RP-EXP 31130A (isoxaflutole) was applied in 1996 to perennial ryegrass turf infested with "Midiron" bermudagrass, as the second season of a two year study to determine the control or suppression capability of this compound on bermudagrass in perennial ryegrass turf. Mild suppression was achieved at the rate of 0.18 lb. AI/A, while moderate control was achieved at the 0.36 lb. AI.A rate. Double applications at the low rate did not favor suppression for very long, over the single application at the same rate (0.18 lb. AI/A). Reversal of rates at the second application did not enhance overall suppression. The 0.36/0.36 lb.AI/A treatment provided the highest level suppression, noting that the main effect of 'chemical treatment' was not significant for suppression during year two of this test. Leaf bleaching and the percent of the bermudagrass foliage affected was rate dependent. All bermudagrass in the test increased considerably among treated plots (since spring 1995 when all bermudagrass was 80 square cm of plot surface area), to astronomically for the checks by the close of the test in year two (1996). Multiple applications appear warranted at the 0.36 lb. AI/A rate every 21 days to promote a high level of suppression of bermudagrass in perennial ryegrass turf under desert conditions.

Bermudagrass (Cynodon spp.) as a cosmopolitan ground cover in areas where it is adapted, is either loved as a turf or revered as a pest. It is more of a pest as a competitor in cool season turfgrasses. Control of bermudagrass in cool season turf is potentially achieved through the diligent processes of either (1) constant cultivation and reseeding of the cool season turf or, (2) non-selective treatments directed to the bermudagrass, usually followed by re-seeding of the cool season turf again, or (3) the use of herbicides or plant growth regulators to suppress the bermudagrass, providing a competitive edge for the cool season grass. This report addresses the use of RP EXP31130A (isoxaflutol) in controlling/suppressing bermudagrass in a perennial ryegrass year round turf under desert conditions. The protocol is exactly as followed during the first year of the study, conducted during the summer of 1995.

The base turfgrass (perennial ryegrass) was maintained at a mowing height of 2.75-3.0 inches, and mowed 3x week during the winter, spring and summer. From October 1995 to October 1996, a total of 3, 2 and 2 lbs./M of nitrogen, phosphorous and potassium fertilizer was applied from five applications. The test was irrigated at 85-95% Ref Eto from the on-site weather station. This level would avoid soil moisture stress to the ryegrass.

The following treatments (identical to 1995) were applied to (5'x4' plots) each which contained two sub-samples of 80 cm² plugs of 'Midiron' (sold locally as E-Z Turf - established early spring 1995). All rates are in lbs. AI/A.

RP EXP31130A 0.18 lb.

RP EXP31130A 0.18 lb./0.18 lb. (split)

RP EXP31130A 0.18 lb./0.36 lb. (split)

RP EXP31130A 0.36 lb.

RP EXP31130A 0.36 lb./0.18 lb. (split)

RP EXP31130A 0.36 lb./0.36 lb. (split)

Prograss (ethofumesate) 1.3 lb.

Untreated Control

Treatments were applied using a three nozzle boom with a backpack Co₂ sprayer, using 8004 nozzles @ 28 psi, delivering a final solution volume of 96 gallons per acre. No surfactant was used. Plots received treatments early during the mornings of July 9, and again (split application only) on August 20, 1996. Plot were irrigated 10-12 hours later with the normal irrigation cycle.

Bermudagrass status (growth) was assessed by measuring the average radius of each of the two bermudagrass plugs per plot in centimeters. The radius values were then used to determine the average occupational area of the bermudagrass among the perennial ryegrass plot. Average areas (both sub-samples/plot) were divided by the mean area occupied of all four check plots on each respective evaluation date, and expressed as percent control/suppression..

Growth measurements were determined on the bermudagrass on July 10, August 8, and September 20, 1996.

Visual response of bermudagrass was determined by assigning each bermudagrass sub-sample with values representing the degree (intensity) of leaf whitening (chlorophyll loss), and/or the percent of the bermudagrass foliage (0-100%) which was bleached white. In addition to the control/suppression values, it is imperative that the actual percent change in the actual area maintained by living bermudagrass be considered. This is noted for all treatments in the appropriate tables.

The test was maintained as a randomized complete block with four replication, and was analyzed using the ANOVA technique via SAS software on a mainframe computer. Treatment mean separation statistics were provided using Tukey's honest least significant values, which calculates an LSD value only if the significance level of the actual P value for the treatment effect (applied herbicides) is 0.05 or less. Two applications of FORE fungicide were applied on July 25 and August 30 to suppress foliar symptoms of brown patch.

Bermudagrass Response to RP EXP31130A
Bermudagrass plants were evaluated for the degree of leaf bleaching and the percent of the foliage which was bleached, on July 15 (6DAT/1) and August 9 (31DAT/1) after the first treatment. The second treatment was applied on August 20, 1996 and the same variables were measured on August 28 (8DAT/2), along with overall turfgrass color of the perennial ryegrass.

July 15 (6DAT/1)
The degree of expression of leaf whitening (leaf bleaching) and the percent of the foliage bleached was most noticeable at 5 DAT on July 15th. Both response variables showed significant effects due to application treatments. RP EXP31130A treatments at the three 0.36 lb. initial applications had moderate bleaching (mean scores of 3.8 to 4.6). The 0.18 lb. treatments scored mean bleaching values of 2.6 to 3.8, which showed rate dependency in general (Table 1). The Prograss turf had no bleaching, while the control showed one or two leaves bleached (volatility or drift?). The percent bleached foliage ranged from 0 to 20%, with the higher rates of EXP causing more foliar bleaching. The bleaching affected roughly 15-20% of the foliage at the 0.36 lb. rate, and 5-12% at the 0.18 lb. rate. It appeared that the bleaching was more severe and intense overall during the previous year (Table 1).

August 9 (31DAT/1)
On August 9, the treatment effect was still significant for these response variables. The percent of the foliage that was bleached remained visible on two of the three treatments which received the initial 0.36 lb. rate, but little bleaching was still evident from those treatments which received the 0.18 lb. (low) rates. The degree of leaf bleaching decreased by this time (approximately one month after application ) with the 0.36 lb. rate treatments showing only slight discoloration remaining (treatment means ranging from 1.6 to 2.3) (Table 1). The bermudagrass was essentially recovered, so the second application was made on August 20, 1996.

August 28 (50DAT/1:8DAT/2)
At 7 DAT, the degree of leaf bleaching and percent bermudagrass foliage showing any discoloration at all was most evident. EXP which now received the second split application at the high rate (0.36/0.36) had the most severely bleached turf with a mean score of 5.0 (moderate-severe). The remaining treatments produced rate dependent responses for the degree of bleaching. The (0.18/0.36), (0.36/0.18) and (0.18/0.18) treatments produced mean bleaching degree scores of 4.1, 3.9 and 3.3, respectively (Table 1). The single application only of EXP (at either rate) showed minor discoloration at this time (1.3-1.5 mean scores only). The Prograss (ethofumesate) produced no bleaching symptoms. These symptoms slowly disappeared as the bermudagrass foliage returned to "green" (Table 1).

Bermudagrass Control/Suppression
Bermudagrass growth was determined on three dates (July 15, August 9, and September 20, 1996) as a percentage of the untreated check (control) mean.

Given the undisputed fact that actual bermudagrass areas increased in all plots (treatments and checks), and that the plant is not eliminated, the growth activity on bermudagrass would (should) be viewed as partial suppression.

Percent suppression (treatment effect) was not significant for applications on these three dates in 1996. Actual F values for the main effect of "treatment" were 0.53, 0.99 and 1.08 for these three evaluations . Note that since 1995, the actual ground area of the bermudagrass had greatly increased with time (Table 2).

Early season suppression (July 15) ranged from essentially 0 to 29%. These values are essentially indicative of bermudagrass status which was "slowed in growth" from previous treatments (1995). The (0.36/0.18) and (0.36/0.36) EXP treated turf showed essentially 27-29% suppression, with cumulative rates and split applications exhibiting rate dependent results. The single applications of 0.18 and 0.36 lbs. showed minimal activity up to the new application schedule for 1996 (year 2). Prograss had 10% suppression. Again, the results were not statistically significant and only trends are discussed as noted from these results.

By August 9, suppression increased among treatments from 17% to 59%, being approximately one month after the first application of the second year. The (0.36/0.36) treatment (which now received the third application at this rate over two years) had 59% suppression, with the second 0.36 application upcoming in eleven days. The Prograss (1.3 lb.) treated turf had 39% suppression, followed by the 0.36 initial application (two applications, one per year at this point) with 33% suppression. The other EXP treatments ranged from 17% to 22% suppression. It is interesting to note that cumulative effects seemed to have had accrued for rates (over years) after the first application of year two (Table 2).

The second application was made on August 20, 1996. Treatments which were designed to receive the second split which included (0.36/0.36), (0.18/0.36), (0.36/0.18), and (0.18/0.18) for RP EXP31130A.

The third and final evaluation of bermudagrass growth was made on September 20, 1996. Suppression values for treatment means ranged from 54% to 76%. However, note that the actual areas of bermudagrass did continue to increase in time, but not at the rate as that of the untreated check (control). Suppression was highest for the (0.36/0.36) split rate application of EXP, at 76%, followed by the 0.36 single year application (69%). The remaining treatments which resulted in less than 65% suppression included the (0.18/0.18) split treatments, and the 0.18 single application, and Prograss at 1.3 lbs. (54%). Again, the treatment mean effect was not significant. Note that in about six weeks time, the bermudagrass increased dramatically in some of the plots, while the control plots were almost taken over by bermudagrass, when the ryegrass was at its weakest hour.

1). Maximum expression for foliar discoloration occurred in 5-7 days (similar to 1995 study).

2). The degree of bleaching and percent foliage which is bleached are correlated (similar to 1995 study)(r = 0.70 - 0.80).

3). Both responses are rate dependent (similar to 1995 study).

4). The intensity and longevity of discoloration appeared to be less than that which occurred in 1995 (field observation).

5). In order to maintain a chlorotic (white leaf) bermuda condition, the 0.36 lb. rate should be repeated every 21 days (field observation and data results).

6). At these rates and two applications per season, bermudagrass is mildly suppressed by RP EXP31130A at 0.09 lbs. AI/A, with moderate suppression (less area infested) at the high 0.36 lb. rate.

7). The advancement of bermudagrass increased greatly from July to September in year two of this study. The summer of 1996 was a hot and very humid summer which is more severe on ryegrass than a hot/dry summer. It is unknown if enhanced soil degradation of isoxaflutole may have taken place in the second year of this test (speculation).

8). If economically viable, repeat applications at every twenty-one days (5-6 per season) may be needed to provide high levels of suppression of bermudagrass, using the 0.36 lb. AI/A.

FN:RPEXPBRT.96 DIR:Q496

Table 1. Response of Midiron bermudagrass to repeat applications of RP EXP31130A in Perennial Ryegrass turf. University of Arizona, Summer 1996.

²Percent foliage bleached = percent of bermudagrass showing bleaching (0-100%).

³Date of evaluation. Number of days after treatment (DAT)/application number. 1 = applied July 9, 1996, 2 = applied August 20, 1996,

⁴Treatment = foliar application of RP EXP31130A and ethofumesate (Prograss).

⁵Rate = lbs. AI/A.

⁶Mean of all plots for each evaluation date.

⁷LSD value = Treatment mean separation statistic. Numerical difference between treatments must be larger than the corresponding LSD value for true treatment differences to occur.

Table 2. Percent suppression of Midiron bermudagrass and cumulative change in occupational area to application of RP EXP31130A in perennial ryegrass turf. University of Arizona, Summer 1996.

²Cumulative change = multiplication increase of bermudagrass from original 80 cm² plant area before treatments in spring 1995. Values are means of four replications.

³Treatment - foliar applications of RP EXP31130A and ethofumesate (Prograss). 1996 applications 1 = July 9, 1996, 2 = August 20, 1996 (split).

⁴Rate = lbs. AI/A

⁵Mean of all plots for each evaluation date.

⁶LSD value = Treatment mean separation statistic. Numerical difference between treatments must be larger than the corresponding LSD value for true treatment differences to occur.