 Abstract:This study was aimed at studying the biological characteristics of the calves of the Hereford and Simmental breeds, and crosses of these breeds in Siberia. The article discusses the peculiarities of metabolism and utilization of nutrients, growth rate, meat productivity and quality, feed conversion in groups of calves up to 15.5 months of age due to the breed identity in the keeping conditions in light facilities and cold climate. Similarity has been noted in the parameters of gas-and-energy metabolism in Hereford and crossbred calves, including their seasonal dynamics. During the experimental period, a seasonal downward trend was noted in the heart rate of Hereford calves by 11.4 %, and respiration rateby 33.8 %. In the Simmental peers, these figures decreased by 12 and 37.2 %, respectively, in hybridsby 6.1 and 29 %. The seasonal decrease in the body temperature in Hereford and crossbred animals by 2.4 -2.5 %, in Simmentalsby 1.3 %, and increased heat production by Hereford animals and their crossesby 20.9 and 20.1 % (P<0.01), compared to the Simmental peers, were noted. In terms of the content of hemoglobin, Hereford calves (116.5 ± 3.6 g/l) and Hereford × Simmental hybrids (114.0 ± 5.1 g/l) tended to be superior to Simmental calves (103.5 ± 5.4 g/l). No significant difference in the erythrocytes count in the blood of calves was found between the groups. In all ages, Simmental calves were inferior to their Hereford and crossbred peers in terms of the content of total protein and albumin in the blood serum. By the content of globulins in the blood serum, no differences were found between the groups. The coefficients of nutrients digestibility due to balanced feeding was relatively high in all groups. Hereford × Simmental hybrids were superior to their Simmental counterparts by the organic matter digestibility by 3.0 % (P<0.01) . The most significant difference in the live weight was observed between Simmental and crossbred calves (P<0.01). The average daily gain of the hybrids over the entire study period was greater by 25.6 and 15.2 % than that of purebred Simmental and Hereford calves. Accordingly, Hereford × Simmental calves had an advantage over Herefords in terms of the slaughtering weight by 5.4 %, and over Simmentalsby 22.1 %. The energy value of the meat of purebred Herefords and crossbred calves was higher by 8.4 % than in purebred Simmentals. The conclusion has been made about the prospects of using Herefords and their crosses with Simmentals for the sustainable development of beef cattle breeding in the conditions of Siberia. Keywords: beef cattle, Hereford breed, Simmental breed, crossbred animals, energy metabolism, meat yield, feed conversion. G.I. Ragimov, K.V. Zhuchaev, M.L. Kochneva, V.V. Gart, B.O. Inerbaev, G.M. Goncharenko, V.S. Deeva Hereford and Simmental Cattle Breeds in Siberia: Implementation of the Adaptive and Productive Potential in the Cold Climate 9632