Evaluation of Redroot Pigweed (Amaranthus retroflexus L.) Control Using Nicosulfuron + Rimsulfuron in Mixture with 2,4-D + MCPA

A few herbicides have been registered to control broadleaf weeds in Iran. Sulfonylureas are dual purpose herbicides to control narrow and broadleaf weeds, but these herbicides control narrow-leaf weeds better than broadleaf weeds. Hence, it is better to mix these herbicides with other broadleaf herbicides by different site of action to control broadleaf weeds effectively. Although the most of herbicide mixtures have been antagonism, but studies have shown that phenoxy herbicides have supplementary effects on broadleaf weeds in mixture with sulfonylureas.
In order to study the effect of nicosulfuron rimsulfuron (Ultima) in mixture with 2,4-D MCPA (U46 Combi Fluid) on redroot pigweed (Amaranthus retroflexus L.) at the four-to six-true leaf stage, an experiment was done in 2012 at the greenhouse of Agricultural Faculty of Ferdowsi University of Mashhad, Mashhad-Iran. Experiment was performed as a randomized complete block design with seven rates of herbicide mixtures and four replicates. The plants were sprayed using a greenhouse bench sprayer equipped by 8002 single nozzle with an even spray pattern delivering 200 L ha-1 at 300 kPa and boom height of 50 cm. Herbicides doses in mixture were considered based on effective dose required for 50% reduction in aboveground dry matter of redroot pigweed in pre-test experiments and joint action model calculations. ED50 doses of nicosulfuron rimsulfuron and 2,4-D MCPA applied alone were 22.33 and 55.98 g a.i. ha-1, respectively. The ratio of the herbicides in binary mixtures were 100:0, 87.5:12.5, 75:25, 50:50, 25:75, 12.5:87.5 and 0:100. The dose-response curves in binary fixed-ratio mixture were fitted simultaneously within each treatment using a three-parameter Gompertz model (with the lower limit equal to zero) available in the drc add-on package to the R programme. A Box-Cox transform-both-sides approach was performed to achieve variance homogeneity. The goodness-of-fit was assessed by graphical analyses of residuals and F-test for lack-of-fit. The results of dose-response curves of herbicides in mixture by 50 and 90 percent reduction in aboveground dry matter of redroot pigweed were plotted on the graph and compared to the ADM isobole. Points above the isoboles indicate that the joint action of a mixture is lower than predicted by ADM, while points below the isoboles indicate a joint action higher than predicted by ADM. In the present study, we examined whether the predicted ED50 and ED90 doses of the herbicide mixture was contained in the 95% confidence interval of the estimated ED50 and ED90 doses. This approach inevitably overestimates the number of significant deviations, because it does not incorporate a variation around the isobole. Significant deviations were termed antagonism if higher and synergism if lower than the corresponding estimated ED50 and ED90 doses. As the results with the herbicide mixtures originate from this experiment, it was necessary to standardize the x- and y-axes so that the ED50 and ED90 doses of the herbicides applied separately were always fixed to 1.
Results revealed that 2,4-D MCPA in mixture with nicosulfuron rimsulfuron has lower effect on redroot pigweed in comparison with where herbicides applied alone. So that all of observations were located outside of isobole line, irrespective of response level and strong antagonism was observed in mixture of two herbicides. Interference was observed more especially in mixtures that nicosulfuron rimsulfuron ratios was higher than or equal with 2,4-D MCPA. In these mixture ratios, observations were located far from isobole line at ED50 and ED90 response levels in comparison with higher mixture ratios of 2,4-D MCPA. There may be interference between two herbicides in mixture for absorption into and translocation to site of action. Herbicides formulation and adjuvants may be led to increase or decrease in efficiency of herbicides in mixture according to ADM. Studies have shown that formulation of one herbicide in mixture has important effect on active ingredient absorption of herbicides into the plants. Therefore, it is possible plants have had a chance to detoxify herbicide molecules in mixture at the same rates that herbicides applied alone. Hence, it was required higher rates in mixture to reach an effective dose at the site of action by 50 or 90 percent reduction in aboveground dry matter of redroot pigweed. Therefore, the efficiency of herbicides has been reduced in mixture compared to applied herbicides alone.
Mixture of 2,4-D MCPA with nicosulfuron rimsulfuron does not recommend to control of this weed, because impose more herbicides rates to environment. Therefore, other binary mixtures must be evaluated using nicosulfuron rimsulfuron in mixture with synthetic phenoxy herbicides or other broadleaf herbicides.