The concentrations of growth hormone (GH), insulin (Ins), tri-iodothyronine (T3) and thyroxine (T4) in blood samples of growing yaks during different bimonthly seasons were determined by radioimmunoassay. The changes of body weight of growing yaks and composition of grass grazed were measured accordingly. The seasonal changes of hormones were significant (P < 0·01 or P < 0·05). Within season, the variances of hormones depended upon the different growing stages. The body-weight gains in the different groups varied in different seasons, increase being significant in May, July and September, decrease being significant from January to May. Correlation analysis indicated that T4 concentration had a significant positive correlation with the body weight of the growing yaks(r = 0·2509, P < 0·05) and other hormones did not have any significant correlation with body weight. The results showed that the annual cycle of weight loss and gain was attributed to the seasonal change of nutrition status. The seasonal change of the assayed hormones depended on the grass growth.

Testicular tissue was studied in domesticated yak and in wild yak x domesticated yak (F1 and F2) bulls at 6, 12, 18 and 24 months of age by stereology for quantitative histology and by comparative studies on the lactate dehydrogenase (LDH) isoenzyme spectrum, LDH activity and the percentage content of LDH isoenzymes in testes. The results indicated that all three types of yak were similar in both the characteristics of ultrastructure of testicular tissue and the degree of germ cell development. The process of spermatоgenesis was initiated and sperm were produced for the first time at the age of 12 months in all three yak types. All yak types exhibited similar age-related increases in weight of testes, volume density of both the seminiferous tubules and seminiferous epithelium and height of seminiferous epithelium but all these indexes were slightly affected by season. The number of LDH isoenzyme bands after electrophoresis varied with age as follows: four bands were present at 6 months of age, five bands at 12 and 18 months and six bands at 24 months of age. The sixth band was LDH-x, which appeared between the LDH4 and LDH5 band. These results indicated that yak bulls reach sexual maturity at 24 months of age and that puberty is not affected by infusion of wild strains.

Female domesticated yaks were artificially inseminated with semen taken from wild yaks and the offspring used to form a nucleus breeding herd of crossbred animals. The meat production of the first generation (F2 × F2) frombreeding within the nucleus herd was assessed by slaughter and carcass dissection of males. One group (no. = 14) was slaughtered at 6 months of age and the second (no. = 12) at 18 months. Crossbred yaks were significantly heavier (P < 0·01) at 6 months (74·7 (s.e. 10·41) kg v. 59·8 (s.e. 10·23) kg) and at 18 months (150·5 (s.e. 56·1) kg v. 117·7 (s.e. 17·4) kg) than domesticated ones. The killing-out proportion (0·47) was similar at both ages and for both groups of animals. Carcass weight, meat weight and the yield of prime cuts were higher for crossbred animals than for domestic yak at both ages.

Wild yak males were crossed with domesticated female yaks to produce a breeding herd of crossbred animals. The milk production of the progeny of this herd was measured in the present study. Yaks were milked once daily after prior suckling of the calf. Yaks were allowed to graze for 15 hiday on high-altitude (3200 to 3500 m) pastures. No supplementary food was given. Average daily milk yield of 1·77 (s.e. 0·16) kg and milk yield over 120 days, 212·2 (s.e. 20·2) kg were significantly higher (P < 0·01) than the corresponding figures for unimproved domestic yaks (1·53 (s.e. 0·10) kg and 184·6 (s.e. 10·5) kg respectively). The fat content of milk from crossbred yaks, 52 (s.e. 2·9) glkg, was not significantly different from that of unimproved animals, 53·5 (s.e. 4·1) g/kg.