Milk production traits of beef cows as affected by horn fly count and sire breed type.

Horn fly infestations negatively impact economic traits of beef cattle. Our objective was to determine the impact of horn flies on beef cattle milk yield and quality. Cows (n = 53) were derived from Brangus dams and sired by Bonsmara (n = 7), Brangus (n = 13), Charolais (n = 8), Gelbvieh (n = 5), Hereford (n = 12), and Romosinuano (n = 8) bulls. Horn fly counts and estimates of milk yield and quality were recorded monthly from May through October on individual cows. Data for milk yield and quality and horn fly count were analyzed by mixed model least squares using a linear model including sire breed, cow in sire breed, month, and month × sire breed. Effects of horn fly count on milk yield and quality were estimated by analysis of covariance using log horn fly count and log horn fly count × sire breed. Horn fly counts varied by month (P < 0.0001), with lowest population recorded in May (99 flies) and peaked in August (520 flies). Bonsmara and Gelbvieh sired cows had greater (P < 0.05) milk yield compared to Hereford sired cows (8.75 and 8.62 vs. 6.02 kg/d, respectively). Milk yield from Charolais, Romosinuano, and Brangus sired cows was intermediate (7.28, 7.00, and 7.06 kg/d, respectively). Regression of milk yield on fly count differed (P < 0.05) among sire breeds. Milk yield was reduced (P < 0.05) by 0.99 and 0.64 kg/d per unit increase in log horn fly count in Gelbvieh and Bonsmara sired cows. Regression coefficient for milk yield on log horn fly count was reduced (P < 0.01) in Gelbvieh sired cows when compared with Brangus, Charolais, Hereford, and Romosinuano sired cows, and coefficients for Bonsmara sired cows were reduced (P < 0.05) in comparison to Brangus sired cows. Increases in log fly count were associated with decreases (P < 0.05) in milk fat, solids-not-fat, and milk urea nitrogen. Milk yield was reduced (P < 0.05) by increased fly numbers depending on month of lactation and interaction with log horn fly count. In May, June, and July milk yield was reduced 0.72, 0.68, and 0.71 kg/d per unit increase in log horn fly count. Our results indicate that horn fly infestations reduce milk yield and quality of spring-calving beef cows depending on sire breed and month of lactation. Development of sustainable beef production systems may include selecting breed types whose milk yield and quality is less influenced by horn flies, allowing for better expression of genetic potential for milk yield in nutritionally challenging environments.