Differences in Milk Production Efficiency and Physiology Between Lactating Holstein Cows and Yaks Born and Kept at About 4000 m of Altitude When Fed the Same Diet.

Yaks (Bos grunniens) have developed numerous mechanisms to cope with hypoxic environments. Still, as their milk yield is low, the use of production systems building on dairy breeds at high altitude might increase efficiency, but this has not yet been explored. The purpose of the study was, therefore, to quantify the differences in performance and physiology between Bos taurus breeds and yaks when kept under hypoxic conditions for their entire lifetime. We compared 20 mid-lactating Holstein cows and yaks each born and kept at about 4000 m of altitude using exactly the same experimental diet. The Holstein cows produced 5.7 times as much energy-corrected milk (ECM) at a feed efficiency (kg ECM/kg dry matter intake) being 2.63 times that of the yaks (both p < 0.001). Ruminal ammonia (p < 0.001) and blood urea nitrogen concentrations (p = 0.008) were higher in Holstein cows than yaks. Neutral detergent fibre digestibility was lower (p = 0.017) in the Holstein cows (p < 0.001), but not organic matter digestibility. Their blood had lower haemoglobin, haematocrit, and red blood corpuscular indices (all p < 0.001), a higher respiratory rate (p < 0.001), lower O₂ saturation (95 vs. 99%; p = 0.001) and higher HCO^3- (p = 0.023) and total CO₂ levels (p = 0.005) than that of the yaks. Immune system-related traits did not differ, and most indicators of metabolic energy supply were similar in both animal types. In conclusion, the Holstein cows outcompeted the yaks in milk production efficiency at high altitude, but this at cost of mild hypoxia during lifetime and the problem that establishing this production system required a diet where all components had to be transported for long distances from the lowlands.