Abstract: The North Tibet plateau is the world highest plateau with a unique alpine grassland and water environment. To obtain a better understanding of the correct supply of Molybdenum (Mo) to livestock in north Tibet, we investigated the content and geographical variation of Mo in different families of pasture plants (n=1017) and water (n=40), then discuss the Cuprum (Cu):Mo ratio in different plant families, and calculate the total Mo intake of the yak in north Tibet. The average Mo concentration in five families preferred for grazing are: Compositae (2.71 μg g^-1), Leguminosae (2.70 μg g^-1), Gramineae (2.48 μg g^-1), Cyperaceae (1.63 μg g^-1), and Rosaceae (1.51 μg g^-1). There was a strong geographical variation in Mo concentration (p < 0.001). The mean value of Mo in north Tibet surface water from 15 sites is 0.89 µg L^-1. The Mo ingestion by yak through these plants and water in north Tibet is about 9586 µg day^-1 which means the toxicity of Mo does not exist in the average daily diet. However, the large geographical variation found may cause some toxicity of Mo in the average daily intake of north Tibet pasture plants in some areas. The Cu:Mo ratio of 2:60 is considered the limit for risk of Mo hyperactivity, while extremely high Cu:Mo ratios may lead to chronic copper poisoning. Our survey of plant samples found 43.29% below and 29.3% above the limiting Cu:Mo ratio of 60 indicating some risk to north Tibet livestock.

Key words: copper, molybdenum, North Tibet, pasture plants, water