Stable flies ( Stomoxys calcitrans L.) and the behavior of grazing beef cattle

A change-over design with complete balance for first residual effects was used with a tethered grazing technique to evaluate the effects of arthropod ectoparasites on the behavior of adult Angus beef cows ( Bos taurus) (body weight (BW), 465 ± 30 kg) grazing vegetative tall fescue ( Festuca arundinacea Schreb. cultivar ‘Kentucky 31’). The three treatments were: T 1, open grazing plots with exposure to natural ectoparasite populations; T 2, grazing plots with exclosures (5 m × 5 m × 2 m) to minimize natural ectoparasite populations; T 3, similar enclosures containing 2500 laboratory-reared adult stable flies ( Stomoxys calcitrans L.). At the start of grazing, herbage dry matter (DM) mass (> 5 cm) of the 25 cm sward was 2882 kg ha −1 and the herbage DM allowance (> 5 cm) was 1.0 kg (100 kg BW) −1 h −1. Nine flies alighted on cows in open plots (T 1), very few (< 1) on cows grazing in the exclosures (T 2) and 108 stable flies alighted on cows in the enclosures (T 3). Of the latter, there were 67 on front legs, 30 on hind legs and seven on the trunk. Stable fly populations on the body surfaces of cows declined logarithmically during grazing meals. During the first part of grazing meals, stable flies induced cows (in T 3) to move their heads (8 times per minute), and to lift their front legs (8 times per minute) and hind legs (3 times per minute). Stable flies also induced ear movements (12 per minute), tail movements (69 per minute) and skin twitches (25 per minute). Rates of stable fly-induced behavior declined logarithmically during grazing meals. Cows in T 1, T 2 and T 3 visited 4.2, 3.9 and 3.1 feeding stations per minute, respectively, but rates of herbage DM intake were not affected by the presence of stable flies ( P > 0.05) and averaged 0.35 kg (100 kg BW) −1 h −1. In the absence of stable flies (T 2), rates of biting of cows declined logarithmically from 46 to 30 bites min −1. Cows subjected to stable flies (T 3), however, started grazing at 19 bites min −1 and ended grazing meals at 26 bites min −1. Mean herbage DM intakes per bite were 833, 694 and 1011 mg for T 1, T 2 and T 3, respectively.