The influence of season of birth and geno- type for GH and LEP genes on meat productivity of Kazakh white headed bulls

The study was conducted in the Orenburg region to scientifically substantiate optimal calving timing and marker-assisted selection strategies for maximising the genetic potential of beef productivity in Kazakh Whitehead bulls, based on assessing the effects of interactions between GH and LEP gene polymorphisms and season of birth. The slaughter qualities and carcass morphology of young animals born in different seasons were assessed in two consecutive experiments. Experiment 1 involved 60 bulls with different genotypes for the GH (LL, LV, VV) and LEP (AA, AV, VV) genes, obtained in autumn calving (October-November), while experiment 2 involved 57 young animals similar to those calved in spring (March-April). Control slaughter was performed at 15 months of age on 9 animals in each experiment (3 animals of each genotype). The GH gene polymorphism was significantly associated with carcass weight and muscle tissue development: in animals with the VV genotype, they were greater than in bulls of the LL variant by 21.0 kg (9.5 %; p < 0.05) and 9.8 kg (13.3 %; p < 0.05), respectively. The LEP gene genotype determined differences in the mass of raw internal fat: the VV genotype exceeded the AA variant by 2.7 kg (30.3 %; p < 0.05). The spring birth season ensured an increase in slaughter weight by 16.9 kg (7.2 %; p < 0.05) and muscle tissue mass in the half-carcass by 5.2 kg (6.9 %; p < 0.05), compared to the autumn one. To confirm the best realisation of the genetic potential of beef productivity in bulls during early spring calving, it is advisable to scale up the study to a larger sample.