Genetic parameters for packed red blood cell volume and daily gain during grazing in Japanese Shorthorn calves facing a natural challenge from Japanese theileriosis

Japanese theileriosis, which is caused by the intraerythrocytic protozoan Theileria orientalis sergenti, is one of the most harmful anemic diseases to grazing calves in Japan. In this paper, we estimated genetic and phenotypic parameters of packed cell volume (PCV) and productivity in 435 Japanese Shorthorn calves, and discussed the feasibility of the genetic improvement for higher anemia tolerance. The heritabilities of PCV at pre-grazing (PrePCV) and minimum PCV during grazing period (MinPCV), age at MinPCV (MinAge), the times of theileriacidal drug treatment (TRT) and daily body weight gain during the grazing period (DG) were 0.54±0.11, 0.13±0.06, 0.21±0.07, 0.35±0.21 and 0.65±0.06, respectively. There were weakly negative genetic correlations between MinPCV and DG. MinAge had a strongly positive genetic correlation with MinPCV, while a strongly negative genetic correlation with DG. TRT had negative phenotypic and genetic correlations with all traits. From the results of simulations of selection, a single improvement in DG would reduce MinPCV and TRT in a population as a correlated response. Although it would lead to a reduction in TRT as a correlated response, it would also lead to temporarily severe anemia. This would require more labor for management of grazing herds and frequent times of sanitary check. On the other hand, a simultaneous improvement in DG and anemia tolerance would be inferior in efficiency of improvement in DG. Instead of a low efficiency of improvement in DG, a simultaneous improvement would decrease TRT more efficiently and increase MinPCV.