ISU Horticulture Publications

1999 Iowa Turfgrass Research Report

Effects of Trinexapac-ethyl on Poa annua Populations

in Green Height Creeping Bentgrass

Barbara R. Bingaman, Melissa C. McDade, and Nick E. Christians

This study was designed to evaluate the growth regulator, Trinexapac-ethyl (Primo), for Poa annua control in creeping bentgrass maintained at green height following a fall conversion program. This study was conducted on a practice green at Veenker Memorial Golf Course in Ames, IA. The turf in this area consisted of creeping bentgrass with an infestation of Poa annua that ranged from 50 to 80% through the 1998 season.

The experimental design was a randomized complete block. Individual plot size was 5 x 5 ft with 3 replications. Treatments were applied to the same plots as in 1997. Primo 1EC was used at 0.3 fl oz/1000 ft² monthly from May through September. Initial application was made on May 14 and subsequent applications were made on June 17, July 14, August 19, and September 23, 1998. Primo 1EC was mixed with 283 ml of water (3 gal/1000 ft²) and was applied at 30 psi with a carbon dioxide backpack sprayer equipped with Teejetâ #8006 flat fan nozzles. All applications were made between 6:30 and 7:00 a.m. Following applications, the plot was watered with the normal watering schedule in the late afternoon.

Visual turf quality data were taken from April 22 through October 27, 1998 (Tables 1 and 2). Visual turf quality was assessed with a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst turf quality. On each of these dates, the plots were checked for phytotoxicity. Phytotoxicity was assessed using a 9 to 1 scale with 9 = no damage, 6 = moderate tip burn (browning) and 4 = severe tip burn.

Poa annua populations were estimated as the percentage of area per plot covered by Poa annua. Percentage cover data were taken on May 13, May 28, June 12, July 14, July 30, August 12, August 26, September 10, September 22, October 1, October 14, and October 27 (Table 3 and 4). Poa annua control is represented by calculating reductions in percentage cover as compared with the untreated control (Tables 5 and 6).

On May 16, 1998, the practice green was core aerified and topdressed. The effects of Primo on the recovery from this procedure were examined on May 28 by estimating the percentage bentgrass recovery. Estimations were based on bentgrass growth into the 'core areas'.

All data were analyzed with the Statistical Analysis System (SAS, Version 6.12) and the Analysis of Variance (ANOVA) procedure. Treatment effects were compared with Fisher's Least Significant Difference (LSD) test.

In general, quality differences between the treated and untreated bentgrass were found throughout the season approximately 14 days following Primo applications (Tables 1 and 2). In October, treated bentgrass consistently was darker green than the untreated bentgrass. No phytotoxicity was found.

Primo treated bentgrass had less Poa annua than the untreated control from May 28 through October 27 (Tables 4 and 5). The differences in percentage cover were significant on all data collection dates except July 30 and August 26. Mean percentage Poa annua cover also was significantly lower in treated bentgrass than untreated. Significant reductions in Poa annua cover ranged from 22.7 to 63.3% as compared with the untreated controls (Tables 5 and 6). The mean reduction in Primo treated bentgrass was 47.6%.

Primo treated bentgrass recovered from core aerification significantly slower that untreated bentgrass (P > F = 0.0339). On May 28, recovery in the Primo treated bentgrass was 65% as compared with 88.3% recovery for untreated bentgrass. By June 12, recovery was 100% for both treated and untreated bentgrass.

Additional percentage cover data will be taken spring 1999 beginning at greenup. Winter damage also will be assessed spring 1999.

Table 1. Visual quality¹ of green height creeping bentgrass treated for the 1997-98 Green Height Poa annua Conversion Study (April 22 - August 12, 1998).

Material Rate
fl oz/
1000 ft²

April
22

May
13

May
28

June
12

June
30

July
14

July
30

Aug
12

1

Untreated control
N/A 6.0 7.0 6.7 6.7 7.0 7.0 7.0 7.0

2

Primo 1EC²
0.3 6.0 7.0 7.0 8.0 8.0 7.0 7.0 7.0

LSD_0.05

-- -- NS 1.4 NS -- -- --

¹Visual quality was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst quality.

²Primo 1EC applications were made on May 14, June 17, July 14, August 19, and September 23.

NS = Means are not significantly different at the 0.05 level.

-- = Means separation tests are not valid for these data.

Table 2. Visual quality¹ of green height creeping bentgrass treated 1998 Green Height Poa annua Conversion Study (August 26 - October 27, 1998).

Material Rate
fl oz/
1000 ft²

Aug
26

Sept
10

Sept
22

Oct
1

Oct
14

Oct
27

Mean
quality

1

Untreated control
N/A 7.0 7.0 7.0 7.0 7.0 6.3 6.8

2

Primo²
0.3 8.0 7.0 7.0 9.0 9.0 9.0 7.6

LSD_0.05

-- -- -- -- -- 1.4 0.5

¹Visual quality was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst quality.