BEHAVIOR OF GRAZING CATTLE EXPOSED TO SMALL POPULATIONS OF STABLE FLIES (STOMOXYS-CALCITRANS L)

Cattle grazing in the presence of low-level infestations of stable fli es (Stomoxys calcitrans L.) may offset reductions of grazing time by i ncreasing their rate of herbage intake, and this may contribute to the determination of economic threshold populations. On four consecutive days, we released four pulses of 50 or 100 laboratory-reared stable fl ies at 15 min intervals over 1 h in screened enclosures, according to a balanced change-over design. Within these enclosures, fasted, adult Angus cows (Bos taurus L.) (body weight (BW) 465 +/- 30 kg) grazed veg etative tall fescue (Festuca arundinacea Schreb. cv. 'Johnstone') at h erbage dry matter (DM) allowances of 1.4 kg (100 kg BW)(-1)h(-1). Alig hted fly numbers on body surfaces of cows were similar during each qua rter of grazing meals; and numbers of alighted flies increased linearl y as more were released. Cows exposed to colonies of 100 stable flies had 16 alighted on fore legs, 5 on hind legs and 5 on trunks. Stable f lies weighed 6.1 mg before and 17.7 mg after feeding. Fly-induced beha vior of grazing cows did not vary significantly (p > 0.05) within graz ing meals, and releases of up to 100 stable flies caused linear increa ses in movements of heads (to 3.3 min(-1)), ears (to 3.7 min(-1)), ski n twitches (to 14 min(-1) side(-1)) and tail swishes (to 36 min(-1)), but had little effect on movements of fore and hind legs (< 1 min(-1)) . Stable fly releases caused linear increases in herbage DM intake (0. 43-0.53 kg (100 kg BW)(-1)h(-1)) and bite DM mass (from 1.2 g to 1.5 g ). Cows prehended 35, 38 and 31 bites min(-1) when 0, 50 and 100 flies were released. The low levels of stable fly infestation used in this experiment had little direct negative interference on the mechanics of herbage ingestion, as indexed by fly-induced head and leg movements. Favorable conditions may increase the rate of stable fly feeding, shor ten their feeding time and minimize interference with their grazing ho sts. Faster rates of herbage intake of grazing cows in the presence of stable flies were attributed to annoyance.