Moderation of grazing behavior of beef cattle by stable flies ( Stomoxys calcitrans L.)

Grazing beef cows ( Bos taurus L.) were exposed to stable fly ( Stomoxys calcitrans L.) and their reactions were related to ingestive behavior. Three concatenated latin squares (4 × 4) were used to evaluate the effects of stable flies on the behavior of Angus cows grazing vegetative tall fescue ( Festuca arundinacea Schreb. cv Johnstone) in screened enclosures. Four populations (0, 50, 100 and 150) of starved, laboratory-reared stable flies were released into each enclosure at 15 min intervals during 1 h grazing meals. Mean numbers of alighted stable flies increased linearly with treatment level up to 125 per animal and up to 46 on forelegs and up to 7 on hindlegs. Dorsal and ventral trunk surfaces each supported up to 35 alighted flies. Alighted fly numbers increased on all major body sectors as grazing meals progressed: when 150 stable flies were released, there were 25, 44, 56 and 63 flies on forelegs during successive quarters. Releases of up to 150 stable flies caused linear increases in fly-induced movements of heads (to 1.9 min −1), ears (to 4.5 min −1), skin twitches (to 18 min −1 per side), tail swishes (to 36 min −1), forelegs (to 1.4 min −1) and hindlegs (to 0.8 min −1). Stable flies caused linear increases in herbage dry matter (DM) intake (0.50–0.58 kg (100 kg body-weight (BW)) −1 h −1), and bite DM mass (from 1.0–1.4 g) and a linear decline in rate of biting (from 41-35 bites min −1). Cows visited 3.2, 3.9, 4.3 and 3.4 feeding stations min −1 ingesting 16.1, 12.6, 10.9 and 16.0 g DM per feeding station when four pulses of 0, 50, 100 and 150 stable flies were released, respectively. Blood intake of engorged stable flies declined from 9.3 to 8.1 mg, either indicating competition between flies for feeding stations or deterrent actions of their hosts disturbed their feeding. Moderation of ingestive behavior of cows was attributed to annoyance of low populations of alighted flies. In the presence of stable flies, cattle may accelerate herbage intake and reduce grazing times.