The Horn Fly and its Association with Milk Yield, Milk Quality, and Staphylococcus Mastitis in Organic Dairy Herds

Summer mastitis continues to be a challenge on many dairy farms. Greater understanding of the factors affecting mastitis will aid in developing management programs. Mastitis and fly populations both increase during the summer months, but the relationship between the two is unknown in lactating dairy cows housed partly or fully on pasture. Our objectives were to 1) determine the association between horn fly numbers, somatic cell count (SCC), and milk yield on organic dairy herds and 2) identify specific Staphylococcus mastitis pathogens (Staph. aureus, Staph. chromogenes, Staph. hyicus and Staph. agnetis) in quarter milk samples and horn fly populations. Four USDA certified organic herds located in Tennessee and Kentucky were enrolled in the study, with an average herd size of 55 lactating cows. Sampling began in May 2019 and continued through October 2019. For the first objective, unexpectedly, no relationship was observed between horn fly numbers and log SCC. Days in milk (DIM) had the most significant effect on the change in SCC throughout the summer. A significant negative relationship was found between horn fly numbers and milk weight. As horn fly populations increased, milk weight decreased. When determining the factors associated with a cow testing positive for Staph. aureus, logSCC (P = 0.08) tended to explain the variation. No factors were associated with a cow testing positive for Staph. chromogenes, Staph. agnetis, or Staph. hyicus. Sex of the horn fly pool and whether it was collected off the back or udder, had a significant association with the type of bacteria present in that pool. Female horn flies were more frequently found with Staph. aureus, Staph. chromgenes, and Staph. agnetis compared to male horn flies. With this information, the reduction in milk weight during the summer might partly be contributed to an increased number of horn flies. All Staph. species tested were carried by horn flies and can be potential vectors for intramammary infections and presence in milk. This study shows the importance of fly control on dairy farms during the summer.