ‘Penncross’ was most dense, May or June, it’s SD was nearly equivalent to ‘A-4’ and ‘L-93’ at their lowest SD.

The agronomic benefits of an adequately fertilized turf have been well documented, but include increased vigor, canopy greenness, rooting, and damage recovery. In response to golfer’s desires for fast green speeds many golf course managers are applying less N than ever, often ≤ 112 kg/ha/year. This management approach is risky, and may compromise bentgrass health on heavily trafficked greens grown in stressful growing environments on sand-based rootzones that retain fewer nutrients. These ultra-low N regimes result in malnourished turf which is more prone to environmental stress and pest damage. An alternative approach might be to apply N at 112 to 196 kg/ha/year and manipulate other inputs such as applying plant growth regulators or using lightweight rollers to achieve green speed.

In this study, N levels of 112 or 196 kg/ha/year had very little effect on the SD for the three cultivars evaluated. The exception, however, occurred in August of both years when a beneficial response of the higher N level for SD was observed for ‘Penncross’ and ‘A-4.’ Although increased N did not enhance SD, it dramatically improved bentgrass visual appearance. This data supports the recommendation that moderate summer N at 49 to 73 kg/ha should be applied to minimize stand loss and ensure rapid recovery at the onset of favorable growing weather.

Due to the increased SD of newer bentgrass cultivars, many golf course managers are on an aggressive sand-topdressing program to maintain a firm, smooth surface. This normally involves the light application of sand every 7 to 10 days throughout the growing season. Although not directly evaluated in this study, we suggest that care should be exercised when attempting this management strategy for older bentgrass cultivars like ‘Penncross,’ especially when maintained using a low-N fertility program. Sand topdressing is a mechanically abrasive practice (6,8), and if improperly timed, may cause deleterious effects, further reducing ‘Penncross’ SD and negatively affecting overall putting green quality.

Where the densest, most aesthetically pleasing and persistent putting greens are desired, modern bentgrass cultivars should be planted and ample annual N should be regularly applied, particularly during the summer months. Future studies providing additional information regarding the affects of mowing frequency, frequent lightweight rolling, turf grooming and plant growth regulators on SD for HSD cultivars would be beneficial.

Acknowledgments

This research was supported by the Midwest Regional Turf Foundation and the Indiana Golf Course Superintendents Association. Grateful appreciation is expressed to Mr. David Weiss and all the undergraduate students for their technical assistance throughout this experiment.

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