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Bruce Burdick
1109 S. Birch St.
Albany, MO 64402
660-726-5610
Email: BurdickB@missouri.edu

Hundley-Whaley Center phone
660-726-3698

Overall weed control, as rated on August 9 (end-of-season), ranged from 78 to 96%. The poorest weed control was provided by Roundup treatments without a residual herbicide. The only significant Roundup rate response (78% vs. 88% control) was between the 16 ounces and 32 ounces per treatment, Roundup treatments applied as a delayed burndown followed by a mid-post Roundup treatment.

As a rule, treatment containing a residual herbicide provided better grassy weed control at end-of-season than treatments without a residual herbicide. Residual herbicides did not provide significantly enhanced broadleaf weed control at end-of-season. Residual herbicide treatments that contained either Dual or Harness, along with atrazine, provided a higher level of grassy weed control than those containing only atrazine.

Situation: Chemical weed control in field corn has traditionally relied on soil applied, residual herbicides. All to frequently, the most commonly used herbicides (atrazine and metolachlor) show up in drinking water sources and become a public health concern. Mechanical cultivation is frequently offered as a method to reduce herbicide use but widespread use of this weed control method increases soil erosion and sedimentation in Missouri's surface waters.

The development and release of glyphosate tolerant (GT of RR) corn varieties offer producers an opportunity to develop a corn weed management program that has the potential to provide excellent, economically viable weed control without a residual herbicide. However, there is concern that the weed seedbank will build with late emerging weeds.

There are still questions about optimum glyphosate rates. While the recent price reduction of Roundup Ultra makes the costs of rate increases less of a financial issue to producers, rates are still an issue in environmental loading. Research conducted in 1998 at the Hundley-Whaley farm suggests that good to excellent weed control can be obtained with reduced glyphosate rates.

Objective: This research will have the following three objectives.

1. To determine glyphosate application timing and use rate for optimum weed control and crop yield of GO field corn and to compare that to the performance of a commonly used standard weed control program in field corn.

2. To evaluate the need for a residual mid to late-post application of atrazine or atrazine plus metolachor or acetochlor at reduced rates, along with glyphosate, for control of late emerging weeds.

3. To transfer the knowledge gained from the study to commercial corn producers throughout northwest Missouri.

Rainfall through the season could be characterized by excessively wet through June then drought. Only 0.95 inch of rain was recorded in July. August received 1.13 inches of rainfall. Summer temperatures were typical. One excessively hot period occurred from July 24 through July 30 where daytime highs were 99 to 103 degrees Fahrenheit. This was the period for pollination of the corn treatments that had received a pre-emergence burndown herbicide treatment.

The study location was located on east Fairchild Road at the Hundley-Whaley farm. The soil is a Grundy silt loam with 3% organic matter. Fertilizer was broadcast applied, with a Gandy Ezee Flow fertilizer applicator, at the rate of 180-0-90 immediately after planting. The nitrogen source was ammonium nitrate.

Weed pressure ranged from moderate to heavy for the following predominant weed species:

1. Marestail
2. Giant foxtail
3. Cocklebur
4. Wild sunflower

Other miscellaneous weeds present in the study were little barley, ivyleaf morningglory, and corn gromwell with variable pressure.

The study was designed as a randomized complete block with six replications. It consisted of nineteen treatments. One treatment was an untreated weedy check and one was a standard pre-emergence Gramoxone plus Bicep followed by Marksman applied mid-post (6-8" tall corn) treatment. The other seventeen treatments consisted of glyphosate applied at various times and rates, with and without atrazine or atrazine plus metolachlor or acetochlor for residual weed control. Applications timings were:

1. At planting - a pre-emergence glyphosate burndown treatment followed by either a mid-post glyphosate treatment (6-8" tall corn), with and without atrazine or atrazine plus metolachlor, or a mid-post treatment of glyphosate and a late, late post (16-20" corn) glyphosate treatment.
2. A delayed glyphosate burndown (2-4" tall corn) treatment followed by a late post glyphosate treatment (10-12" corn) with and without atrazine or atrazine plus acetochlor.

Roundup Ultra was the source of glyphosate. It was applied at rates ranging from 16 to 32 ounces of product at each application. Ammonium sulfate at the rate of 17 pounds/100 gallons of spray solution was mixed with all Roundup treatments. All applications were made with 20 GPA of spray solution.

Evaluations included the recording of weed pressure, by species, prior to each glyphosate application; a visual evaluations of weed control 10-14 days following each herbicide application and an "end-of-season" (August 9) visual evaluation for treatment efficacy. The evaluation of weed pressure prior to treatment should encourage scouting and the "end-of-season" evaluation should provide indications of weed seed production that would build seed bank. Crop yields were taken. A planned economic analysis of results was not done due to very poor yields resulting from a mid and late summer drought in Gentry county.

Extension of Results: The study was discussed at the 1999 Hundley-Whaley farm field day on September 9, 1999. Approximately 650 people stopped by this study to hear what was being done. In addition, results of the test will be distributed to producers and dealers during 2000 as follows:

Treatments receiving delayed burndown herbicides (post 2-4" corn) were under stress early in the study. Notes taken on June 14, a little over two weeks after planting, showed the corn in these treatments to be shorter and less robust than where a pre-emergence burndown treatment was applied. Crop growth reduction ratings were taken on June 17 (19DAP) and the ten delayed burndown treatments showed crop growth reductions (biomass reduction estimates) of from 36-45%. This compared to no crop growth reduction for the nine treatments receiving a pre-emergence burndown treatment. The delayed burndown treatments showed nitrogen deficiency type symptoms despite the heavy rate of nitrogen applied at planting time.

There was no apparent crop injury from any of the Roundup plus residual herbicide post-emergence treatments.

All treatments provided 100% control of wild sunflower at the end-of-season rating (August 9). However, there was a difference in giant foxtail and cocklebur control. The table below shows results of selected treatments.

Table 1

Table 2

The table below shows that there was no significant difference in weed control between two and three applications of Roundup, each applied at 16 ounces/acre.

Table 3

The best overall "end of season" weed control (96%) was provided by the standard Gramoxone + Bicep followed by Marksman treatment (T19) and the multiple Roundup application (T6).

The effect of delaying the first Roundup application from a pre-emergence to a 2-4" corn, very early post did have an adverse effect upon both early and late growth of the corn. The delayed application in a normal season might not have been evident but due to the delay in planting we had 2-4" corn competing with 4-8" giant foxtail. Measurements were taken of corn height, 45 days after planting. The date for initiation of tasseling was also recorded. The table below gives an indication of the effect of early weed competition upon corn eight and tasseling.

Table 4

Table 5

The 45 DAP corn height for all nine of the pre-emergence burn down treatments ranged from 50 to 55 inches. The height of the ten delayed burn-down treatments ranged from 34 to 39 inches.

The "days to tassel" for the nine pre-emergence burn down treatments ranged from 59 to 60 days. The "days to tassel" for the ten delayed burn-down treatments ranged from 64 to 68 days after planting.

Corn yields were taken on October 19. Corn yields were severely affected by the summer drought. All treatments yielded under 40 bushels/acre and there was considerable variation in treatment yields. No meaningful data on yields could be reported.

Conclusion: The data that shows the severe reduction in early season crop growth due to delayed burndown treatments can be of benefit to producers. It indicates that corn growth is reduced by early season weed competition, even if that competition is eliminated within two weeks following corn emergence (Table 4). University research has repeatedly shown that if corn appears to be stunted and N deficient early in the year (by 3-6 WAP) due to weed interference, significant yield reductions will occur.

Tasseling date was significantly retarded in the delayed burndown treatments. In a normal year, if tasseling is delayed, corn pollination is attempted in hotter, drier weather. This generally results in reduced corn yields.

The data also suggests that a late post applications of Roundup will be required for giant foxtail control if no residual herbicide is applied early post-emergence (Table 5). It also suggests that residual herbicides used at lower than labeled use rates, applied early-post, is beneficial in reducing end-of-season grassy weed pressure when compared to post treatments without the residual herbicide (Table 2).