Estudo da infestação de fêmeas bovinas de corte pelo Rhipichepalus (Boophilus) microplus, Haematobia irritans e Dermatobia hominis
Abstract
In tropical regions, resistance of cattle to external parasites is an
important factor determining efficiency of the production systems. The objectives in
this study were to evaluate the degree of resistance of Nelore (NE), Canchim x
Nelore (CN), Aberdeen Angus x Nelore (AN) and Simmental x Nelore (SN) females
of several ages to cattle tick (Rhipicephalus (Boophilus) microplus), horn fly
(Haematobia irritans) and beef-worm (Dermatobia hominis), and the possibility of
selection to increase resistance of beef cattle to these parasites, through the
estimation of genetic parameters of the degree of natural infestation, in order to
furnish information to beef cattle breeding programs in Brazil. Two experiments were
carried out. In experiment 1, 16 NE, 18 CN, 16 AN and 16 SN 16.5 months old
females, were artificially infested with 20,000 larvae of cattle tick, four times 14 days
apart each, were done, and from day 18 to day 22 of each infestation the number of
engorged female ticks (≥ 4.5 mm) was counted on the left side of each animal. In this
experiment, data were analyzed as the percentage of return (PR = percentage of
ticks counted relative to the number infested), transformed to (PR)1/4, using the least
squares method with a model that included effects of genetic group (GG), animal
within GG (error a), infestation number (I), GG x I, and the residual (error b). In
experiment 2, from six to ten cattle tick, horn fly and beef-worm countings on NE
(184), CN (153), AN (123) and SN (120) naturally infested females of several
physiological states (calves, pregnant and open heifers, primiparous cows with and
without a calf and pluriparous cows with and without a calf), from July 2003 to
December 2004, were done. In this experiment, data, transformed to log10 (n + 1),
were analyzed by the least squares method with a model that included effects of
genetic group (GG) of female, animal within GG (error a), year-season of counting
(YS), physiological state, and GG x YS interaction. Besides that, genetic parameters
of the degree of infestation by the parasites were obtained by the restricted maximum
likelihood method, using models that included fixed effects of contemporary group
(genetic group, year-season of counting) and physiological state, and additive direct,
animal s permanent environmental, and residual random effects. In artificial
infestation, despite the GG x I interaction, in general, NE animals were more
resistant, followed by CN, and, at last, by SN and AN ones, which showed,
respectively, the following percentage of return: 0.35 ± 0.06, 0.54 ± 0.05, 0.89 ± 0.06
and 0.85 ± 0.06. In natural infestation, the difference among genetic groups
depended on year-season of counting, however, in general, NE females were less
infested by ticks than females of the other genetic groups, while AN females showed
higher infestation by horn flies and beef-worm than females of the other genetic
groups. Heritability and repeatability estimates based on one-trait analyses were 0.12
and 0.12, 0.30 and 0.30, and 0.04 and 0.12, for infestation by cattle ticks, horn flies
and beef-worms, respectively, indicating that it is feasible to obtain genetic progress
for infestation by horn flies. The genetic correlations among these traits were low,
except that between infestations by horn flies and beef-worms (0.60).