PERSONNEL: Hongfei Jiang and Jack Fry
INTRODUCTION:
Plant growth regulators (PGRs) traditionally have been used to suppress turfgrass vegetative or reproductive growth on sites where high turf quality was not required. Increased use of trinexapac ethyl (Primo) on golf course turf in the southern U.S. has led to questions regarding the feasibility of using PGRs on golf course fairways in Kansas. Some studies have suggested that PGRs may be effective in enhancing turfgrass drought resistance by reducing evapotranspiration or enhancing rooting.
MATERIALS AND METHODS:
This study was conducted on a blend of perennial ryegrass at the Rocky Ford Turfgrass Research Center in Manhattan. Turf was mowed three times weekly at 0.75 inches, recieved 3 lbs. N/1,000 sq ft/yr, and was irrigated to prevent stress prior to PGR application. PGRs and application rates are presented in Table 1. PGRs were applied initially on 2 June, and turf response without irrigation was evaluated until 11 July. For 1 week, turf was watered well, a second PGR application was made on 18 July, and irrigation ceased. PGRs were applied using a CO2-pressurized backpack sprayer equipped with 8004 flat-fan nozzles and calibrated to deliver 60 GPA (1.4 gal/1000 ft2) at 35 psi. Plots measured 2 by 3 meters and were arranged in a randomized complete block with four replications.
Data were collected on turf quality, clipping dry weight, canopy height (3 days after mowing), canopy minus air temperature, relative water content leaf osmotic adjustment, and soil moisture extraction between 0 and 15 inches. Turf quality was rated visually in 13 separate weeks using a 0 to 9 scale, where 0 = dead turf; 7= acceptable quality for a golf course fairway; and 9 = optimum color, density, and uniformity. Clipping dry weight was determined in 12 separate weeks by making one lengthwise pass over each plot with a push-type reel mower and collecting clippings, drying, and weighing. Canopy height was measured at three locations in each plot prior to clipping collection by dropping a small cardboard disc over a ruler standing on the soil surface perpendicular to the turf canopy. Canopy minus air temperature was measured at three locations per plot using a hand-held infrared thermometer in 6 separate weeks. Leaf relative water content and osmotic potential were measured in 4 separate weeks using standard methods. Soil moisture extraction was determined gravimetrically in 4 separate weeks by removing two 1-inch-diam by 15-inch-deep cores from each plot. Data were subjected to analysis of variance, and means were separated using the least significant difference T test with P=0.05.
RESULTS:
Turf Quality: Mean perennial ryegrass quality was acceptable for all PGRs except mefluidide. Mefluidide-treated turf commonly was chlorotic within several days after treatment, and significant turf loss occurred during dry down.
Clipping Weight and Canopy Height: Mean clipping weights and canopy heights of perennial ryegrass treated with mefluidide and trinexapac-ethyl were lower than those of untreated turf.
Canopy minus Air Temperature: A higher canopy minus air temperature generally is observed if the turf is under stress and unable to cool itself effectively by transpiration. Paclobutrazol- and trinexapac-ethyl-treated ryegrass had mean canopy temperatures equal to or lower than that of untreated turf.
PGRs did not significantly affect relative water content and osmotic adjustment of grass leaves or soil water extraction.
SUMMARY:
Of the PGRs evaluated, only trinexapac-ethyl significantly suppressed perennial ryegrass shoot growth, resulted in mean acceptable turf quality, and did not increase canopy minus air temperature over that of untreated turf. Greenhouse and field studies will continue in 1996.
Table 1. Effect of PGRs on perennial ryegrass quality, growth, and canopy temperature.
| PGR | Application Rate (lbs. a.i/A) | Turf Quality | Clipping Weight (g/m2) | Canopy Height (cm) | Canopy Minus Air Temp. (C) |
| Untreated | ---------- | 7.7 ab* | 0.52 a | 3.2 ab | 3.1 c |
| Ethephon | 3.0 | 7.6 bc | 0.49 a | 3.2 a | 3.6 b |
| Mefluidide | 0.12 | 4.9 d | 0.28 b | 2.9 d | 5.5 a |
| Paclo butrazol | 0.50 | 7.8 a | 0.48 a | 3.1 bc | 2.7 d |
| Trinexapac ethyl | 0.17 | 7.5 c | 0.33 b | 3.0 c | 2.9 cd |