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© 2007 Plant Management Network.
Accepted for publication 8 December 2006. Published 18 April 2007.

Color and Shoot Regrowth of Turf-type Crested Wheatgrass Managed Under Deficit Irrigation

Bradley S. Bushman, Blair L. Waldron, Joseph G. Robins, and Kevin B. Jensen, USDA-ARS Forage and Range Research Laboratory, Logan, UT 84322-6300

Corresponding author: Bradley S. Bushman. sbushman@cc.usu.edu

Bushman, B. S., Waldron, B. L., Robins, J. G., and Jensen, K. B. 2007. Color and shoot regrowth of turf-type crested wheatgrass managed under deficit irrigation. Online. Applied Turfgrass Science doi:10.1094/ATS-2007-0418-01-RS.

Abstract

An increased demand on limited water supplies has led to a growing interest in turfgrasses tolerant of lower irrigation regimes. One potential source of drought-hardy turfgrass is the perennial Triticeae wheatgrasses. These grasses have thrived in semi-arid regions, mainly through dormancy in the dry periods. However, dormancy is undesirable in residential and commercial settings where irrigation can be applied regularly. In this study we used a line-source gradient of irrigation on seven species of turf-type grasses, with a primary focus on crested wheatgrass. Our objective was to determine what level of irrigation will prevent dormancy in crested wheatgrass, and compare the color and shoot regrowth after mowing between it and the other turfgrass species at different irrigation levels. The crested wheatgrasses did not enter dormancy under the moderate irrigation level of approximately 60% evapotranspiration replacement. Under all irrigation levels, tall fescue and Kentucky bluegrass had the highest color ratings, due to genetically darker color. Shoot regrowth differences between the species were minimal, and also indicated dormancy only under the lowest irrigation levels.

Introduction

An increased demand on limited water supplies in semi-arid climates has led to an increased interest in turfgrasses tolerant of lower irrigation regimes. Under heat and drought stress grasses can exhibit leaf water potential decline (34), stomata closure (22), an increase in canopy temperature (24), and a production of reactive oxygen species (32). Plant abscisic acid levels increase and induce stomata closure, reduce growth, and elicit cellular responses to maintain turgor (17,22,33). The visual results of these processes are leaf rolling, lower tiller counts, slower shoot growth, leaf firing, browning, and senescence. These visual parameters have accurately represented the cumulative effects of drought damage (6,11,16,32,34,35), and are useful in selecting for tolerance of drought conditions (7,28).

One drought tolerance and avoidance mechanism is the development of dormancy, which maintains auxiliary and terminal buds while leaves brown and senesce (7). Many perennial grasses originating from semi-arid environments have evolved using dormancy as a drought avoidance mechanism, yet typically resume active growth during the fall when water is available and temperatures are cooler (20). This dormancy characteristic of wheatgrasses, however, is undesirable in residential or commercial turf settings because the browning and leaf senescence results in lower aesthetic turf quality. Diesburg et al. (8) evaluated the Triticeae grasses for low-input turf potential based upon persistence and uniformity, but did not consider color, texture, and density as important traits for such turf purposes. However, their study suggested that Triticeae grasses used in conventional turf settings, even under low levels of management, would only be acceptable if they displayed the "fresh green color" typical of actively growing grass stands (8).

Robins et al. (25) found that crested wheatgrass (Agropyron cristatum L. Gaertn.) turf color and shoot regrowth declined during the hot summer months when irrigated at either 7- or 14-day intervals. Hanks et al. (13) reported that crested wheatgrass turf color declined in early and midsummer, but recovered in later summer when irrigated at 50% evapotranspiration (ET) replacement. These authors suggested that the decline in color was mostly due to dormancy, which critically limited the turf potential of crested wheatgrass. The purpose of this study, therefore, was to assess the amount of irrigation required to prevent dormancy, as determined by color and shoot regrowth, in crested wheatgrass.

Experimental Design

Eighteen grass cultivars and breeding lines were planted under a line-source irrigation gradient at Evans Research Farm, Millville, UT (41°45’N, 111°08’W, 1350 m above sea level). Soil series was Nibley silty clay loam (fine, mixed, mesic Aquic Argiustolls). The majority of the entries were perennial Triticeae grasses, including nine crested wheatgrasses, three RS-hybrids, one thickspike wheatgrass, and one intermediate wheatgrass (Table 1). The RS-hybrids were derived from quackgrass (Elytrigia repens [L.] Gould) × bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love) crosses (18). Standards included Kentucky bluegrass, tall fescue, and hard and sheep fescue cultivars (hard and sheep fescue cultivars were grouped together as fine fescue for analyses). The experiment was planted fall of 1994. During the 1995 establishment year, the plots were irrigated uniformly to field capacity (0.34 mm water per mm soil with available water capacity of 0.18 mm water per mm soil) on a weekly basis, as verified by gravimetric soil moisture determinations. Fertilizer was applied in early June and late September at 56 kg of N per ha. The line-source gradient was applied from 1996 to 1999, and data were recorded for the 1996 and 1999 seasons.

Table 1. Cultivars and breeding lines evaluated for color and shoot regrowth under a line-source irrigation gradient.

The experiment was arranged using the split-plot modification of a randomized complete block design. Whole plots (entries) were 1 m wide × 14 m long and were planted perpendicular and on both sides of the line-source irrigation pipe using a drop spreader. Each whole plot was replicated four times. Alleyways (0.5 m wide) were mowed parallel to the line source at 2-m intervals leaving seven 1 × 1.5-m sub-plots.

Irrigation and Maintenance

Plots were irrigated weekly during the growing season, with the line-source gradient resulting in subplots closer to the sprinkler receiving higher amounts of irrigation (12). The amount of water applied to the sub-plots was captured in rain gauges with an oil overlay to prevent evaporation, and is presented as cm per week applied (Table 2). Reference evapotranspiration (ET_o) values were obtained from the Utah Climate Center, using the Hargreaves equation (15), at a weather monitoring station approximately 3.2 km from the Evans research farm. The approximate ET_o values were 4.3, 4.75, 4.14, 2.52, and 1.52 cm/week for June, July, August, September, and October 1996, respectively. In 1999, ET_o values were 3.77, 4.51, 4.0, and 2.73 cm/week for June, July, August, and September, respectively. Data was not available for October 1999. Plots received on average 35% more water in 1996 than in 1999 (Table 2), but regression slopes of the two years were homogeneous at P = 0.08 (29). Irrigation level one represented approximately 99% ET replacement during June through September of 1996, and 85% ET replacement June through September of 1999. Whereas, irrigation level seven, which was rainfall only, was approximately 17% and 11% ET replacement during June through September for 1996 and 1999, respectively (Table 2). Fertilizer was applied in early June and late September at 56 kg of N per ha each year and mowing was done to a height of 7.6 cm (3 inches) at an interval that removed approximately 33% of growth.

Table 2. The amount of water and percent ET_o replacement at seven
irrigation levels.

Data Collection and Statistical Analysis

Color ratings and regrowth measurements were taken prior to each mowing. No data were recorded for June of 1996 because of high rainfall in May, such that June data represents only the 1999 season. Color was scored visually on a scale from 1 to 9 with 9 being a dark green and 1 being completely brown (28). The first color rating of each season from subplots under the highest irrigation level were considered representative of genetic color due to the presence of adequate moisture and fertilizer. Regrowth measurements were recorded as total plant height in cm.

Average turf color and regrowth were analyzed across and within years with the MIXED procedure (23). Species, irrigation level, and year were considered fixed, while replicate, species × replicate, and species × irrigation level × replicate were included as random variables. Spatial correlations between irrigation levels were accounted for by analyzing irrigation level with the repeated function of MIXED (SAS Institute Inc., Cary, NC), after determining the appropriate covariance structure (compound symmetry) based upon goodness of fit tests. Data were also analyzed within irrigation level. Mean separations were based on Fisher’s protected LSD at the 0.05 level of probability, and Pearson correlations were computed with the CORR procedure (SAS Institute Inc.).

Effects of Irrigation Level on Color

All interactions with year were highly significant (Table 3); therefore, appropriate further analyses were conducted within each year. Differences between years were mainly due to magnitude, especially at the higher irrigation levels, where color was on average 30% higher at irrigation level one in 1996 as compared to 1999 (Table 4). Within each year, species × irrigation level was also significant for color; however, species rank rarely changed in response to the measured irrigation levels (Table 4). Skogley and Sawyer (28) previously proposed a score of "6" as an acceptable threshold of color. In this study we arbitrarily set a color score in a range of five to six as a minimum for aesthetic quality, and as an indication of active growth versus dormancy. In 1996 crested wheatgrass color declined significantly with each successive lower irrigation level (from 8.2 to 2.0), and exhibited a score of "4" indicating unacceptable dormancy at irrigation level five. Color was minimally maintained at irrigation level four or approximate ET replacement of 60% (Tables 2 and 4). The range of color scores over all irrigation levels was smaller in 1999, but again color in crested wheatgrass was minimally maintained at approximate ET replacement of 60% (irrigation level three) (Tables 2 and 4). This is the first study to report that dormancy in crested wheatgrass may be minimally circumvented by irrigating at ET replacements rates of greater than 60%. Our findings coincide with Hanks et al. (14) that irrigation of crested wheatgrass turf at ET replacement of 50 to 60% resulted in stand thinning and loss of turf quality due to "strong summer dormancy responses." Our results also agree with Hanks et al. (13) average color score of 4.8 for a crested wheatgrass population irrigated at 50% ET replacement.

Table 3. Species and irrigation level effects on color and shoot regrowth.

** Significant at P < 0.01

Table 4. Color ratings (1 to 9, where 1 is brown and 9 is dark green) of seven turfgrass species across a line-source irrigation gradient.

 ^x Abbreviations: TF = tall fescue; KBG = Kentucky bluegrass; FF = fine fescues; RS = RS-hybrids; CWG = crested wheatgrasses; IWG = intermediate wheatgrass; TSWG = thickspike wheatgrass.

 ^y Irrigation levels (IL) from one to seven, where one received the most and seven received only rainfall. Ranges are included for FF, RS, and CWG, because more than one cultivar or breeding line was tested.

 ^z Ranges of values are included for species in which more than one variety was tested.

The standards, tall fescue and Kentucky bluegrass, exhibited the highest color ratings at all irrigation levels, especially when irrigation was greater than 60% ET replacement (Table 4). This is in part due to darker genetic color, but is also indicative of less proneness to dormancy during deficit irrigation in these species. Ervin and Koski (10) found that acceptable turfgrass quality, in part influenced by color and leaf firing, could be maintained in semiarid regions by irrigating every three days at ET rates of 60 to 80% for Kentucky bluegrass and 50 to 80% for tall fescue.

In general, other species performed as expected. Fine fescue color response was comparable with tall fescue and Kentucky bluegrass (Table 4). The cultivar Covar has a slightly blue color that lowered the overall color score for fine fescue. In comparison to crested wheatgrass, the RS-hybrids exhibited higher color in 1996 and similar color in 1999 (Table 4). The leaf of the RS-hybrid is broad and more closely resembles their quackgrass ancestry, but their drought tolerance more closely resembles the hardier caespitose bluebunch wheatgrass. Under supplemental irrigation, our results were not consistent with the common observation that as a forage grass RS-hybrids maintain green color longer than the crested wheatgrasses. However, this would have been true at the irrigation level representing natural precipitation only. Thickspike and intermediate wheatgrass always displayed lower or equivalent color as compared to crested wheatgrass (Table 4). The low color of thickspike wheatgrass in particular is partially due to a genetic blue appearance, and to shredded leaves as a result of mowing. Intermediate wheatgrass segregates for the genetic blue (glaucous) appearance.

Our experiments were irrigated once per week, allowing for surface drying and promoting deep root growth. Ervin and Koski (10) reported that tall fescue turf quality could be maintained at irrigation rates as low as 50% ET by irrigating every three days. Fry and Butler’s (11) found that tall fescue needed to be irrigated at 50% ET every two days or conversely 75% ET at three to seven day intervals. The application of these findings to crested wheatgrass needs to be characterized, but these reports do suggest that more frequent irrigation at lower ET replacement rates may ameliorate dormancy in crested wheatgrass turf.

Effects of Irrigation Level on Shoot Regrowth

Similar to color, significant year interactions resulted in examining shoot regrowth data on a yearly basis. Interactions with irrigation level were also mostly due to magnitude with very minor rank changes among species (Table 5). Crested wheatgrass is noted for its vigorous spring growth, and Hanks et al (13) reported that this overly abundant spring regrowth is a critical weakness to its potential as a low-maintenance turf. In this study, height at mowing was not overly discriminant of crested wheatgrass dormancy, inasmuch as each successively lower irrigation level resulted in reduced regrowth (Table 5). This was true for most species with the exception of intermediate and thickspike wheatgrass (Table 5). Hanks et al. (13) reported minimal correlations (0.09 to 0.26) between reduced spring regrowth and turf quality of crested wheatgrass. We found a moderate correlation of 0.47 (P < 0.001) between regrowth and color across irrigation levels, indicating that crested wheatgrass regrowth followed a similar downward trend with color as irrigation level decreased. However, crested wheatgrass regrowth at ET replacement of approximately 60% (irrigation level four in 1996 and irrigation level three in 1999) was inconsistent in comparison to Kentucky bluegrass and tall fescue between the two years (Table 5), but was not excessively greater than these standards. Overall, we concluded that regrowth declined with decreasing irrigation, but was not an adequate assay of dormancy in crested wheatgrass.

Table 5. Shoot regrowth (cm) of seven turfgrass species under a line-source irrigation gradient.

 ^x Abbreviations: TF = tall fescue; KBG = Kentucky bluegrass; FF = fine fescues; RS = RS-hybrids; CWG = crested wheatgrasses; IWG = intermediate wheatgrass; TSWG = thickspike wheatgrass.

 ^y Irrigation levels (IL) from one to seven, where one received the most and seven received only rainfall. Ranges are included for FF, RS, and CWG, because more than one cultivar or breeding line was tested.

 ^z Ranges of values are included for species in which more than one variety was tested.

Conclusion and Management Implications

These results are the first to document the possibility of maintaining active green growth in crested wheatgrass using weekly deficit irrigation levels greater than or equal to 60% ET replacement. This and other studies have clearly shown that dormancy and unacceptable browning occurs in crested wheatgrass at irrigation of less than 60% ET replacement. Additional research is needed to examine the effect of frequency and duration of irrigation interval on crested wheatgrass dormancy.

Due to genetic color differences, tall fescue and Kentucky bluegrass had higher color ratings than crested wheatgrass at all irrigation levels. This suggests that developing genetically darker green types of crested wheatgrass will be necessary before its widespread utility in low-maintenance landscapes. However, the range among crested wheatgrass entries in this study suggests that there is potential for improvement in genetic color (Table 4). These results do not preclude long-term survival advantages of crested wheatgrass over traditional turfs when grown under reduced irrigation in the semiarid western United States. Therefore, these wheatgrasses may best be used in situations where turf is desirable but irrigation is limited or absent.

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