Growth and Reproductive Performance of Rangeland Beef-Cattle as Influenced by Controlled and Uncontrolled Populations of Horn Flies (Diptera: Muscidae).

A 4-yr study was conducted on native rangeland to assess the growth and reproductive performance of cows (Bos taurus) infested with naturally occurring seasonal populations of horn flies (Haematobia irritans). One hundred five Angus × Hereford cow-calf pairs were evaluated as a randomized complete block that was replicated across 4 yr. Cows were approximately 39 d postpartum at the beginning of each yearly trial and were randomly allocated to either an untreated control (UTC) or an insecticide-treated (TRT) herd. Horn fly populations were monitored throughout each yearly replication and blood serum progesterone levels were used to estimate postpartum interval lengths and days to pregnancy. Initial body weights of cows were collected in May with final body weights and calf weaning weights acquired in October of each year. Monthly horn fly control ranged from 85.55 to 99.57% throughout the 4 yr. Cows within UTC herds maintained on average 530.10 ± 94.74 more (P = 0.0015) flies per animal than TRT. However, no differences were detected between treatment groups for any of the reproductive parameters evaluated (P > 0.05). Despite a lack of difference in the reproductive parameters measured, TRT cows gained more (P = 0.0492) weight throughout the fly seasons when compared to UTC cattle. Furthermore, calves paired with insecticide-treated cows tended (P = 0.0680) to wean 16.28 ± 8.04 kg heavier than calves paired with cows exposed to naturally occurring horn fly populations.