Influence Of Glyphosate-Fungicide Combinations On Weed Control, Spray Penetration, And Yield In Glyphosate Resistant Soybean

Introduction:
Until recently, only two foliar fungicides have been registered for use on soybeans in the United States, chlorothalonil and azoxystrobin. Even though these products have been registered for some time, they have only rarely been utilized for the control of foliar diseases in soybeans. Since the discovery of Asian soybean rust (Phakopsora pachyrhiza) in the United States, however, several fungicides have now received federal labels for use on soybeans.

It is likely that some soybean growers may decide to apply one of these fungicides in combination with a postemergence glyphosate treatment. Little is known about the effects of these various glyphosate-fungicide tank-mix combinations on weed control, soybean response, or yield. Similarly, few studies have investigated the degree to which these fungicides are able to penetrate into the soybean canopy. This is likely to be an important factor in the management of Asian soybean rust, as this disease first appears in the lower portions of the soybean canopy and progresses upwards.

Objectives:

1. To determine the influence of glyphosate-fungicide tank-mix combinations on weed control, soybean response, and soybean yield compared to applications of glyphosate alone.
2. To evaluate spray penetration into the soybean canopy with glyphosate-fungicide combinations applied at two application timings.

Materials and Methods:
General Trial Information

• 2 field trials were established near Columbia and Novelty, MO in 2004
• Pioneer 93B04 soybean planted in 15-inch rows at 170,000 seed/A
• All treatments were applied with a CO2-pressurized backpack sprayer with TeeJet® 8001XR flat fan nozzles delivering 10 gpa at 24 psi
• Individual plots were 10 by 45 feet and arranged in a RCBD with 4 replications

Treatment Information

• Roundup WeatherMax was applied alone at 22 fl ozs/A or in combination with various fungicides at 2 application timings (specific treatments and rates shown in Table 1)
• "EARLY" glyphosate-fungicide combinations were applied when the average size of weed escapes reached 6 to 8 inches following a preemergence application of 1.5 pts Boundary/Acre
• "LATE" glyphosate-fungicide combinations were applied when the average size of weed escapes reached 6 to 8 inches following an early postemergence application of 22 fl ozs of Roundup WeatherMax
• At the early application timing, soybeans ranged from 12- to 16-inches in height and were in the V6 stage of growth at Columbia and Novelty
• At the late application timing, soybeans ranged from 24- to 26-inches in height and were in the R1 stage of growth at Columbia and Novelty

Weed Control Evaluations

• Total late-season weed density was determined by counting all weed species within two 1-m2 quadrats in each plot (weeds encountered at each site included ivyleaf morningglory, giant foxtail, common ragweed, and tall waterhemp)

Evaluation of Soybean Response

• Leaf area index (LAI) was measured in each plot with a Li-Cor® LAI 2000 Plant Canopy Analyzer at two-week intervals after treatment
• Soybeans were harvested from the middle 1.5 m of each plot with a small plot combine and seed yields were adjusted to 13% moisture

Evaluation of Spray Coverage/Penetration

• Water-sensitive spray cards were placed in each plot at heights corresponding to 75, 45, and 15% of the total soybean canopy height at the time of each application
• All cards were positioned approximately 4 inches from the center of the soybean row
• Spray cards were collected after each application and analyzed with APS Assess© software for determination of % coverage at each height

Results:
Weed Control

• Within application timings, glyphosate alone provided similar late season weed density as all glyphosate-fungicide combinations at both Columbia and Novelty (Figure 1)
• When averaged across all treatments, greater late season weed density was observed at the early compared to the late application timing in Columbia but not Novelty

Soybean Response

• Leaf area index (LAI) was similar for all treatments at 15, 30, 45, and 60 days after application at Columbia and Novelty (data not shown)
• Few differences in soybean yield were observed among the glyphosate or glyphosatefungicide combinations at either Columbia or Novelty (Table 1)
• Early versus late application timing did not influence soybean yield at either Columbia or Novelty

Spray Penetration

• No differences in spray coverage were observed between glyphosate and any glyphosatefungicide combination at Columbia or Novelty
• At the early application timing, spray coverage decreased from the high to the medium to the low spray card heights (Figure 2)
• At the late application timing, spray coverage decreased from the high to medium spray card heights, but no differences in coverage were observed between the medium and low spray card heights (Figure 2)

Conclusions:
These results indicate that the addition of the fungicides Bravo, Eminent, Folicur, Headline, Laredo, Pristine, Quilt, Stratego, and Tilt to glyphosate does not influence weed control compared to applications of glyphosate alone. These glyphosate-fungicide combinations did not cause soybean injury or reductions in leaf area index, and in our trials the addition of these fungicides neither significantly increased nor decreased soybean yield. Additionally, spray penetration was reduced with all treatments as depth in the soybean canopy increased, which may be a critical factor in the management of Asian soybean rust.

Figure 1. Influence of glyphosate and glyphosate+funfgicide combinations on total late season weed density at Columbia and Novelty, MO. *Plus signs (+) indicate fungicides that were applied in combination with glyphosate (Roundup WeatherMax at 22 fluid ounces per acre). Total late season weed density in untreated plots was 27 weeds/m2 at Columbia and 25 weeds/m2 at Novelty. Error bars represent the standard error of mean.

Figure 2. Influence of application timing on percent coverage of spray cards placed at three heights in the soybean canopy at Columbia (A) and Novelty (B). No differences were detected between glyphosate and glyphosate-fungicide combinations therefore data were pooled and means represent the average response observed across all treatments. Error bars represent the standard error of the mean.

Figure 3. Illustration of watersensitive cards and spray coverage of these cards when placed at high (75% of total canopy height), medium (45% of total canopy height), and low (15% of total canopy height) positions within the soybean canopy at early and late application timings.

Table 1 . Influence of glyphosate and glyphosate-fungicide tank-mix combinations on soybean yield at Columbia and Novelty, Missouri.