By Randy Buhler
Logan County Extension Agent
As we look for ways to cut production costs this year, a common thought is to use lower rates of fertilizer, chemical and maybe even seed.
But how much can you cut back and not ruin your yield. This column will answer the part about chemicals, or more specifically, herbicides.
A study reported in the latest issue of Weed Technology set out to discover how much weed control is influenced by herbicide rates as a percentage of the label rate. The study created a database from many studies of rates and weed control effectiveness, or what weed scientists call efficacy. The study looked at rates that were less than 20% of the label rate, 20% to 40%, 40 to 60%, 60 to 80%, 80 to just under 100% of the label rate.
The researchers found that reduced rates were more likely to be successful in corn than in small grains, soybeans, sugar beets or other field crops. They also determined that pre-emerge chemicals had better results with reduced rates than post-emerge chemicals. Post-emerge chemicals had better results than the pre-plant incorporated chemicals. Results were compared as to percentage of control and variability of result at each rate category. When between row cultivation was used as part of the weed control system, the pre-plant incorporated and pre-emerge herbicides had better results than post-emerge chemicals.
Use of surfactants did not show reliable weed control enhancement sufficient to allow reduced rates. Not all chemicals in the study required the use of label rates of surfactant. Their conclusion was you should not rely on a surfactant to improve efficacy of below-label rates. Some chemicals do require surfactants, in order to perform. You must follow the label on the surfactants
There was no significant difference in weed control efficacy between conventional and conservation tillage systems of production. In both systems, an increase in herbicide rate coupled with between row cultivation improved efficacy and decreased variability of results.
Weed control efficacy is dependent on soil type. For any rate category, the weed control efficacy was better on coarse textured soils than on fine textured soils.
For small grains, weed control efficacy was high when label rates were used. A reduction of rate to 80% or less resulted in substantial reduction in weed control achieved. For corn, when between row cultivation was included in the system, weed control efficacy was 80% in at least 90% of the case studies with herbicide rates at 20% or higher of the recommended rate. Within each crop group studied, using a herbicide at label rate did not achieve better results than using 60% of label rates. Over 70% control was maintained in over 90% of cases when rates were at 30 to 60% of label rates.
An alternative method, not reported in the study, is to use the full label rate on a band over the row, and use between row cultivation to complete your weed control program. Look at costs of chemicals and your cost of cultivation to find the lower cost system. You need to know the spectrum of weed species present in your field. Certain weed species may require a specific control method for success. For row crops, other than corn, the banding method will be less risky than reducing label rates of broadcast applications.
Remember, when you use less than label rates, the chemical company is not obligated to service a performance complaint.