Behavioural and physiological responses to pest flies in pastured dairy cows treated with a natural repellent

Abstract The effects of reducing fly attack intensity, using an essential oil fly repellent, on fly avoidance behaviour, grazing, milk production and stress was evaluated. Twenty Holstein dairy cows on pasture were divided into two groups of 10, which were alternatively treated with the repellent, or left untreated, over a 9-wk period. Once a week, fly counts and observations of defensive behaviours (full and partial tail flick, skin twitch, head throw, leg stamp, lick side) were performed. Grazing, rumination time and milk weights were recorded and milk cortisol measured. Treated cows had lower fly densities than untreated ones. Treated cows had lower rates of tail flicks (2.73 vs. 5.15), skin twitches (0.96 vs. 3.53), head throws (0.06 vs. 0.53), and leg stamps (0.04 vs. 0.22) per min. Treated cows spent more time grazing (122.7 vs. 104.4 min), tended to spend less time ruminating (11.5 vs. 20.0 min) and travelled shorter distances on pasture (0.27 vs. 0.30 km) per 180-min period. There were no differences in milk yield or milk cortisol between treated and untreated cows. The treatment of cattle with repellent reduced pest fly presence and defensive behaviours, and increased grazing time.