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The indirect effects of horn flies and sire breed on calf preweaning and postweaning performance traits
Horn flies negatively impact weaning weight and average daily gain (ADG) of beef cattle, but the indirect effect of horn flies on preweaning and postweaning performance traits is less understood. Therefore, the objective of this study was to assess the indirect effects of horn flies on calf performance traits. Angus sired calves (n=51) from crossbred cows (n=53) from Brangus dams sired by either Bonsmara (BONS; n=7), Brangus (BRAN; n=13), Charolais (CHAR; n=8), Gelbvieh (GELV; n=5), Hereford (HERF; n=12), or Romosinuano (ROMO; n=8) bulls were evaluated. Total horn fly counts were recorded on individual cows while in pasture from 0700 to 0900 h beginning in May and ending in October. Horn fly counts were transformed to natural log horn fly counts prior to data analysis. Data for preweaning ADG, postweaning ADG, 365-d adjusted yearling weight (YWT), and birth to yearling ADG were analyzed by mixed model least squares. The linear model included sire breed, calf gender, and sire breed x calf gender. Effects of horn fly count on these traits were estimated by including a linear covariate of log horn fly count and log horn fly count x sire breed. Preweaning ADG was affected (P < 0.002) by sire breed of dam. Romosinuano, BONS, and CHAR calves had greater preweaning ADG (1.00 ± 0.05, 0.99 ± 0.04, 0.99 ± 0.04 kg/d; respectively) compared to BRAN and HERF calves (0.88 ± 0.03 and 0.81 ± 0.03 kg/d), with GELV calves intermediate to ROMO, BONS, CHAR and BRAN (0.98 ± 0.05 kg/day). Preweaning ADG was dependent upon an interaction of cow sire breed and log horn fly count (P < 0.10), with results indicating preweaning ADG was reduced 0.19 kg/d per unit increase in log horn fly count in BONS calves (P < 0.05). A one unit increase in log horn fly count resulted in 0.07 kg/d (P < 0. 10) increase in postweaning ADG, 19.52 kg increase (P < 0.10) in 365-d adjusted YWT, and 0.05 kg/d (P < 0.02) increase in birth to yearling ADG. Horn flies indirectly had a negative impact on preweaning performance of calves from certain cow sire breeds. However, a positive indirect effect on postweaning calf performance was documented in this study. Postweaning management and compensatory gains may explain the results reported, but continued research of indirect effects of horn flies on calf performance traits is needed.
Keywords: average daily gain, calf performance, horn fly