Face flies ( Musca autumnalis De Geer) and the behavior of grazing beef cattle

Abstract

Ectoparasites, at or above certain levels, usually reduce productivity of infested livestock, however the causative processes are often poorly defined. Irritation from non-biting flies may, for example, interfere with grazing behavior and reduce herbage intake while increasing the energy diverted to support fly-induced behavior. Zero, 400 or 800 laboratory-reared adult face flies ( Musca autumnalis De Geer), which had been deprived of nutrients and water for 12 h, were released in large (5 m×5 m×2 m) screened enclosures each containing one tethered adult Angus beef cow ( Bos taurus) grazing tall fescue ( Festuca arundinacea Schreb. cultivar ‘Johnstone’). At the start of grazing, the herbage dry matter (DM) mass of the vegetative sward was 3861 kg ha −1, sward surface was at 21 cm and the pseudostem surface plane was at 6 cm. The herbage DM allowance was 1.4 kg (100 kg) −1 h −1. Surfaces of the heads of grazing cows supported about 30 alighted face flies. In addition, 20 and 30 flies alighted on the backs of cows grazing in the presence of 400 and 800 face flies per enclosure, respectively. Few face flies were present on other body surfaces. Heads of cows moved 0.0, 1.0 and 0.7 min −1 in response to the activity of 0, 400 and 800 face flies per cow, respectively. Ear movements were initiated 0.1, 22.2 and 14.3 min −1 per cow exposed to 0, 400 and 800 face flies, respectively. Panniculus reflexes (on one side of the cow's body) were triggered 0.3, 3.5 and 4.1 min −1 per cow when exposed to 0, 400 and 800 face flies, respectively. Face flies (0, 400 and 800 per cow) activated cow tail movements 0.4, 3.2 and 10.1 min −1, but did not induce movements of the front or hind legs. Rates of herbage DM intake increased linearly from 1.69 to 2.91 kg per cow h −1 (0.37–0.59 kg (100 kg) −1 h −1) as the numbers of face flies increased up to 800 per cow, largely because herbage DM intakes per bite increased from 0.74 to 1.36 g. Rates of biting in grazing cows (37 bites min −1) and the time spent at feeding stations (19 s per station) were not significantly affected by face flies. It appears that cows grazed deeper into the sward in an attempt to dislodge face flies from the muzzle and this resulted in heavier bites and, consequently, higher rates of herbage intake.