Abstract
ABSTRACT: This study evaluated the effects of seasons and latitude on tick counting and determined the best model to estimate genetic parameters for tick count and hair coat. Records of animals naturally exposed to ticks on farms in several Brazilian states and in Paraguay were used. The ANOVA was used to verify the effects of seasons and latitude on the tick count trait. Spring was the season with the highest average, followed by summer and autumn, which showed no differences between them. The winter presented the lowest average values. Latitude -11° had the highest mean value followed by latitude -18°. The Bayesian approach was used to evaluate tick count and hair coat and to identify a suitable model for estimating genetic parameters for use in genetic evaluations. The data were analyzed using an animal model with four different specifications for “fixed” purposes. The inference was based on a Markov chain Monte Carlo (MCMC). The criteria for selection of the Bayesian model indicated that the M1 model, which considered the breed composition in the contemporary group, was superior to the other models, both for tick count and hair coat. Heritability estimates for tick count and hair coat obtained using the M1 model were 0.14 and 0.22, respectively. The rank correlations between the models for tick count and hair coat were estimated and reordering was verified for tick count. The estimated genetic correlation between tick count and hair coat traits was negative (-0.12). These findings suggest that different genes regulate tick count and hair coat.
Highlights
Brazil is the largest beef exporter and producer in the world
The resistance to the tick Rhipicephalus (Boophilus) microplus is among the traits of economic importance for genetic evaluation in beef cattle
We evaluated four models differing in the composition of the contemporary groups: M1 – contemporary group (CG) formed by the combination of the effects of year and season of birth, sex, farm, breed composition, management group, latitude and date of counting; M2 – the breed composition was considered in classes outside the CG; M3 – the CG was equal to M2, but the breed composition was defined as a covariable; M4 – considering two CGs: CG1 - considering the year and season of birth, sex, farm, management group and date of counting; and CG2 – considering only latitude and breed composition
Summary
Animals with productive potential and adapted to the environment are needed. In this context, the resistance to the tick Rhipicephalus (Boophilus) microplus is among the traits of economic importance for genetic evaluation in beef cattle. V.51, n.9, diseases affecting livestock, pets and humans because of their ability to host and transmit disease-causing organisms (GASPARIN et al, 2007). These include pathogenic protozoa, rickettsia, spirochaetes and viruses (JONGEJAN & UILENBERG, 2004). The tick causes a losses are more likely to be in the vicinity of US$22–30 billion dollars in livestock yearly, these economic losses were estimated considering the total number of animals at risk and the negative effects of parasitism on cattle productivity, based on known yield losses in untreated animals (LEW-TABOR & RODRIGUEZ VALLE, 2016)
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