The effect of pyrethroid use in beef cattle on reproductive function and steroid biosynthesis was investigated. In the first study, BSEs were taken 5 d prior to and 14 d after treatment with label-dose of a commercial pyrethroid (permethrin)-based pour-on fly control product. Motility did not differ as a result of treatment. Although morphology improved across treatments between the first and second BSE, permethrin treated bulls had less improvement in percent of head sperm abnormalities and a tendency to have less improvement in midpiece sperm defects compared to control bulls, resulting in less improvement of primary abnormalities. Nonetheless, overall outcome for satisfactory breeder status was not impacted by treatment. In addition, there was no difference in the change of testosterone concentration as a result of treatment. Histopathological examination determined that testicular degeneration and tubule diameter did not differ as a result of treatment. However, it should be noted that regardless of treatment, degeneration score (higher score having more degeneration) was positively correlated with primary abnormalities and negatively correlated with normal sperm cells.

In the second study, superovulated beef heifers were flushed at 17 and 51 days after treatment with label-dose pour-on pyrethroid (permethrin) or saline control. No differences were noted in total embryos recovered regardless of treatment. However, total embryos did decrease in the second flush compared to the first. Embryo quality grade, total transferrable quality embryos (TQE), and overall flush success did not differ due to treatment. However, TQE did decrease in flush 2 compared to flush 1 regardless of treatment. Estradiol per ovulated follicle and estradiol per total ovarian structure was greater in flush 2 but did not differ due to treatment. Furthermore, permethrin treated heifers tended to have reduced progesterone concentrations and lower progesterone per corpus luteum proportion at embryo recovery.

In summary, the data from the two studies indicate that permethrin administration at one-time label-dose in yearling bulls and superovulated beef heifers has minimal effects on reproductive physiology, but not to a degree that would indicate compromised reproductive function or fertility.