Genetic Analysis of Count-Based Horn Fly Abundance Phenotypes in Beef Cattle.

Horn flies are a major irritant to livestock. In cattle, horn flies can cause increased heart and respiratory rates, decreased feed efficiency, decreased weaning weights, and reduced milk production, resulting in substantial economic losses. In the United States, horn flies are estimated to cause more than $1 billion in economic losses on pastured cattle, annually. Reliance on management and insecticides has proven inadequate. Additionally, intensive use of insecticides has led to increased resistance in horn fly populations. Thus, there is an increasing need for a long-term solution to the horn fly problem in cattle. Assessing horn fly abundance related phenotypes under commercial conditions is costly and logistically complex. Consequently, the genetic basis of these traits remains largely unknown due to the lack of sufficient phenotypic data. Although several studies reported clear differences in fly abundance within and across breeds, only a few heritability estimates are available based on small-scale studies. This study was conducted during the summers of 2019 and 2022 (two data collections per year) on two University of Georgia associated farms, Northwest Georgia Research and Education center and Eatonton Beef Research Unit. A total of 1,047 cows and heifers were assessed by trained evaluators and several digital images were taken for each animal. Each animal was scored at least twice with some cows scored both years for a total of 4 times during the study period. During each scoring period, the best image for each animal was used to assess fly abundance using direct and sampling-based counting approaches. After edits, the data consisted of 2,197 records collected on 936 cows and heifers. The pedigree consisted of 1,226 animals. A mixed linear model that included year, farm-pen, month, and age (in years) as fixed effects and the random additive and permanent effects was used to jointly analyze three fly abundance related phenotypes. Estimates of heritability of horn fly abundance was between 0.15 and 0.21 for subjective and image-based phenotypes. Genetic correlation was high (0.83) between the image-based phenotypes and moderate between the image-based phenotypes and the subjective assessment (0.61 to 0.65). Repeatability ranged between 0.28 and 0.35 across the three traits. The lower heritability for the subjective assessment points towards added variability due to variation between evaluators. Although the results of this study support the potential to improve horn fly resistance using genetic tools, alternative phenotypes to assess fly abundance are needed. Non-count-based phenotypes could provide an attractive alternative for selecting against horn fly abundance. [ABSTRACT FROM AUTHOR]