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Acute toxicity studies in 24 newborn calves and 12 two-year-old steers were conducted with a microencapsulated formulation of diazinon (O-O-diethyl O-(2-isopropyl-6-methyl-4-yrimidinyl) phosphorioate) after dermal applications. For calves, the maximum safe dosages of a diazinon wettable powder formulation were identical with those of the new formulation (0.05% concentration); for older cattle, safe dosages were 0.25% concentration. Thus, because of the slow release and absorption of diazinon, the safety factor for mature cattle is fourfold. In the present study, there was cholinesterase depression in calves at the toxic concentrations of 0.1% and in steers at concentrations of 2.0%. As determined by the mean cholinesterase depression activity, the dose-response relationship was true in the 4 treatment groups of older cattle, but was erratic in the 6 groups of calves. The use of modified and improved insecticidal formulations, such as the encapsulated type, is recommended. Their use may prolong acaracidal activity due to effective residues, eliminate the need for repeated applications at shorter intervals of time, and possibly allow increased dosages to mature cattle, without toxicity, and thus reduce acquired resistance of ticks and other insects.

Acute toxicity studies in newborn calves and 2- to 3-year-old cattle were conducted with methyl parathion (O,O-dimethyl O-(p-nitrophenyl) phosphorothioate) in the form of a 22% microencapsulated slurry used in a manually operated sprayer. Twelve calves and 9 cattle were tested. The minimal toxic dosages were 4 L of a 0.5% concentration for calves and 10 L of a 0.25% concentration for mature cattle. At all toxic dosages, cholinesterase activities were decreased--that of the calves to 0 and that of the cattle to 50% of base line or less. Weight losses 2 weeks after single exposures to the treated cattle ranged from 6.5% to 11.5% of total body weight. Absorption of the formulation in its entirety or its metabolic products resulted in a possible interference with feed utilization.