Steve D. Ruhl, Agriculture and Natural Resources Extension Agent
Jeff Stachler, Horticulture and Crop Science Extension Program Specialist
Previous Ohio State University small plot research has shown an effect of time of day for effectiveness of herbicide applications. These studies also used lower than normal rates on larger than normal weeds to assure a time of day effect. This study investigated the effect of the time of day of glyphosate using recommended rates on recommended sizes of target weeds.
| Cooperator: | Tom Weiler | Soil Test: | pH 7.0, P 23 ppm, |
| County: | Morrow | K 154 ppm | |
| Nearest town: | Chesterville | Fertilizer: | None |
| Drainage: | Systematically tiled | Herbicides: | Roundup UltraMax at 26 |
| Soil type: | Sloan silty clay loam | oz/A | |
| Tillage: | Conventional | Planting Date: | May 22, 2002 |
| Previous Crop: | Corn | Planting Rate: | 203,000 seeds/acre |
| Variety: | Golden Harvest H-3243 | Row Width: | 10 inches |
| Harvest Date: | October 11, 2002 |
The field chosen had high giant ragweed and moderate to high common lambsquarters pressure. Annual grass and smooth pigweed pressure was light and variable. The study used six time treatments — Roundup UltraMax applied at 26 fluid ounces per acre at 6 a.m., 9 a.m., 12 p.m., 6 p.m., and 9 p.m. and an untreated check. Ammonium sulfate was added at 17.0 pounds/100 gallon of spray mixture. Experimental design was a randomized complete block with four replications and a plot size of 10 feet wide by 40 feet in length. Applications were made on June 19th when the giant ragweed was six to 10 inches tall. Weed control was visually evaluated on August 22 on a scale of 0 to 100 percent, with zero indicating no control and 100 percent indicating complete weed control. The center 6.6 feet of each plot was harvested with a plot combine.
Table 1. Effect of Application by Time of Day on Giant Ragweed and Common Lambsquarters Control and Soybean Yield.a
| Weed Controlbc | |||
| Treatment | Common | ||
| (Time of Application) | Giant Ragweed | Lambsquarters | Soybean Yieldc |
| % | % | bu/A | |
| 6:00 a.m. | 78 b | 98.0 b | 63.5 bc |
| 9:00 a.m. | 100 a | 98.5 ab | 70.9 a |
| 12:00 p.m. | 100 a | 100.0 a | 68.6 ab |
| 6:00 p.m. | 98 a | 99.3 ab | 66.7 abc |
| 9:00 p.m. | 79 b | 99.3 ab | 61.0 c |
| LSD (0.05) | 9.8 | 1.7 | 6.1 |
| F test | 13.3 | 2.2 | 2.7 |
| a Roundup UltraMax was applied at 26.0 ounces/A plus AMS at 17.0 lb/100 gallon of spray mixture on June 19 at a spray volume of 20 gallons/A at 30 PSI. | |||
| b Plots visually evaluated on August 22, 2002. | |||
| c Treatment means followed by the same letter are not significantly different. | |||
There was a significant effect of time of day of application of glyphosate with the control of giant ragweed. Giant ragweed control was significantly lower when the glyphosate was applied at 6 a.m. and 9 p.m. In fields with high giant ragweed pressure, glyphosate should be applied somewhere between 9 a.m. and 6 p.m. to maintain maximum control.
The only significant difference in timing for common lambsquarters control was between the 6 a.m. and 12 noon applications. However, lambsquarters control was excellent no matter when glyphosate was applied.
The reduced weed control significantly lowered soybean yield when glyphosate was applied at 6 a.m. and 9 p.m. compared to being applied at 9 a.m. There appeared to be a time-of-day effect for smooth pigweed and not for annual grasses, but due to the variable and light pressure of these two species, evaluations could not be made.
The authors would like to extend their thanks to cooperating farmer, Tom Weiler. Also thanks to Dr. Jim Beuerlein for harvesting the plots and Golden Harvest for furnishing the soybean seed.
For additional information, contact:
Steve Ruhl
Ohio State University Extension, Morrow County
871 W. Marion Rd., Suite 102
Mt. Gilead, OH 43338
419-947-1070
ruhl1@postoffice.ag.ohio-state.edu