Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Animal Science (PhD)

Degree Level



Animal Science


Charles F. Rosenkrans, Jr.

Committee Member

Nicholas B. Anthony

Second Committee Member

Michael A. Brown

Third Committee Member

Hayden A. Brown, Jr.

Fourth Committee Member

Michael L. Looper


Biological sciences, Beef cattle, Calf performance, Horn flies, Milk production, Pasture behavior, Temperament


An experiment conducted in El Reno, Oklahoma during the fly season (May - September/October) evaluated the effects of horn flies on milk production, calf performance, and pasture behavior and temperament measurements of beef cow calf pairs. Cows (n = 53) sired by Bonsmara (BONS; n = 7), Brangus (BRAN; n = 13), Charolais (CHAR; n = 8), Gelbvieh (GELV; n = 5), Hereford (HERF; n = 12), and Romosinuano (ROMO; n = 8) from Brangus dams and their Angus sired calves (n = 51) were used in the study. Horn fly counts (HFC) and milk yield and quality estimates were collected every 28 d from May to October. Pasture behavior (grazing, standing or lying) was recorded monthly twice a day (AM and PM). Exit velocity (EV) and chute score (CS) were obtained from cows and calves monthly. Monthly HFC differed (P < 0.0001), with populations lesser in May (94 ± 42 flies) and greater in August (503 ± 41 flies). The regression coefficients for milk yield on log HFC were not consistent across sire breed (P < 0.05), with milk yield reduced 0.99 and 0.64 kg/d per unit increase in log HFC in GELV and BONS. The regression coefficients of preweaning ADG on log HFC depended on sire breed (P < 0.10), with results indicating preweaning ADG reduced by 0.19 kg/d per one unit increase in HFC in BONS calves (P < 0.05), but not other breeds. A one unit increase in log HFC resulted in 0.07 kg/d (P < 0. 10) increase in postweaning average daily gain (ADG), 19.52 kg increase (P < 0.10) in 365-d adjusted yearling weight (YWT), and 0.05 kg/d (P < 0.02) increase in birth to yearling ADG. Pasture PM behavior was associated with HFC (P < 0.05), with standing cows having fewer flies than those grazing and lying (319 ± 27 vs. 468 ± 52 and 419 ± 38 flies). Exit velocity of cows (P < 0.0001) and calves (P < 0.05) differed monthly. Results from study clearly demonstrated HFC affected milk production, calf performance, and pasture behavior and temperament measurements.