Effect of Face Flies on Grazing Time and Weight Gain in Dairy Heifers12

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Abstract
Automatic recorders attached to halters of Holstein heifers were used to sample the grazing behavior of comparable 15-heifer herds, one protected from feeding by Musca autumnalis De Geer, the other exposed to ambient pest intensity. Grazing time and face fly population levels were monitored on 27 days over a 6-week period in 1978, and on 24 days over a 5-week period in 1979. The rate of weight gain for each herd was also determined. To avoid pasture bias, respective herds were switched between comparable pasture swards on a regular basis. The results, which were similar both years of study, indicate that pest-intensity levels averaging 10–11 face flies/heifer had little effect on the diel pattern of grazing but did reduce grazing time by ca. 20 to 25 min/day. Differences in grazing time were greatest on ‘high’ fly days, up to 47 min/day, and during ‘diurnal’ hours. Nocturnal grazing was minimal, averaging 11 to 17 min/heifer/day, and did not differ significantly between herds. The rate of weight gain for heifers in respective herds did not differ significantly either year of study.

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