Weed control by and the phytotoxicity to corn (Zea mays L.) of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) as influenced by various phytobland oils was studied at three locations in Virginia. Postemergence control of grasses (Digitaria sanguinalis (L.) Scop., Panicum dichotomiflorum Michx., and Setaria spp.) was increased by single and repeated applications of atrazine in oil. However, no atrazine-oil combinations provided season long control of grasses. Injury to corn was observed most frequently with oils of higher viscosity (70, 100, 150 versus 200 sec viscosity) and with repeated applications.

Reduction of ¹⁴CO₂ incorporation and transpiration by atrazine was increased when oil was used in the treatment solution. Inhibition of photosynthesis increased with oil and/or atrazine rate and decreased with time following treatment. Oil type or viscosity did not significantly affect photosynthesis or transpiration although the 100 sec oils tended to be most inhibitory when used alone or in combination with atrazine.

Atrazine uptake and translocation by corn in both petroleum and vegetable oil-water emulsions (10% oil) was greater than when applied in water or Atlox 109. Uptake of atrazine increased with increasing concentration of petroleum oil (1.25 to 20%), especially at 16 to 32 hr. Emulsifier concentration (0.5 to 8.0%) had little effect except at 12 hr in which case 0.5 and 1.0% emulsifier concentrations resulted in less acropetal translocation of atrazine. At 24 and 48 hr the percent emulsifier appeared to be less important.

Both oil type (paraffinic and naphthenic) and viscosity (70, 100, 150, and 200 sec) influenced uptake and translocation of atrazine by corn leaves. The 100- sec viscosity oils were better penetrant aids than comparable 70, 150, or 200-sec viscosity oils. Naphthenic oils were generally better penetrant aids than the paraffinic oils.

All vegetable oils were better penetrant aids than Atlox 109 but less effective than the 100-sec paraffinic and naphthenic petroleum oils. No apparent differences existed between the various vegetable oils.

Acropetal movement of atrazine⁻¹⁴C following 10 µc drop application was consistently greater when applied in petroleum oil (Sun 11E or Sun 100E) than when applied in vegetable oil (corn, peanut, soybean, or safflower). No apparent specificity, either synergistic or antagonistic, existed between the vegetable oils tested as penetrant aids for atrazine and the species of their origin. Nor was any one vegetable oil consistently the best or poorest penetrant aid for atrazine uptake by corn, peanut, soybean, and safflower. Uptake of 2,4-D was greater when applied in petroleum oil than when applied in vegetable oils at both 48 and 96 hr. Soybean oil appeared to be a better penetrant aid than corn, peanut, or safflower oil for uptake of 2,4-D by soybean.

Petroleum oil not only increased uptake of atrazine by corn but this increased uptake resulted in greater quantities of unaltered atrazine in corn. The route of metabolism of atrazine in corn was apparently little affected.