Field and greenhouse studies of 2,4-D [(2,4-dichlorophenoxy)acetic acid] and glyphosate [N-(phosphonomethyl) glycine] absorption, translocation, and metabolism were initiated to explain field observations which indicated susceptibility of common milkweed (Asclepias syriacaL.) but not hemp dogbane (Apocynum cannabinumL.) to glyphosate and the reverse response to 2,4-D. Glyphosate was absorbed less than 2,4-D in both species with absorption of both herbicides being greater in common milkweed. Greater herbicide absorption by common milkweed was attributed to less epicuticular wax, less cuticle, lower contact angle of the herbicide spray, and the presence of stomata and trichomes on the adaxial leaf surface. No major translocation differences of the herbicides were noted between species. Translocation of glyphosate was more rapid than that of 2,4-D. More glyphosate than 2,4-D accumulated in areas of high meristematic and metabolic activity. Rapid 2,4-D metabolism occurred in common milkweed leaves above treated leaves. There was no detectable 2,4-D metabolism in hemp dogbane roots after 20 days, while 60% of the 2,4-D in common milkweed roots was metabolized. Limited absorption of glyphosate but not 2,4-D by hemp dogbane and metabolism of 2,4-D but not glyphosate by common milkweed were considered the primary factors involved in explaining observed susceptibility differences.

Surfactants were evaluated in the greenhouse for their ability to enhance glyphosate [N-(phosphonomethyl) glycine] toxicity to common milkweed (Asclepias syriacaL.) and hemp dogbane (Apocynum cannabinumL.). Ethoxylated amines were among the most effective groups of surfactants. Nonionic ether and ester ethoxylates combined with a dimethyl amine or a quaternary ammonium salt were more effective than any of these surfactants alone. Effectiveness of ethoxylated amine surfactants was altered by pH changes. Cat-ionic surfactants tended to be more effective than nonionic surfactants. With exceptions, surfactants were more effective with increased ethoxylation. Amine surfactant effectiveness increased with increasing concentrations up to 0.3% (w/v). Surfactant effectiveness on a molar basis was more closely related to the ethylene oxide content of the surfactant than was effectiveness on a percentage basis. Contact angle was not related to surfactant effectiveness at high or low surfactant concentrations. Ethoxylated stearyl ether and amine surfactants gave optimum effectiveness at hydrophile-lipophile balance (HLB) values of 15 to 16 and 19 to 20, respectively. Surfactants with a low HLB were usually less effective. Effectiveness of surfactant combinations was quite variable and difficult to predict. Therefore, the indiscriminate addition of surfactants to glyphosate spray mixtures which already contain a surfactant should be avoided.

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