Seeds of a putative 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide–resistant tall waterhemp biotype from Henry County, IA, were collected from a seed corn field in fall 2009 after plants were not controlled following a POST application of mesotrione plus atrazine. The response of this biotype to various herbicide modes of action was evaluated in greenhouse and field tests. Under greenhouse conditions, the suspect biotype showed an eightfold decrease in sensitivity to mesotrione with a 50% control rate of 21 g ha−1compared with 2.7 g ha−1for the susceptible biotype. The biotype also had a 10-fold decrease in sensitivity to atrazine and a 28-fold decrease in sensitivity to thifensulfuron. Under field conditions, tall waterhemp was not controlled POST at the label rate of 1,100 g ha−1of atrazine. Tall waterhemp control was less than 60% at the label rates of three commonly used POST HPPD-inhibiting herbicides in seed corn: 105 g ha−1of mesotrione, 92 g ha−1of tembotrione, or 18 g ha−1of topramezone. Thus, this new tall waterhemp biotype is resistant to three herbicide modes of action: HPPD inhibitors, photosystem-II inhibitors, and acetolactate synthase (ALS) inhibitors.
Utility adjuvants are adjuvants that are tank-mixed in the spray solution to improve the spray application process, but do not directly influence herbicide efficacy. However, by improving the spray application process, utility adjuvants can indirectly improve herbicide efficacy. There are five primary utility adjuvant types: compatibility agents, deposition agents, drift control agents (sometimes referred to as antidrift agents or drift retardants), defoaming agents, and water conditioning agents, and three secondary utility adjuvant types: acidifying agents, buffering agents, and colorants (dye markers). Herbicides can react either physically or chemically with other spray mixture components to form an unsprayable mixture. Compatibility agents prevent these reactions from occurring. Drift control agents and deposition agents increase the amount of herbicide deposited on target surfaces. The primary function of drift control agents is to reduce the amount of spray solution that moves off-target. Indirectly, the amount of herbicide reaching target surfaces can be increased. A defoaming agent will reduce or prevent foam produced in the spray mixture. Ions in the spray solution can interact with various herbicides, decreasing efficacy. Water conditioning agents will counteract the effect of the ions on herbicides. Water conditioning agents must be added before the herbicide to prevent herbicide–ion interaction. Acidifying and buffering agents function in a similar fashion, reducing or increasing spray solution pH. A buffering agent will maintain a pH range, whereas an acidifying agent will not. Colorants are dyes that are added to the spray solution to produce a visible color on the sprayed area to assist the applicator in applying the herbicide.
Research was conducted at the Brandon Research Centre to determine the effect of spray solution pH or adjuvants on the efficacy of two cyclohexanedione (CHD) herbicides, clethodim and tralkoxydim, and two aryloxyphenoxypropionate (APP) herbicides, fenoxaprop-P and quizalofop, and if ultraviolet light is a factor influencing their activity. CHD herbicide efficacy decreased as spray solution pH increased above 7. APP herbicide efficacy was not affected by spray solution pH, probably due to their formulation as proherbicide esters. The choice of adjuvant affected CHD herbicide efficacy but not APP herbicide efficacy. Amigo®, Canplus-411® plus ammonium sulfate, and Merge® were usually the most effective adjuvants with the two CHD herbicides. Physical removal of ultraviolet light did not improve APP herbicide efficacy but did improve CHD herbicide efficacy.
. 1994. Effect of wild mustard (Brassica laber) competitioT 9l VigF g9 quality of triazine-tolerant and triazine-susdeptible canola (Brsssl'co nnpus anll Bmssica rapa). Can. J. Plant Research was conducted at two sites near Brandon, Manitoba, in 1990 and l99l to determine the influence of time of removal of wild mustard from triazine-tolerant (TT) or triazine-susceptible (TS)
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