Avaliou-se o grau de infestação natural por ectoparasitas em fêmeas bovinas de corte de três grupos genéticos. Foram realizadas contagens, de julho de 2003 a dezembro de 2004, de carrapatos (Rhipicephalus (Boophilus) microplus), moscas-dos-chifres (Haematobia irritans) e bernes (larvas de Dermatobia hominis) em fêmeas dos grupos genéticos Nelore (NE), Angus x Nelore (AN), Canchim x Nelore (CN) e Simental xNelore (SN) infestadas naturalmente. Foram feitas de 6 a 10 contagens em cada animal, totalizando 5.384 observações em fêmeas de sete estádios fisiológicos (bezerras, novilhas vazias, novilhas prenhes, vacas primíparas, com ou sem bezerro, e vacas pluríparas, com ou sem bezerro). Os dados, transformados por log10 (n + 1), foram analisados pelo método dos quadrados mínimos, com um modelo estatístico que incluiu os efeitos de grupo genético da fêmea, de animal dentro de grupo genético (erro a), de ano-época da contagem, de estádio fisiológico e da interação grupo genético x ano-época da contagem. A diferença entre os grupos genéticos dependeu do ano-época da contagem, contudo, em geral, as fêmeas nelores foram as menos infestadas pelo carrapato e as Angus x Nelore, as mais infestadas pela mosca-dos-chifres e pelo berne. Houve efeito de estádio fisiológico da fêmea em todas as três características estudadas, e as diferenças variaram com a característica. Apesar da interação grupo genéitco x ano-época de contagem, existem diferenças entre os grupos genéticos Nelore, Canchim x Nelore, Angus x Nelore e Simental x Nelore quanto ao grau de infestação natural por carrapatos, por moscas-dos-chifres e por bernes.

It was evaluated in this work, the degree of natural infestation by external parasites in beef cattle females from three genetic groups. Countings of cattle tick (Rhipicephalus (Boophilus) microplus), horn fly (Haematobia irritans) and beef-worm (Dermatobia hominis) were performed from July 2003 to December 2004 in females from the genetic groups Nellore (NE), Angus x Nellore (AN), Canchim x Nellore (CN), and Simmental x Nellore (SN) naturally infested. It was realized from 6 to 10 countings in each animal, totalizing 5,384 observations in females at seven physiological stages (calf; empty heifers, pregnant heifers, primiparous cows with or without calf; and pluriparous cows, with or without calf). Data, which were transformed by log10 (n + 1), were analyzed by the least square method with a statistical model that included effects of the female genetic group, of the animal within a genetic group (error a), year-season counting, physiological state, and year-season x genetic group interaction. Differences among genetic groups depended on year-season counting; however, in general, Nellore females were the least infected by ticks, and Angus x Nellore females were the most infected by horn flies and by beef-worms. There was a physiological stage effect of the female in all the three studied characteristics and the differences changed accordingly to the trait. Despite of the counting year-season x genetic group, there are differences among Nellore, Canchim x Nelore, Angus x Nellore and Simmental x Nelore genetic groups regarded to the degree of natural infestation by cattle tick, horn fly and beef-worm.

Resistance of beef cattle heifers to the cattle tick Boophilus microplus was evaluated by artificial infestation of 66 beef cattle heifers of the following genetic groups: 16 Nelore (NE), 18 Canchim x Nelore (CN), 16 Angus x Nelore (AN) and 16 Simmental x Nelore (SN). The animals, with a mean age of 16.5 months, were maintained with no chemical tick control in a Brachiaria decumbens pasture. Four artificial infestations with 20,000 B. microplus larvae were carried out 14 days apart 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 heifer. Data were analyzed as the percentage of return (PR = percentage of ticks counted relative to the number infested), transformed to (PR)¼, and as log10 (Cij + 1), in which Cij is the number of ticks in each infestation, using the least squares method with a model that included the effects of genetic group (GG), animal within GG (error a), infestation number (I), GG x I and the residual (error b). Results indicated a significant GG x I interaction, because AN and SN heifers had a higher percentage of return than CN and NE heifers, while CN heifers showed a higher percentage of return than the NE heifers only in infestations 3 and 4. Transformed percentages of return were NE = 0.35 ± 0.06, AN = 0.89 ± 0.06, CN = 0.54 ± 0.05 and SN = 0.85 ± 0.06.

O objetivo deste trabalho foi estimar as correlações genéticas dos pesos à desmama (PD), aos 12 (P12) e aos 18 (P18) meses de idade com a idade (IPP) e o peso (PPP) ao primeiro parto, o peso adulto (PAD) e os parâmetros A (peso assintótico) e k (taxa de maturação) da curva de Von Bertalanffy de fêmeas, em um rebanho Canchim. Utilizou-se o método da máxima verossimilhança restrita, em análises bicaráter, com modelos que incluíram os efeitos fixos de grupo de contemporâneos e o efeito aleatório genético aditivo direto. Para PD, o modelo incluiu também a covariável idade da vaca ao parto (efeitos linear e quadrático) e os efeitos aleatórios genético aditivo materno e de ambiente permanente. Para PAD, o modelo incluiu também os efeitos fixos de idade da vaca. As médias das estimativas de herdabilidade foram iguais a: 0,37 (PD); 0,31 (P12); 0,35 (P18); 0,39 (A); 0,29 (k); 0,13 (IPP); 0,39 (PPP); e 0,42 (PAD). As correlações genéticas de PD com as outras características foram iguais a: 0,46 (parâmetro A); 0,02 (parâmetro k); -0,12 (IPP); 0,66 (PPP); e 0,42 (PAD). Com P12 as correlações genéticas foram iguais a 0,39 (parâmetro A); 0,31 (parâmetro k); -0,32 (IPP); 0,77 (PPP); e 0,66 (PAD) e com P18 foram iguais a 0,21 (parâmetro A); 0,42 (parâmetro k); -0,29 (IPP); 0,65 (PPP); e 0,60 (PAD). Estes resultados indicam que PD, P12, P18, PPP, PAD e os parâmetros A e k das fêmeas possuem variação genética aditiva suficiente para responderem à seleção massal e que a seleção para PD, P12 e P18 deve resultar em respostas correlacionadas desejáveis em IPP e k, mas indesejáveis em PPP e PAD.

The objective of this study was to estimate the genetic correlations of female body weights at weaning (BWW), 12 (W12) and 18 (W18) months of age with age at first calving (AFC), body weight at first calving (WFC), adult body weight (ABW), and the parameters for mature weight (A) and maturation rate (k) obtained using the Von Bertalanffy model, in a Canchim herd. The restricted maximum likelihood method, with two-trait analyses, was used with models that included the fixed effects of contemporary group and the random additive direct effect. For BWW the model also included the covariate age of cow at calving (linear and quadratic effects) and the additive maternal and the permanent environmental random effects, and for ABW the model included also the fixed effect of age of cow. The means of the heritability estimates were equal to: 0.37 (BWW), 0.31 (W12), 0.35 (W18), 0.39 (A), 0.29 (k), 0.13 (AFC), 0.39 (WFC), and 0.42 (ABW). The genetic correlations between BWW and the other traits were: 0.46 (A), 0.02 (k), -0.12 (AFC), 0.66 (WFC) and 0.42 (ABW). With W12 they were equal to 0.39 (A), 0.31 (k), -0.32 (AFC), 0.77 (WFC) and 0.66 (ABW), while with W18 they were equal to 0.21 (A), 0.42 (k), -0.29 (AFC), 0.65 (WFC) and 0.60 (ABW) These results indicate that selection to increase BWW, W12, W18, WFC, ABW, and parameters A and k should result in direct genetic responses, and that selection to increase female body weights from weaning to 18 months of age should result in favorable responses in AFC and parameter k, but in unfavorable correlated responses in WFC and ABW.