Reducing couch grass irrigation with a PGR and soil surfactants

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Despite the numerous benefits provided by turfgrass, the amount of water required to maintain the turf is a major concern in many communities.

For this reason, multipronged approaches to conserve irrigation water — such as applying deficit irrigation and using warm-season grasses, alternative water sources and efficient irrigation systems — have been investigated and recommended in recent years (3).

Approaches to conserving irrigation water
Deficit irrigation — defined as the application of water below full crop water requirements (evapotranspiration, or ET) — has been widely adopted as a strategy for conserving irrigation water (4). However, such an approach exposes turfgrasses to chronic stress, and recovery usually occurs only after periods of sufficient rainfall. Bermudagrass [Cynodon dactylon (L.) Pers.] is one of the most widely used warm-season turfgrasses in arid and semiarid climate zones because it requires significantly less water than cool-season grasses do, tolerates traffic, and can be maintained at all mowing heights needed for turf areas (1).

Chemical aids
Soil surfactants and plant growth regulators are two products that have been used to maintain turf quality under reduced irrigation or drought conditions. Soil surfactants, also called wetting agents, are chemicals that decrease the interfacial tension between hydrophobic and hydrophilic surfaces of soil particles (2). Soil surfactants are commonly used to prevent and treat localized dry spots and areas of water repellency in the soil. Although the effects of surfactants on water repellency and localized dry spots have been widely studied, their use in large-scale efforts to help conserve irrigation water has been investigated only recently (4). Soil surfactants increase water uniformity and water availability in the root zone, resulting in better turfgrass quality under drought conditions.

Plant growth regulators are another class of compounds that have been shown to reduce water use in plants. Since its introduction in 1990, the use of the plant growth regulator trinexapac-ethyl (Primo Maxx, Syngenta) has become standard practice, particularly on golf courses, to maintain high-quality turfgrass. Primo Maxx is used primarily to reduce plant growth and thereby reduce mowing frequency, as well as to increase turfgrass density, color and resistance to stress. More recently, researchers have reported that bermudagrass maintained under drought conditions and treated with trinexapac-ethyl, had higher turfgrass quality than untreated controls (5).

Soil surfactants and plant growth regulators have both shown the potential to lower irrigation requirements and increase turfgrass quality under drought conditions. We hypothesized that combining a surfactant with a plant growth regulator would more effectively increase turfgrass quality and lower water requirements than using each chemical on its own.

Materials and methods

A three-year study was conducted from 2014 to 2016 on mature Princess 77 bermudagrass (Cynodon dactylon L.) maintained at a fairway height of 0.5 inch (1.27 centimeters). The research area was located adjacent to New Mexico State University’s golf course in Las Cruces, N.M. (arid, elevation of 4,150 feet [1,265 meters]). Irrigation water was provided from an overhead sprinkler system.

The treatments were:

  • Non-treated control
  • The surfactant Revolution (Aquatrols) at 6 ounces/1,000 square feet (20 liters/hectare)
  • The plant growth regulator Primo Maxx (a.i. trinexapac-ethyl) at 0.25 ounce/1,000 square feet (1.6 liters/hectare)
  • Revolution + Primo Maxx (tank-mixed)

Revolution and Primo Maxx were applied at 28-day intervals from May until October, and irrigation was withheld from the plots for two hours following application to allow Primo Maxx to enter the plant system. Irrigation levels were 50% ETos (severe drought), 65% ETos (moderate drought) and 80% ETos (unstressed control treatment).

Results and discussion

Cover
Plots receiving a combination of the surfactant and trinexapac-ethyl exhibited greater green cover at 50% ETos in 2014, at 65% ETos in 2015, and at 50% and 65% ETos in 2016 when compared with the untreated controls (Figure 1).

Percent green cover was higher in treated plots (chemicals applied separately or combined) irrigated at 65% or 50% ETos than in the control plots. However, percent green cover never dropped below 70% for any of the treatments (including the control). The differences in green cover among treatments were relatively small, ranging from 4% to 15%. Although these differences are statistically significant, they may be less important to the turf manager than turf quality differences among treatments.

Dark green color index
With the exception of plots irrigated at 50% ETos in 2015, bermudagrass receiving trinexapac-ethyl, either in combination with Revolution or alone, exhibited darker green color when compared with untreated controls at all irrigation levels throughout the research period (Figure 2). Plots treated with Revolution alone did not differ in color from control plots on seven of nine sampling dates (Figure 2).

Even though chemical treatments were made until October, dark green color index declined from September to October and November for all treatments. Such a loss in green color can be attributed to the beginning of winter dormancy.

Turfgrass quality
Plots receiving trinexapac-ethyl (alone or in combination with Revolution) exhibited better visual turf quality than the untreated controls at 50% ETos in 2014 and 2016, and at 65% and 85% ETos in 2014, 2015 and 2016 (Figure 3). The application of Revolution alone increased turfgrass quality at 50% ETos in 2014, at 50% and 80% ETos in 2015, and at all three irrigation levels in 2016. During the three years of the study, untreated control plots only showed an acceptable visual quality rating of 6 when irrigated at 80% ETos (Figure 4). With the exception of plots treated with trinexapac-ethyl and trinexapac-ethyl + Revolution and irrigated at 50% ETos in 2015, all plots receiving chemical applications were given acceptable ratings of 6 or higher during the three years for all irrigation treatments (Figure 3).

Bermudagrass drought
Overview of the block area irrigated at 50% ETos during summer, showing some of the treatments that exhibit signs of drought but are otherwise relatively healthy.

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