MULTIPLE RELEASES OF STABLE FLIES (STOMOXYS-CALCITRANS L) AND BEHAVIOR OF GRAZING BEEF-CATTLE

A change-over statistical design with complete balance for first resid ual effects was used with a tethered grazing technique to evaluate the effects of arthropod ectoparasites on the behaviour of adult Angus be ef cows (Bos taurus L.; body-weight (BW), 465 +/- 30 kg) grazing veget ative tall fescue (Festuca arundinacea Schreb. cv Kentucky 31) in scre ened enclosures (5 m x 5 m x 2 m). Starved laboratory-reared stable fl ies (Stomoxys calcitrans L.) were released into each enclosure at 15-m in intervals during 1 h grazing meals. Alighted flies were counted on body surfaces and rates of fly-induced movements recorded every 15 min . Treatments were: T1, 4 x 0 stable flies; T2, 4 x 125 stable flies; T 3, 4 x 250 stable flies. Herbage dry matter (DM) mass (more than 5 cm) of the 25 cm tall sward was 3499 +/- 259 kg ha-1 and the herbage DM a llowance (more than 5 cm) was 1.19 +/- 0.13 kg (100 kg BW)-1 h -1 when grazing started. Surfaces of cows assigned to T3 supported 42 alighte d flies in the first quarter and their numbers increased linearly to 5 3 in the last quarter of grazing meals. Front legs supported 24 alight ed flies in the first quarter and their numbers rose linearly to 35 fl ies in the last quarter. Nine flies alighted on hind-leg surfaces in t he first quarter and their numbers increased linearly to 13 in the las t. Alighted flies on the trunk declined linearly from seven in the fir st to five in the last quarter. Few flies alighted on cows' heads. Rat es of movement of heads, ears, tails, and front and hind legs, and ski n twitches increased (P < 0.05) as more flies were released (T1-T3). F ly-induced movement rates of tail, hind-leg and skin twitches did not vary (P > 0.05) throughout grazing meals. Front-leg movement rates inc reased linearly (P < 0.05) as grazing meals progressed, whereas head a nd ear movement rates increased markedly (P < 0.05) during the last qu arter of grazing meals. Rates of biting in grazing cows declined linea rly (P < 0.05) as more flies were released and as grazing meals progre ssed. Stable flies also caused a linear decline (P < 0.01) in the time spent at grazing stations, and the number of bites and DM mass of bit es taken at each feeding station. Stable flies became more disruptive of grazing processes as grazing meals progressed; increasing numbers o f alighted flies, especially on the front legs, caused head and front- leg movements that interfered with prehension and biting. Herbage DM i ntake rates were 0.49 kg (100 kg BW)-1 h-1 and were not significantly (P > 0.05) affected by treatments, possibly because declining rates of biting were associated with increasing bite mass.