Crop Science 43:282-287 (2003)
© 2003 Crop Science Society of America
TURFGRASS SCIENCE
Effect of Irrigation Frequency on Turf Quality, Shoot Density, and Root Length Density of Five Bentgrass Cultivars
J. E. Jordana,
R. H. White*,a,
D. M. Vietorb,
T. C. Haleb,
J. C. Thomasb and
M. C. Engelkec
a Texas A&M University, Soil and Crop Sciences Dep., UMS 2474, College Station, TX 77843-2474
b Texas A&M University, Soil and Crop Sciences Department, UMS 2474, College Station, TX 77843-2474
c Texas A&M Dallas, Agricultural Research and Extension Center, 17360 Coit Rd., Dallas, TX 75252
* Corresponding author (rh-white{at}tamu.edu)
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ABSTRACT
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The culture of creeping bentgrass (Agrostis palustris Huds.) at low mowing heights on putting greens in the hot humid region of the USA presents numerous water management problems. Frequent irrigation of greens to prevent water stress has been credited with producing shallow rooted turf with reduced tolerance to environmental stress. The present study was conducted to determine the relationship between irrigation frequency and turf quality, shoot density, and root length density for five cultivars of creeping bentgrass grown on a sand-based root zone and maintained to putting green standards. A total of 81 plots, 1.5 by 1.5 m each, were established on a USGA-type root zone mixture and organized into nine irrigation cells of nine plots each. Each irrigation cell could be irrigated individually. One plot in each irrigation cell was planted to the following bentgrass cultivars: A-4, Crenshaw, Mariner, L-93, and Penncross. The remaining four plots in each irrigation cell were also planted with bentgrass but were not a part of this study. Irrigation frequency treatments of 1-, 2-, and 4-d were imposed on three irrigation cells each. After establishment, measurements of turf quality, shoot density and root length density were made over a 2-yr period. In both years, Crenshaw and L-93 had the best turf quality. At the end of the study, shoot densities for Crenshaw, L-93, and A-4 were 37 to 42% greater than Mariner and Penncross. Because of frequent rainfall events in 1997, there were no significant (P 
0.05) effects of irrigation frequency on shoot density or root length density. However, in 1998, turf was more dependent on irrigation and bentgrass irrigated every 4 d had significantly greater turf quality, shoot density, and root length density than that watered every 1 or 2 d. The data show that even under putting green management conditions, reduced irrigation frequency of bentgrass produces a larger and deeper root system resulting in greater overall plant health, turf quality, and shoot density.
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INTRODUCTION
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WHEN PROPERLY MAINTAINED, creeping bentgrass provides a high quality turf surface for golf course putting greens. Because of this high quality surface, bentgrass use has spread far into the southern USA, well beyond its area of adaptation (Beard, 1973). Summer decline of roots during July and August is a major problem encountered with bentgrass in the southern USA (Murphy et al., 1994). The decreased total number of roots and average root length at high soil temperatures reduces plant access to soil water and nutrients. In addition, the dead roots serve as a food source for soil microbes, which results in increased respiration (Kuzyakov et al., 2001) and reduced oxygen concentrations. Both high soil temperature (Beard and Daniel, 1965; Huang and Xu, 2000; Xu and Huang, 2000a,b) and low soil aeration (Huang et al., 1998) decrease root growth and increase root mortality. In addition, a synergistic effect between high soil temperature and low soil aeration further contributes to bentgrass decline (Huang et al., 1998).
Very short mowing heights also reduce root growth of creeping bentgrass (Beard and Daniel, 1965; Madison, 1962; Salaiz et al., 1995) and increase turf susceptibility to temperature and water stresses (Madison, 1962). Therefore, careful water management, including watching the turf for signs of water stress and making adjustments in irrigation practices in response to water stress, is critical when growing bentgrass in the hot, humid South. To accomplish this, turfgrass managers have resorted to daily irrigation of greens combined with hand watering of dry spots and syringing the turf during the hottest period of the day. However, over irrigation can also be detrimental to bentgrass turf and may lead to increased disease occurrence, algae formation, and thatch accumulation. Provided aeration does not become limiting, prolonged periods of high soil moisture reduce the number and depth of roots (Madison and Hagan, 1962) while increasing total water consumption. In contrast, less frequently irrigated turf used less water (Brian et al., 1981; Doss et al., 1964; Mantell, 1966) because of reduced evapotranspiration. Daily irrigation of bentgrass putting greens contradicts the recommendation to irrigate turfgrasses deeply but infrequently (Madison and Hagan, 1962; Beard, 1973), although it may be required for greens built with sand-based root zone mixtures.
Infrequent irrigation fostered better root development on both cool-season (Bennett and Doss, 1960) and warm-season forage grasses (Doss et al., 1960; Madison and Hagan, 1962). In contrast, St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] showed no response of root length density to increasing irrigation frequencies of up to 6 d (Peacock and Dudeck, 1985).
The objective of this study was to determine the relationship between irrigation frequency and bentgrass turf quality, shoot density, and root length density for five cultivars of creeping bentgrass.
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METHODS AND MATERIALS
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A total of 81 plots (1.5 by 1.5 m each) were established on a putting green constructed according to the United States Golf Association (USGA, 1993, p. 1–3) recommendations. The plots were organized into nine irrigation cells containing nine plots each. Each irrigation cell was equipped with four 90°, 3-m stream spray irrigation heads and had independent irrigation control. One plot in each irrigation cell was randomly assigned to each of five creeping bentgrass cultivars: A-4, Crenshaw, Mariner, L-93, and Penncross. The remaining four plots in each irrigation cell were planted to bentgrass, but were not used for this study. A mixture of 2.44 g m-2 pure live seed and 2.44 g m-2 of a processed sewage sludge (Milorganite, Milwaukee, WI) was applied on 19 Nov. 1996. Fertilization, mowing, and irrigation were applied uniformly across all plots before the initiation of the irrigation treatments on 21 July 1997. A total of 29.3 g N m-2 yr-1 was applied to all plots. Application rates were 3.27 g N m-2 every 2 wk during October to December and 1.22 g N m-2 every 2 wk during March–June. Elite polymer/sulfur coated urea (21-4-11 from Lesco Inc., Strongsville, OH) was the N source. Potassium was applied at rates equal to N by supplementing with a slow release potassium source. The plots were hand brushed twice monthly and occasionally lightly dusted with topdressing sand during the fall and spring growth periods. Plots were not verticut or core aerified. Turf quality was rated as excellent for putting green use and the turf was mature prior to beginning the irrigation treatments. Turf was mowed 6 d wk-1 at a height of 4 mm. Irrigation was applied between 0100 and 0600 h when the wind velocity was 2 m s-1 or less to assure uniformity.
Irrigation treatments were imposed from 21 July to 4 Sept. 1997 and 22 June to 1 Sept. 1998. Treatments replaced the full amount of water lost from Class A pan evaporation from the previous 1-, 2-, or 4-d intervals. All plots received the same total amount of irrigation water. Each irrigation treatment was replicated three times (three cells) for all five cultivars. Nine collection gauges placed in a grid pattern were used to monitor irrigation uniformity of each irrigation cell at the beginning of the treatment period each year. The average irrigation uniformity was 82, 83, and 83% for the 1-, 2-, and 4-d irrigation cells, respectively. During the fall, winter, and spring seasons, maintenance irrigation was applied twice per week in amounts equal to the Class A pan evaporation.
Weather data were collected by an automated weather station (Campbell Scientific, Logan, UT) located within 100 m of the experimental site. Measurements included minimum and maximum air temperatures, relative humidity, wind speed and precipitation. Pan evaporation was measured manually by means of a Class A evaporation pan and a hook gauge.
Three soil core samples were collected from each plot in June before beginning the irrigation treatments and again in late August before terminating the irrigation treatments each year. Each sample was tested individually and the data were averaged prior to statistical analysis. Soil cores were obtained by inserting a 2.5-cm-diam soil corer to a depth of 15 cm. The number of shoots in each core sample was counted to determine shoot density. The core samples were divided into 1.0- to 7.5-cm and 7.5- to 15-cm depth increments. Each increment was placed on a 3-mm mesh screen and washed with a gentle stream of water. Total root length was determined with a Comair Root Length Scanner (Commonwealth Aircraft Corp. Ltd, Melbourne, Australia) calibrated to a range of known lengths and widths of thread. Root length density was calculated by dividing the total root length in cm by the volume of the sample in centimeters cubed. The change in root length density over the treatment period was calculated for each depth and each year by subtracting the June root length density (RLD) from the August RLD. Positive change values indicate increasing RLD over the treatment period while negative change values indicate decreasing RLD over the treatment period.
Turfgrass quality was visually assessed and rated on a scale of 1 to 9 representing poor and excellent quality turf, respectively. A rating of 5 was considered to be the minimum acceptable turf quality for a putting surface. Ratings were conducted weekly during irrigation treatments and monthly during the remainder of the year. Pesticides were applied on a curative basis only.
All data were statistically evaluated by ANOVA for a split-plot arrangement of a randomized complete block design. The combined data from both years was analyzed followed by an analysis of each year's data separately. When ANOVA indicated significant differences (P = 0.05), the minimum significant difference was computed by means of Tukey's Studentized Range Test (Zar, 1996, p. 197–269). Error bars on figures were computed from the standard error around the mean (Steel and Torrie, 1960).
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RESULTS AND DISCUSSION
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Environmental Conditions
The mean average air temperature, relative humidity, wind speed, and total rainfall for the summer months (May–August) that included the periods when irrigation treatments were imposed in 1997 and 1998 were summarized (Table 1). Mean average air temperatures were higher for each month in 1998 than in the corresponding months in 1997. Mean wind speeds were 1.1 m s-1 greater in June and July of 1998. While the relative humidity readings for the two years were fairly similar, approximately twice as much precipitation occurred during the experimental period of 1997 as in 1998. June 1997 was a very wet month with 140.2 mm of precipitation as compared to only 5.6 mm during the same month in 1998. Thus, the environmental conditions at the experimental site may be considered to be warmer, windier, and drier during the summer of 1998.
Turf Quality
Turf quality over the entire 2-yr period was significantly (P = 0.001) affected by year and cultivar; however, there were no significant interactions among any of the parameters (Table 2). To remove the effect of year, the data were analyzed separately for each year. Mean turf quality ratings for Crenshaw and L-93 were the highest of the five cultivars in 1997 and exceeded the minimum acceptable turf quality rating of 5.0 (Table 3). However, the quality ratings were relatively low. Quality ratings for A-4 and Mariner were intermediate and lowest for Penncross.
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Table 2. Analysis of variance for the turf quality measurements over 2 yr, five cultivars, and three irrigation frequency treatments.
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Table 3. Mean turf quality for five bentgrass cultivars in 1997 and 1998. Each value is the average of 99 individual measurements.
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The mean turf quality ratings for 1998 were greater than those for 1997, but the cultivar ranking was similar. Mean turf quality ratings for Crenshaw and L-93 were the highest of the five cultivars in 1998. Quality ratings for A-4 and Mariner cultivars were intermediate and lowest for Penncross. All turf quality ratings for 1998 exceeded the minimum acceptable turf quality of 5.0. The improved turf quality was due to increased shoot density during 1998 (data presented below).
During the 1997 treatment period (Day 200–247), turf quality did not differ significantly (P = 0.05) among irrigation treatments (Fig. 1). This is probably due to frequent rains and high humidity overriding the treatment effects on turf quality or possibly there were no treatment effects. Turf quality slowly improved through the fall of 1997 and the early part of 1998 until it reached a quality of approximately 8 by Day 150 in 1998 (Fig. 2). Heat and water stress contributed to a gradual decline in turf quality after treatments began in 1998. The final two measurements of 1998 indicated that the turf quality of the 4-d irrigation frequency treatment was significantly greater (P = 0.05) than the other treatments. These data indicate that during hot and dry periods when bentgrass is dependent on irrigation for water, less frequent irrigation can provide improved turf quality without increasing total water use.
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Fig. 1. Mean turf quality from Day 200 (16 July) through Day 280 (7 October) of 1997 for the three irrigation frequency treatments. Bars represent the standard error around the mean.
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Fig. 2. Mean turf quality from Day 145 (27 May) through Day 235 (24 August) of 1998 for the three irrigation frequency treatments. Bars represent the standard error around the mean.
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Shoot Density
Shoot density over the entire 2-yr period was significantly (P = 0.001) affected by year, month, replication, and cultivar. In addition, there was a significant interaction between year and cultivar (Table 4). To remove the effect of year, the data were analyzed separately for each year. Shoot density for each irrigation treatment increased throughout the two years of this study (Table 5). The data indicate that as of Aug. 1998, the plots probably had not yet reached their maximum sustainable shoot density. Shoot density differed significantly (P = 0.05) among cultivars near the end of the irrigation treatments in 1997 and 1998, except for the 1-d frequency in 1997. Shoot densities of L-93 were significantly greater than Mariner and Penncross for all three irrigation frequencies in 1998 and the 2- and 4-d frequencies in 1997. These differences in shoot density are consistent with cultivar differences in quality ratings.
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Table 4. Analysis of variance for the shoot density measurements over 2 yr, five cultivars, and three irrigation frequency treatments.
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Table 5. Shoot density of creeping bentgrass cultivars as affected by irrigation frequency in early June and late August of 1997 and 1998.
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There were no significant differences (P = 0.05) among irrigation treatments for mean shoot density of the June 1997 and June 1998 samples. This indicates that there was no carry-over effect of irrigation treatment on turf quality in 1998.
For August 1997, there were no significant (P = 0.05) effects of irrigation treatment (Table 5) on shoot density, but a significant (P = 0.05) increase in mean shoot density was observed for bentgrass irrigated every 4 d as compared with that watered every 1 or 2 d in 1998. Larger amounts of precipitation during the treatment period in 1997 could have masked the effects of irrigation frequencies. In 1998, the turf was more dependent on irrigation and responsive to the irrigation treatments.
Root Length Density
The overall analysis of the root length density data showed significant effects due to year, month, replication, irrigation, cultivar, and depth (Table 6). The significant effects of month and depth follow the general concepts that the amount of roots decreases with depth and is greatest in the spring and fall seasons for cool season grasses. Significant two-way interactions were found for month and year, month and replication, month and depth, and month and irrigation frequency.
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Table 6. Analysis of variance for the root length density measurements over 2 yr, five cultivars, two depths, and three irrigation frequency treatments.
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In June 1997, RLD in the 1- to 7.5-cm depth of the 1-d irrigation frequency treatment ranged from 25.3 to 39.2 cm cm-3 (Table 7). By August 25, the RLD at this sampling interval had declined 13.6 to 26.9 cm cm-3, a decrease of approximately 60%. It is thought that the lack of significant (P = 0.05) cultivar differences was due to large random variation of RLD measurements during 1997. The reduction of RLD in the upper 7.5 cm of soil was likely due to a combination of increased root mortality under high soil temperatures and related stresses, and decreased production of new roots. At the 7.5- to 15-cm depth, RLD for turf irrigated daily increased slightly from June to August. Environmental constraints on root growth and survival in the 1- to 7.5-cm depth could have increased the availability of photoassimilates for root growth in the more favorable environment at 7.5 to 15 cm. Similar patterns of reduced RLD in the 1- to 7.5-cm depth and slightly increased RLD at 7.5 to 15 cm occurred for bentgrass irrigated at 2-and 4-d frequencies in 1997.
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Table 7. Mean root length density and the change in root length density at two depth intervals for five creeping bentgrass cultivars under three irrigation frequencies during 1997.
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In 1998, changes in RLD were similar among cultivars at the 1- and 2-d irrigation frequencies in the 1- to 7.5-cm layer (Table 8). In contrast to 1997, RLD decreased at both sampling depths at the 1- and 2-d irrigation frequencies. Higher temperature and evaporative demand during 1998 could have reduced root growth and survival at the deeper sampling depth as compared to 1997. Unlike the reductions in RLD observed at 1- and 2-d irrigation frequencies, RLDs increased at both sampling depths for the 4-d irrigation frequency in 1998. In addition, the change in RLDs differed significantly (P = 0.05) among cultivars at 1 to 7.5 cm. Less frequent irrigation could have resulted in greater root uptake of water and increased drainage of excess irrigation water which would have improved soil aeration and stimulated root growth. The L-93 cultivar performed best at the 1- to 7.5-cm depth, but increases in RLD did not differ among cultivars at the 7.5- to 15-cm depth.
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Table 8. Mean root length density and the change in root length density at two depth intervals for five creeping bentgrass cultivars under three irrigation frequencies during 1998.
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These findings are in agreement with previous reports (Madison and Hagan, 1962; Bennett and Doss, 1960; Doss et al., 1960). Bluegrass (Poa pratensis L., ‘Merion’) grown on a Yolo clay loam soil and subjected to 2-, 10- and 20-d irrigation frequencies exhibited smaller numbers of plants, but deeper root systems, as irrigation frequency increased (Madison and Hagan, 1962). The relatively high mowing heights of 1.3 to 5.1 cm used in the study may account for the different response in terms of the number of plants per unit area. However, the response of the root system to irrigation is similar to that of the present study. Bennett and Doss (1960) also documented deeper rooting with less frequent irrigation in six out of eight cool season grasses. In a similar study using five warm season forage grasses, Doss et al. (1960) reported deeper rooting of the grasses that received less frequent irrigation.
While the previous studies employed a variety of both cool and warm season forage grasses, they did not include bentgrass nor were the grasses maintained at low mowing heights and managed intensively as is typical of putting greens. The present study confirms that even under putting green management conditions, bentgrass responds similarly by producing a larger and deeper root system when subjected to reduced irrigation frequency. This results in more efficient exploration of the soil for moisture and plant nutrients, resulting in improved turf quality and increased shoot density.
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CONCLUSIONS
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During the 1997 treatment period, turf quality did not differ significantly (P = 0.05) among irrigation frequency treatments. In 1998 turf quality was similar until the latter half of the treatment period during which time the 4-d treatment had a significantly greater turf quality. When turf quality was averaged over irrigation treatments, the Crenshaw and L-93 cultivars consistently had the highest quality and Penncross had the lowest quality in both years. Because of the large amounts of precipitation in 1997, there were no significant (P = 0.05) effects of irrigation treatment on shoot density or root length density. However, in 1998 the turf was more dependent on irrigation and the 4-d irrigation frequency treatment had a significantly greater shoot density and root length density. The results of this study show that during growing seasons when precipitation is limiting and turf growth is heavily dependent on irrigation, turf quality, shoot density, root length, and root length density can be improved by increasing the time interval between irrigation events.
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ACKNOWLEDGMENTS
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We gratefully acknowledge the generous assistance and support from Mr. Mark Hall in helping with establishing and managing this study. This research was supported in part by the United States Golf Association and the Texas Turfgrass Association. The work was conducted at Texas A&M University, Soil and Crop Sciences Dep., College Station, TX 77843-2474.
Received for publication January 7, 2002.
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REFERENCES
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ABSTRACT
INTRODUCTION
