The Effect of Interaction between Imazethapyr Herbicide Dose and Bean Cultivars (Phaseolus vulgaris) on Weed Control

Weed management is an important agronomic practice. It would be takes 40% of time of small holder farmers, if carried out by hand. Farmers have being interested in more comprehensive programs for weed management that reduce weed populations over time and the use of reduced herbicide doses in order to diminish their production costs. Integrated weed management takes advantages of various methods to suppress weeds in a more common and environmental natural way. Today, it is understood that chemical herbicides are not free of problem. So researchers are working on integrated weed management, which brings all possible methods to control weeds. The use of reduced herbicide doses will cost a fraction of full dose and make them even more affordable to poor farmers who have a limited amount of resources. Researches indicate that there is good potential to reduce the number of herbicide applications and utilize lower herbicide doses within competitive cropping systems. Crop competitiveness advantage is a great potential for weed suppression that provides reducing herbicide dose to avoid its environmental and economic suffers. Therefore, a field experiment was conducted to evaluate the effect of reduced dose of imazethapyr on weed control when two bean cultivars grown in pure stand and intercropping together.
The field experiment was conducted as split plot based on randomized complete block design at the Research Farm of University of Tehran located in Karaj during 2013–2014. Main plots were five doses of imazethapyr (0, 25, 50, 75 and 100 percent of the recommended dose) and sub-plots comprised pure stands and intercropping of two bean cultivars include: Akhtar (the growth habit is upright) and Goli (the growth habit is creeping). Also there is a plot as weed free (control). When the plants were at the third trifoliate stage, determined doses of imazethapyr were sprayed. Weed dry weight and bean yield were analyzed by nonlinear regression. In order to obtain the response curves to the doses of imazethapyr, data were fitted by SigmaPlot software (version 11) according to the standard dose-response equation:Where W is the dependent variable, ED50 is equivalent to a dose of herbicide that reduces weed dry matter by 50 percent and b is the curve slope where the trend is linear. The yield data were fitted using a three-parameter logistic equation:where a is maximum yield, x0 represents a dose of herbicide in which the yield reaches 50 percent and b is the slope of the curve.
The community of weeds in the field mostly consisted of three species; Amaranthus retroflexus, Chenopodium album and Solanum nigrum that all are summer annual weeds. The dose-response model well described the effect of doses of herbicide on weed dry weight. Results indicated that weed species response to herbicide doses differently in bean cultivars and their intercropping. Among dominant weeds, A. retroflexus was determined high sensitive to increasing doses. Bean cultivars were solely different in competition with weeds, as creeping cultivar i.e. Goli with more competitive ability (like: fast early growth and area coverage) was more successful in weed suppression. Herbicide application at reduced dose could also be efficient when we used 75 percent of the recommended dose because led to a yield more than 4000 kg/ha in pure stand of Goli. Pure stand of Akhtar and intercropping showed yields of 3617 and 3641 kg/ha, respectively at 75 percent of the recommended dose. Relative yield total (RYT) for doses of 75 and 100 percent of the recommended dose and weed free control were 0.95, 0.87 and 1.03, respectively.
The results of experiment showed that weed sensitivity to herbicide doses vary differently by species. So that, A. retroflexus dry weight was more affected than C. album and S. nigrum in response to reduced doses of imazethapyr. The application of 25 percent of the recommended dose of this herbicide reduced A. retroflexus dry weight more than 50 percent. It seems that if the dominant weed in a field was the A. retroflexus, the farmer could greatly reduce dose of imazethapyr. However, usually weed community of fields are consisted of several species, therefore, to use reduced herbicide doses, sensitivity of all species should be considered. Also, using bean cultivars that potentially have high competitive ability, it could be a good supplement in combination with reduced doses of herbicide. In this experiment, Goli cultivar presented more potential to combine with reduced doses of imazethapyr.