THE EFFECTS OF SELECTION FOR SIZE IN CATTLE ON HORN FLY POPULATION-DENSITY

Statistically significant differences were observed in the population density of the horn fly, Haematobia irritans irritans L., on Angus cow s having significantly different frame sizes. Angus cows, averaging <1 12.5 cm in height at the hip, had significantly lower numbers of horn flies than Angus cows that measured 112.5-117.5 cm, 117.5-120 cm, 120- 126 cm and >126 cm in height at the hip. The Angus I cows (<112.5 cm i n height at the hip) were significantly shorter in length (mean distan ce from the withers to the hip bone) and were smaller in girth than th e Angus II (112.5-126 cm) and Booneville Angus cows (>126 cm). The est imated heritability (h(2)) of horn fly resistance was 0.43 +/- 0.07 an d 0.95 +/- 0.31 for 1989 and 1990, respectively. Horn fly counts on th e Angus I herd (<112.5 cm in height) was 118.1 (probable breeding valu e, PBV = -20.69) to 165 horn flies per cow (PBV = 26.9 flies per cow i n 1989) and from 75.9 (PBV = -29.1) to 134.5 (PBV = 29.5) flies per co w in 1990. Angus I bulls had PBV = -23.7 to 13.4 and from -26.5 to 14. 75 in 1989 and 1990, respectively. The Angus II cows had horn fly coun ts that ranged from 159.6 (PBV of -23.5) to 208.1 (PBV of 25) per cow in 1989 and from 232.3 (PBV of -56.2) to 378.7 (PBV of 90) per cow in 1990. Angus II bulls had PBVs that ranged from -17.1 to 18.9 in 1989 a nd from -28.1 to 48.8 in 1990. The Angus I cows had significantly (P < 0.0001) lower numbers of horn flies (mean of 63.8 horn flies per m(2) ) than the small, medium or large Angus II cows (mean of 129.4, 149.6 and 145.5 horn flies per m(2), respectively). The data indicated that some specific factor(s) associated with cow size contribute(s) to inna te resistance of cattle to the horn fly.