Comparisons of the resistance to Japanese theileriosis among Santa Gertrudis × Japanese Shorthorn F1, Japanese Shorthorn and their reciprocal crosses

Abstract

ABSTRACTTo improve the resistance of Japanese Shorthorn (JS) calves to the hemoprotozoan parasite Theileria sergenti (Ts), Santa Gertrudis (SG) × JS F1 and its reciprocal backcross (F1 × JS and JS × F1) calves were produced. All the calves were born from February to April and kept on pasture with their dams from May to October. Blood samples were collected biweekly until 18 weeks of grazing and two indices of resistance to the parasite, percentage of parasitized erythrocytes (PE) and hematocrit value (Ht) were monitored. There was a weak negative correlation between daily gain (DG) on pasture and the average PE (P < 0.1), and was a significantly positive correlation between DG on pasture and the average Ht (P < 0.01) in the experimental period. Although maximum PE observed at six or eight weeks of grazing period was not significantly different among the four mating types, SG × JS F1 calves exhibited a significantly larger decrease in PE compared to purebred JS after the peak PE. F1 calves also showed maximum Ht through the grazing period. There were no apparent differences in the two indices among the two reciprocal backcrosses and purebred JS. These results indicated that Ts infection and subsequent decrease in Ht affected the DG of calves in the grassland. The present study also demonstrated that the introduction of the SG gene into JS was effective at the F1 level in regard to the resistance to theileriosis, and that in the backcrosses an improvement in resistance could not be expected.