Apply when weeds are actively growing is a common statement found on labels of postemergence herbicides. While intended to help users obtain consistent results, these statements provide little guidance since post products are inherently applied while plants are actively growing (it is the growing season, after all). In spite of a large volume of research conducted to determine the effect of environmental conditions on herbicide effectiveness, we still have limited understanding of how weather and other factors influence a weed's susceptibility to herbicides. Neil Harker and Robert Blackshaw, researchers with Agriculture and Agri-Food Canada in Alberta, took an interesting approach to determine the relationship between plant vigor and herbicide effectiveness (Weed Technology 17:829-835). A ruler was used to determine the growth rate of wild oat during the 24 hours prior to herbicide application, and then the performance of two herbicides as affected by wild oat growth rate was determined.
Twenty four experiments were conducted on the Canadian prairie to evaluate the performance of imazamethabenz (an ALS-inhibitor, same family as Pursuit) and ICIA 0604 (an ACC-ase inhibitor, same family as Poast Plus) on wild oat, a serious weed in wheat production. The herbicides were applied at 25, 50 and 100% of the label rate when wild oat was at the three to four leaf stage. At each experiment the height of ten wild oat plants was precisely measured the day prior to and the day of application, allowing the growth rate of the weed to be determined.
Wild oat growth rates ranged from approximately 6 to 44 mm/day due to differences in environmental conditions during the different experiments. At the 0.25% rate of both herbicides wild oat control was directly related to growth rate (Figure 1). Performance of imazamethabenz increased 13.9% for every 10 mm (approx. 0.4 inch) of growth in the 24 hr preceding application, whereas ICIA 0604 increased 5.7% for each 10 mm of growth. However, the relationship between wild oat growth rate and herbicide susceptibility was not significant for either herbicide at the 50% and 100% label rate. The lack of response at higher rates is due to the simple fact that label rates are selected to insure the products provide acceptable control under a wide range of conditions. The research validates the recommendation for spraying weeds during active growth since the wild oat was significantly more susceptible to both products during conditions that favored rapid growth.
The long term goal of this type of research is to develop the ability to predict consistently when acceptable control can be achieved with below normal rates. Although successful reduced rate programs have been developed in the past, these programs are typically based on factors other than weed susceptibility (e.g. early application to small weeds; narrow crop row spacing to enhance crop competition; etc.). No systems have been developed that fine tune herbicide rates based on current environmental conditions.
So how can this information used to improve weed management in the Cornbelt? First, run down to your local hardware store and purchase a metric ruler and practice measuring weeds. But seriously, the research documents the importance of monitoring growing conditions and the vigor of weeds at the time of application. Currently we do not have the ability to accurately predict how rapidly weeds are growing based on current weather conditions, and it is unlikely that persons in weed management will be receptive to repeatedly measure weeds to determine growth rates.
Rather than use the information to fine tune herbicide rates, it may better serve applicators by providing insights on when to delay applications, or in situations with flexibility in herbicide rates (i.e. Roundup Ready crops), when to increase rates. Each spring parts of the Midwest encounter conditions that reduce plant growth due to unfavorable temperatures or soil moisture. Weeds of agronomic crops generally have similar optimum growing conditions as the crop, thus if the corn or soybean are struggling, there is a good likelihood that the weeds are not rapidly growing either. If the herbicide label provides the opportunity to increase the rate, fields in which weeds are not growing at peak levels would provide a good opportunity to increase the rate to reduce the risk of reduced performance.
It is not unusual to encounter fields in which some cataclysmic event (frost, hail, failed herbicide application, etc.) has placed weeds under severe stress. In these situations it makes sense to postpone herbicide applications until the weeds have recovered and have resumed active growth. The time required for active growth to resume will vary from field to field, and can only be determined using a systematic scouting program (and perhaps putting that metric ruler to work).
Harker, K.N. and R.E. Blackshaw. 2003. Leaf expansion rate may help determine when low wild oat herbicide rates will be effective. Weed Technol. 17:829-835