ABSTRACT:
Hypomagnesemia is a major cause of death in grazing beef cows in countries of the Southern Hemisphere such as Argentina, Australia and New Zealand. Here we review the literature on hypomagnesemia in beef cattle and describe an outbreak in Uruguay. The disease occurred in late autumn, affecting 6 to 11-year-old Aberdeen Angus and Hereford x Aberdeen Angus lactating multiparous cows in good body condition, grazing on natural grasslands that had been improved by fertilization and sowing of seeds of ryegrass and oat. Approximately 40 out of 225 cows were affected and 24 (10.7%) died. A presumptive diagnosis of hypomagnesemia was established based on sudden death, acute neuromuscular clinical signs, epidemiological data, and the response to Mg administration. The diagnosis was confirmed by detecting low Mg concentrations in serum (0.47-0.57mmol/L), vitreous humor (0.47-0.80mmol/L), aqueous humor (0.66mmol/L) and cerebrospinal fluid (0.59mmol/L). The largest component of the diet corresponding to fast-growing and exuberant forages of ryegrass and oat had high concentrations of K (3.48%), N (4.38%) and P (0.94%), suggesting secondary hypomagnesemia. In addition, the K/(Ca+Mg) ratio was 2.38 in forages of ryegrass and oat (reference value: 2.2), and 0.15 in the soil (reference value: 0.09), which represent risk factors for hypomagnesemia. In conclusion, hypomagnesemia is an important cause of mortality in beef cattle grazing improved natural grasslands in Uruguay and it can be easily prevented by correct seasonal Mg supplementation.
INDEX TERMS:
Hypomagnesemia; grazing beef cattle; improved natural pastures; mortality; ryegrass; oat; Uruguay; cattle