Blood hormonal and metabolite levels in grazing yak steers undergoing compensatory growth

Abstract Yaks ( Bos grunniens ) are adapted to a harsh alpine environment above the altitude of 3000 m, and show strong compensatory growth ability. The objective of this study was to investigate the plasma hormonal status and metabolite levels of growing yak steers when either offered supplements (SP) or not supplemented (NS) during the annual season of severest pasture shortage in late April and early May. The experiment was conducted between 23 April (day 0) and 8 July 2013 (day 76). Twelve 3-year-old yak steers were allocated to one of two treatments, either to receive supplements for 25 days or to remain unsupplemented. The supplement, which was composed of ground maize (75%) and local rapeseed meal (25%), was offered at maintenance level of energy and protein to the group of yaks receiving supplements from 23 April to 18 May (day 25) at evening. All the yaks were released to pasture during the daytime. Rectal temperature was measured fortnightly using an electro thermometer. Heart rate was recorded over four 4-day periods using modified heart rate monitors. Live weights (LW) were weighed weekly in the morning. Afterward, blood samples were drawn before release to the pasture to measure blood glucose (GLU), blood urea nitrogen (BUN), nonesterified fatty acids (NEFA), β-hydroxybutyrate (BHBA), growth hormone (GH), cortisol, the thyroid hormones (triiodothyronine [T 3 ] and thyroxine [T 4 ]), insulin and insulin-like growth factor 1 (IGF-1). The supplemented yaks showed a higher growth rate and compensation ability compared with the unsupplemented yaks. The overall mean heart rates of the unsupplemented yaks were higher than those of the supplemented yaks, and were higher in both treatments in July than in June. There were significant treatment effects on the concentrations of IGF-1, insulin, NEFA and BUN. Whilst supplementation reduced the concentration of IGF-1, it increased the blood concentration of insulin. The differences of blood NEFA between treatments during feed restriction period disappeared in forage growing period. BUN concentrations of SP were significantly lower than NS from day 8 to day 28. Thereafter, BUN concentrations of yaks on the two treatments were similar. The results indicate that supplementary feeding to alleviate nutritional stress in yaks during the period of lowest herbage availability is critical for improving their ability to replace body tissue mobilized over the previous winter months.