Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School


First Advisor

W. E. Arnold


Selective herbicides are highly specific chemicals. They must control weeds at rates which do not injure crops, they must leave no residue in the soil to injure succeeding crops, and they must leave no hazardous residue on crops. It has been extremely difficult for researchers to develop selective herbicides that adequately meet all of these goals. For example, an ideal herbicide spray effectively controls grass and broadleaf weeds simultaneously. Therefore, when farmers are plagued with a variety of weeds including both annual grasses and broadleaf weeds, there is seldom a single economical herbicide available that will completely control the weed population. An important area of current herbicide research is the investigation of herbicide tank mixtures. It is common field experience that use of a broad spectrum herbicide exerts a selective pressure on the field population because of differential effectiveness on various weed species. Combining herbicides offers possible solutions. The use of mixtures will allow a herbicide to overcome the weaknesses of other herbicides in the combination. Crafts has recently suggested that a judicious combination of herbicides is like making up prescription drugs. Properly chosen herbicide mixtures can have many advantages. Using mixtures of toxicants will provide more effective control of populations of mixed weed species exceptionally tolerant of the herbicide program, especially when the toxicants kill by affecting different physiological processes. Tank-mixing can reduce costs. With current high costs to produce it is important that, in the process of crop production, farmers make as few trips over the field as possible in order to keep costs down and maintain or increase profits. Properly chosen combinations provide insurance against weather conditions unfavorable to one of the herbicides. In addition, herbicide combinations offer the possibility of broadening selectivity by synergistic action on weeds. Tank-mixing herbicides may be both effective and profitable but possible disadvantages should not be overlooked. Erratic weed control may also result. The objectives of these studies were: (1) Determine the type of interaction involved in a common tank-mix of quizalofop (±) --2-[4[(6-chloro-2-quinoxalinyl )oxy]phenoxy] propanoic acid and bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4-(3H)-one 2,2-dioxide]. (2) Compare the responses of application factors such as: growth stage, herbicide rate, sequential applications, carrier volumes, and spray additives and how they affect the tank-mix treatment of the two herbicides. (3) Monitor and record stomatal conductance of velvetleaf and yellow foxtail plants treated with the various herbicide treatments.

Library of Congress Subject Headings


Weeds -- Control



Number of Pages



South Dakota State University