Abstract:
The objectives of the study were to estimate genetic parameters for tick resistance and
to evaluate the effect of the level of tick infestation on the estimates of genetic
parameters in South African Bonsmara cattle. Field data of repeated tick count records
(n = 11 280) on 1 176 animals were collected between 1993 and 2005 by ten breeders
participating in the National Beef Recording and Improvement Scheme. The
distribution of tick count records were normalized using a Box-Cox transformation.
Data were divided into 7 sub-data sets based on the mean tick count per contemporary
group, to facilitate the investigation of the effect of level of tick infestation on the
derived genetic parameters. A repeatability animal model including the fixed effects
of contemporary group and age of animal at tick counting and random effects of the
direct additive genetic, permanent environmental and residual effects was used to
estimate genetic parameters using REML procedures. The additive genetic variances
for tick count ranged from 0.01 to 0.08. Variances for the permanent environment
ranged from 0.00 to 0.03. Phenotypic variance decreased with increasing mean tick
count level while additive genetic variance increased with increasing mean tick count
level. The heritability also increased with mean tick count level until a mean tick
count level of ≥30. The highest heritability estimate obtained in the current study was
0.17 for data with mean tick count level ≥25. These results suggest that sufficient
genetic variation for tick count exists in the Bonsmara cattle. Therefore genetic
selection for tick resistance is feasible even though genetic progress may be slow.
