Reducing bermudagrass irrigation with a PGR and soil surfactants

Can applying a combination of trinexapac-ethyl and a soil surfactant produce better turf quality under drought conditions than applying the products individually?

|

Bermudagrass drought research
The area for this study was Princess 77 bermudagrass maintained at fairway height and located adjacent to New Mexico State University’s golf course in Las Cruces, N.M. Photos by Matteo Serena


Editor’s note: This research was funded in part by the United States Golf Association.

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.


At 50% ETos, plots treated with any of the three chemical treatments had greater quality than control plots from July to September, with quality ratings of 6 or greater from June to August (Figure 4). The combination of Revolution and trinexapac-ethyl resulted in mean quality ratings of 7.2 in July, 6.7 in August and 6.2 in September. Average ratings of plots treated with trinexapac-ethyl alone and irrigated at 50% ETos were 7.4 in July and 6.0 in August (Figure 4).

Plots receiving trinexapac-ethyl or the combination of Revolution and trinexapac-ethyl and irrigated at either 65% or 80% ETos exhibited turfgrass quality ratings of 6 or greater throughout the research period (Figure 4). Bermudagrass treated with Revolution alone exhibited greater quality when compared with controls in June and July at 65% ETos and from June to November at 80% ETos (Figure 4).

Soil volumetric water content and volumetric water content uniformity
The application of Revolution alone increased soil moisture levels in plots irrigated at 50% ETos in 2015 (Table 1). Plots irrigated at 65% ETos and treated with both Revolution and trinexapac-ethyl had higher volumetric water content than control plots did during all three experimental years (2014, 2015 and 2016). The combination of Revolution and trinexapac-ethyl also increased soil moisture levels at 80% ETos in 2014 and 2015. Trinexapac-ethyl alone influenced volumetric water content only in 2014 at 80% ETos. When data were averaged over all ETos levels, sampling months and years, volumetric water content was greater in plots treated with trinexapac-ethyl (26.2%), Revolution (26.3%), and trinexapac-ethyl + Revolution (27.4%) than in the control plots (21.2%).

With the exception of plots irrigated at 80% ETos in 2015, bermudagrass plots receiving Revolution in combination with trinexapac-ethyl exhibited greater soil moisture uniformity compared with untreated controls at all irrigation levels throughout the research period. Standard deviation (used to calculate moisture uniformity) values on plots treated with Revolution + trinexapac-ethyl ranged from as low as 2.2 for 65% ETos in 2014 to as high as 4.6 for 65% ETos in 2015. Soil moisture uniformity was lower on the control plots and ranged from 3.4 for 65% ETos in 2014 to 6.8 for 65% ETos in 2015. Plots receiving Revolution alone had a more uniform volumetric water content compared with the untreated controls throughout the research period for all irrigation levels except for 65% ETos in 2014 and for 50% and 80% ETos in 2015. Similarly, the application of trinexapac-ethyl alone resulted in a more uniform moisture distribution throughout the investigative period, with the exception of plots irrigated at 80% ETos in 2015 and at 50% ETos in 2016.

Conclusions

Our results indicate that the use of a plant growth regulator or a surfactant or the combination of both is an effective drought management strategy that helps conserve water without a significant loss of quality of the turf stand.

Applying a combination of surfactant and trinexapac-ethyl allowed us to irrigate bermudagrass at 50% ETos and still maintain acceptable quality and dark green color. Our results indicated that applying a tank mix of trinexapac-ethyl and Revolution and, to a lesser extent, applying the two products separately led to acceptable turfgrass quality (6 or better on a scale of 1-9, where 1 is dead grass and 9 is turf of the highest quality) for the majority of the growing months throughout the three years of the study.

Our studies did not include physiological measurements, but soil moisture measurements indicated that loss in soil water content was reduced only when trinexapac-ethyl was applied in combination with Revolution. The effects of trinexapac-ethyl alone on soil moisture and water use have been extensively studied for cool-season grasses, with conflicting results. Our VWC results do not indicate a difference in soil water loss or water use between the untreated bermudagrass and the bermudagrass treated with trinexapac-ethyl.

Applying only Revolution did not lead to consistent or significant increases in volumetric water content at the two lower irrigation levels. However, when Revolution was applied in combination with trinexapac-ethyl, volumetric water content increased in plots irrigated with as little as 50% ETos in 2015 and in plots irrigated at 65% ETos throughout the three-year research period. Although volumetric water content was not consistently enhanced by all treatment combinations, applications of Revolution, trinexapac-ethyl and a combination of both resulted in increased volumetric water content uniformity and apparently greater irrigation use efficiency.

Our study documented that bermudagrass quality can be maintained with 15% to 30% less irrigation water than the optimal rate (80% ETos) by using a surfactant, a plant growth regulator or both. Overall, during the hottest period of the year (June-September), the application of Revolution + trinexapac-ethyl resulted in turf quality and color that was equal or superior to that obtained when each chemical was applied separately. When compared with the control, plots treated with Revolution + trinexapac-ethyl rated higher in quality on 13 of the 18 sampling dates and rated higher in color on 14 of the 18 dates. When products were applied alone, Revolution resulted in greater turf quality 11 times, and trinexapac-ethyl produced better turf quality 13 times. Plots that received both Revolution and trinexapac-ethyl ranked higher in turfgrass quality than all other treatments on two occasions at 65% ETos and on one occasion at 50% ETos.

The combination of Revolution and trinexapac-ethyl was more effective at increasing volumetric water content than each chemical applied separately, but all treatment combinations had similar effects on soil moisture uniformity. It appears that the combined effect of Revolution, which improves soil moisture conditions, and trinexapac-ethyl, which enhances stress tolerance through improved photosynthetic activity, provides optimal results in bermudagrass under deficit irrigation. The question is whether the cost of applying both products will prevent their use. Although rigorous water restrictions are in effect in many communities, the available budget may preclude the use of such products.

Funding

Financial support of this study was provided by Aquatrols Corp., New Mexico State University’s Agricultural Experiment Station, the Southwest Turfgrass Association, Syngenta AG and the United States Golf Association. We are also grateful for donations from Helena Chemical Co., and we appreciate the help of Karl Olson and John Kimmel, golf course superintendents at New Mexico State University’s golf course.

Acknowledgments

The information in this article was originally published as “Combining trinexapac-ethyl with a soil surfactant reduces bermudagrass irrigation requirements” by M. Serena, M. Sportelli, E. Sevostianova, R. Sallenave and B. Leinauer in Agronomy Journal 110:2180-2188 (2018).


The research says ...

  • Using trinexapac-ethyl and Revolution alone or in combination allowed turf quality to be maintained under reduced irrigation.
  • Applying the plant growth regulator and the soil surfactant in a tank mix produced better turf quality than applying the products individually, but individual application also produced acceptable turf quality.
  • Applying Revolution alone did not increase volumetric water content, but the combination of the surfactant and the plant growth regulator increased volumetric water content in plots irrigated at 50% ETos in 2015 and plots irrigated at 65% ETos throughout the study.

Literature cited

  1. Hanna, W., P. Raymer and B. Schwartz. 2013. Warm-season grasses: Biology and breeding. In: J.C. Stier, P.B. Horgan and S.A. Bonos, eds. Turfgrass: Biology, use, and management. Madison, Wis.
  2. Kostka, S.J. 2000. Amelioration of water repellency in highly managed soils and the enhancement of turfgrass performance through the systematic application of surfactants. Journal of Hydrology 231-232:359-368. doi:10.1016/S0022-1694(00)00208-0
  3. Leinauer B., and D.A. Devitt. 2013. Irrigation science and technology. Pages 1075-1131. In: Stier, J.C., P.B. Horgan and S.A. Bonos, eds. Turfgrass: Biology, use, and management. Madison, Wis.
  4. Leinauer, B., E. Sevostianova, M. Serena, M. Schiavon and S. Macolino. 2010. Conservation of irrigation water for urban lawn areas. Acta Horticulturae 881:487-492.
  5. Schiavon, M., B. Leinauer, M. Serena, B. Maier and R. Sallenave. 2014. Plant growth regulator and soil surfactants’ effects on saline and deficit irrigated warm-season grasses: I. Turf quality and soil moisture. Crop Science 54:2815-2826. doi:10.2135/cropsci2013.10.0707

Matteo Serena and Elena Sevostianova are research assistant professors in the Department of Plant and Environmental Sciences at New Mexico State University, Las Cruces, N.M. Mino Sportelli is a doctoral student at Universita degli Studi di Pisa Dipartimento di Scienze Agrarie, Alimentari e Agro-ambientali, Dipartimento di Scienze Agrarie, Pisa, Toscana, Italy. Rossana Sallenave is a professor in the Department of Extension Animal Sciences and Natural Resources, and Bernd Leinauer is a professor in the Extension Plant Sciences Department at New Mexico State University, Las Cruces, N.M.

You may also like: Soil surfactants: Vertical water distribution and surfactant longevity