Turfgrass Tolerance of Buffalograss to Rhone Poulenc
EXP31130A Herbicide

D.M. Kopec, J.J. Gilbert and K.B. Marcum
University of Arizona

Abstract

RP EXP31130A herbicide was applied to buffalograss on two dates at various combinations of 0.18 and 0.36 lb. AI/A rates. Buffalograss turf showed bleaching of the leaves in 3-5 days, which was maximized at about ten days later after application. The bleaching was rate dependent. Irrigated buffalograss withstood a single application in a two month period at the 0.18 lb. AI/A rate. The high rate of 0.36 lb. AI/A or combinations of any of those rates in sequential applications decreased turfgrass performance significantly.

Introduction

The experimental compound, RP-EXP31130A was tested for turfgrass tolerance under irrigated conditions in the Southwest. Rates and timing applications were evaluated for this new herbicide to assess the potential effects on buffalograss grass. Turf tolerance is an important function as well as efficacy, and tolerance influences the type of turf a herbicide(s) can be safely used on, or if effects are realized, to what extent do they occur and for what length of time. This information is necessary for developing use rates, timing, and product applications in the turfgrass industry.

Materials and Methods

The material applied to the turf was the 75 WG formulation of RP-EXP31130A. The turf was irrigated with secondary effluent to prevent moisture stress. The turf was mowed two times weekly at three inches. Based on the above objectives, the RP-EXP31130A herbicide treatments were as applied at the following AI/A rates.

INITIAL APPLICATION SEQUENTIAL APPLICATION

0.18 lb + 0. lb

0.18 lb + 0.18 lb

0.18 lb + 0.36 lb

0.36 lb + 0. lb

0.36 lb + 0.18 lb

0.36 lb + 0.36 lb

All tests included a control. Each treatment appeared four times in a completely randomized block design. Initial and sequential applications were made on August 1 and September 4, 1995, respectively. Materials were applied using a Co₂ compressed air sprayer, using 8004 size nozzles on a three nozzle boom hand held sprayer (20 inch spacing). A total volume of 51 gallons per acre was applied between 500-600 a.m. and watered in within 3 hours after applications via standard turf overhead sprinklers.

The turf was observed for any signs of discoloration, turfgrass damage, or notable changes in density, etc. For this turfgrass species, response observations of turfgrass color, percent plot bleaching (loss of chlorophyll), degree of bleaching, quality and visual density were noted throughout the course of development. Observations were noted as days after treatment for each application (response/DAT/N), where N = (1) initial or (2) sequential application. Observation dates were based on expression development, which may have varied in type and number based on the particular turf in question.

Results and Discussion

Buffalograss did exhibit visual phenotype responses to RP EXP31130A from both applications. In a short time span (3DAT/1) buffalograss developed a bleaching of the leaves, and overall color change. Color scores ranged from 4.8 to 7.0, with a significant linear rate effect, as the 0.36 lb. treated turf had the lightest color turf (Table 1). Buffalograss is usually not inherently rich in color, however, treatments were noticeable. Low rate treatments (0.18 lb.) had a slight decrease in color (5.5-6.0) which was basically acceptable color. High rate treatments (0.36 lb.) were noticeably lighter in color (means 4.8-5.0). The check received a mean color score of 7.0 at 3DAT/1.

Initial bleaching was scored using a progressive subjective scale of 1-6, 1 = none and 6 = sever bleaching. Leaf tips developed symptoms first (on the youngest leaf) and progressed downwards to 50% or 65% of the entire leaf, then repeating the condition on the next youngest leaf. Mean bleaching scores ranged from 1.0 to 1.8, with the initial 0.36 treatments causing slightly more bleaching. The linear effect of rate was significant for bleaching at 3DAT/1 (Table 2).

At 8DAT/1, color effects were more pronounced, and the linear effect was again significant. Buffalograss treated at 0.36 lb. had mean color scores of 2.8-3.8, while 0.18 lb. treated turf had mean scores of 4.5-5.0. The check remained at a 7.0 mean color score (Table 1). At this time, the bleaching symptom was strongly pronounced, and the means for 0.36 lb. treated buffalograss now ranged from 4.5-5.0 (moderate to severe bleaching). The 0.18 lb. treatments ranged from 1.8 to 2.3 (slight to slight/moderate). Values above 3.0 for bleaching using this scale would be noticeable to lay persons (Table 2).

Symptoms remained similar at 11DAT/1 for both color and turf bleaching. Mean color scores were still lower for the 0.36 lb. treatments (3.5-4.0) than for those at the 0.18 lb. rate (5.3-5.5) (Table 1). Leaf bleaching was slightly less, but still most noticeable. Bleaching was acceptable for the 0.18 lb. rate (1.8-2.3), but was moderate for 0.36 lb. turf. Again, the effects were affected by initial dose rates (linear contrast significant) (Table 2).

At 18DAT/1, turfgrass color and the percent plot bleaching (0-100% scale) scores were assigned to plots. Color had improved at this time, as mean scores ranged from 4.8 to 6.5 for treated buffalograss (Table 1). Recovery (return of color) was rate dependent here, as was initial damage. The percent bleaching was between 4 to 13% for treated turf (Table 3). The check received a value score of 3%, which upon close inspection was from tip damage from rotary mower damage. It was anticipated that through leaf expansion, bleached leaves would be removed as phytomere emergence developed as a cultured turf.

At 28DAT/1, color intensity was near normal, while the percent bleaching was roughly the same as at 18DAT/1. Mean color scores for color (Table 1) ranged from 5.5 to 7.3 for treated turf, and bleaching was extremely marginal for the 0.18 lb. treatments and was evident at 0.36 lb. only under close observation (Table 3). Both color and bleaching were rate dependent in response at 28DAT/1, which were the last measurements assigned before the second (sequential) treatments were made on September 9, 1995.

At 5DAT/2, percent bleaching, color, quality and density scores were assigned. Bleaching ranged from 1-8%, which was minimal at this time. Treatments which received the initial 0.36 lb. treatment, followed by either the 0.36 lb. or 0.18 lb. sequential application had the greatest amount of bleaching (Table 3).

Treatments which received no sequential application had essentially no bleaching at this time.

Mean color scores ranged from 3.0 to 4.5 for those plots receiving the sequential application, showing a true decrease in color. Plots which received the initial treatment at the 0.36 lb. rate had a lighter color, compared to those sequential treatments which received 0.18 lb. up-front. The single initial treatments at either rate were now totally acceptable (mean color scores of 5.5-6.0) (Table 4).

Turfgrass density was visually assessed using a scale of 1-9, 1 = no turf, 9 = greatest amount. Here again, those turfs which received the initial 0.36 lb. rate, followed by either sequential rate, showed a light but noticeable decrease in visual density assessment (Table 5). The difference between (0.18+0.36) and (0.18+0.18) treatments was negligible and non-significant. A slight difference was realized between the one time initial applications between the 0.18 and 0.36 treatments.

Overall turfgrass quality scores were assigned at 5DAT/2, using the 1-9 scale (1 = dead turf, 9 = best). Quality mean scores ranged from 4.8 to 7.5 for treated turf (Table 5). The sequentially treated turf which received the initial high rate (0.36 lb.) had exhibited lesser quality turf than those which received the initial 0.18 lb. rate. The (0.36+0.36 lb.) turf had the lowest quality at 5DAT/2 (Table 5).

The same field variables were evaluated at 14DAT/2. Percent plot bleaching was now in full expression, as treated turf ranged from 10%-75% bleaching (Table 3). The treatments which now received a second application at the high rate (0.36 lb) had extreme bleaching (75%), regardless of the first treatment rate previously applied. Those treatments receiving the 0.18 lb. sequentially, had 42%-45% plot bleaching, and the former rates were not of consequence by this time (Table 3). Single initial treatments only had 10%-15% bleaching only, similar to the leaf tip drying (from mowing) on the check.

Color scores at 14DAT/2 ranged from 2.3 to 4.3 for turf receiving a second (sequential) application, while treatments formerly applied once had high color scores similar to the check (5.8-6.0 mean) (Table 4). Color reduction was reduced by the rate of the second application, with the 0.36 lb. sequential having the lowest color scores.

Density scores followed similar treatment ranking as above for color, however, scores on average were slightly higher than for the previous density evaluations (Table 5). Either this is observation bias, or from regrowth. Note that stolon/shoot densities were not taken, but visual estimates of relative canopy densities were used for assessments.

Turfgrass overall (general) quality scores ranged from a low of 2.3 to 5.0 for turf which received the second treatment sets. Once again (as with color and density described above) the (0.36+0.36) and (0.18+0.36) treatments had lowest quality scores which were unsatisfactory (Table 5). This was followed next by the (0.18+0.18) and (0.36+0.18) treatment, which had marginally acceptable turf quality scores. Values less than 5.0 represent compromised turf. The single applications applied previously had turf quality scores equal to those of the check.

Final evaluations of color and percent plot bleaching were made on August 28, 1995 (24DAT/2). Color mean scores ranged from 3.7 to 5.5 for those turfs which received various combinations of treatment rates sequentially (Table 4). Treatments which previously received the single initial application at either rate had color scores equal to, or greater than that of the check (Table 4). Of the treatments which received two treatments, the (0.18+0.18) had fully acceptable turf color (mean = 5.5), while those that received at least one application at 0.36 lb. had turf of noticeably lesser color.

At 24DAT/2, percent plot bleaching ranged from 4% to 22% (Table 3). Turfs treated twice had mean scores of 9% to 22%. Bleaching was related to the rate applied at the second (sequential) treatment, as (0.36+0.36) and (0.18+0.36) treated turfs had the greatest bleaching (19%-22%). This was followed by the (0.36+0.18) and (0.18+0.18) treatments, which had percent plot bleaching scores of 14% and 9%, respectively (Table 3).

Based on rates and timing conducted here, it appears that irrigated buffalograss maintained at 3.0 inches would have an acceptable (maximum) tolerance of 0.18 lbs., with a drop in quality (from enhance bleaching) from a sequential application lasting 2-3 weeks. A single application at 0.18 lb. seems the tolerable limit.

Some buffalograss plots had a prostrate spurge spp. infestation during this test. There was no affect on this weed post emergence.

FN:RPBUFF DIR:Q495

Table 1. Mean color¹ scores of buffalograss turf after applications of RP EXP31130A herbicides. University of Arizona, Summer 1995.

RATE² 3DAT/1³ RATE 8DAT/1 RATE 11DAT/1 RATE 18DAT/1 RATE 28DAT/1

check 7.0 check 7.0 check 7.0 check 7.0 check 7.3

.18+.18 6.0 .18+.18 5.0 .18+.18 5.5 .18+.18 6.5 .18+.36 7.3

.18+.36 5.5 .18+0 5.0 .18+.36 5.3 .36+0 5.8 .18+.18 7.0

.18+0 5.5 .18+.36 4.5 .18+0 5.3 .18+0 5.8 .18+0 6.8

.36+0 5.0 .36+0 3.8 .36+0 4.0 .18+.36 5.5 .36+.18 5.5

.36+.18 4.8 .36+.36 3.3 .36+.36 4.0 .36+.36 5.0 .36+.0 5.5

.36+.36 4.8 .36+.18 2.8 .36+.18 3.5 .36+.18 4.8 .36+.36 5.5

Test Mean⁴ 5.1 Test Mean 4.5 Test Mean 5.0 Test Mean 5.8 Test Mean 6.4

LSD Value⁵ 0.7 LSD Value 1.1 LSD Value 1.0 LSD Value 1.2 LSD Value 1.1

¹Color scores = (1-9) 1 = dead turf 9 = dark green. Values are means of four replications.

²Rates expressed in lbs. AI/A

³DAT/N = numbers of days after treatment/sequence, respectively.

⁴Mean of all treatments on test date.