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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

Objective: To determine the effect of weed interference, and duration of competition, on crop performance in Roundup Ready corn.

Materials and methods: The study was designed as a randomized complete block with six replications. Plot size was 12' wide by 30' long, but only the middle 6' width of plot was treated. Treatments consisted of the following:

Asgrow RX 738RR brand of seed corn was planted at the rate of 25,500 seeds per acre in 36" wide rows on May 3. The plot area had produced soybeans in 1999 and was conventionally tilled prior to planting.

Fertilizer, at the rate of 210-0-60 was applied and incorporated into the soil prior to planting the crop.

Weed pressure in the plots ranged from moderate to heavy and consisted of the following weed species - giant foxtail, shattercane, wild sunflowers, cocklebur, ivyleaf morningglory and velvetleaf.

Spray applications were made with a C Farmal equipped with a front mounted boom. Boom height at application was set at 12" above the tallest plants in the field. Spray nozzles were 11002 flat fans spaced 18" apart. Spray propellant was compressed air. Pressure at the nozzle was 38 PSI.

The weather for the year could be characterized as a very dry spring, followed by a 10 day wet period in late June and early July, then very dry through harvest.

Results: The pre-emergence application of Harness Xtra held back weed growth to the point that the first post-emergence application to 2" weeds was made on May 30, two weeks later than the 2" weed treatment in treatment 2 and 7.

Weed control ratings were taken on June 29, seventeen days after the 12" weed treatment was made to the 12" foxtail treatment (#6). At that date, a second flush of shattercane had developed in the early-applied Roundup treatments (T 2-4). The single Roundup treatment to 2" tall weeds also had a heavy re-emergence of giant foxtail and sunflower. The late, single treatment to 12" giant foxtail (T6) failed to control sunflower (18-24" tall) at acceptable levels, but giant foxtail and shattercane (18-24" tall) were controlled well. The following is the weed control ratings for treatments 1-6 on June 19.

The height of the corn at tasseling, percent of final weed control and the corn yield are shown in Table 3. The height of the corn at tasseling, and the very low yield of the weedy check is an indicator of weed pressure in the plots. The crop yield obtained from treatment number 4 would suggest that if a post Roundup treatment were limited to one time during the season, the application should be made to corn when the giant foxtail was 6" tall. The crop did not respond positively to very early weed control in 2000 in treatments where multiple applications of Roundup were made. This differs from work done in 1999, where the highest yields of corn from multiple control treatments resulted in significantly higher yields where the first flush of giant foxtail was controlled at a height of 2".

Figure 1

The bar chart in Figure 1 shows the crop performance for two years of testing. Both years suggest that the best time for a single application of Roundup would be to those treatments with 6" tall giant foxtail. In 1999 the corn was in the V5 growth stage at that time (30 DAP). In 2000 the corn was in the V3 growth stage (27DAP). Part of the difference in corn growth stage might be attributable to the difference in dates of planting between years. Planting was delayed to June 7 in 1999 due to a very wet spring.