Considering the time of year I would say 99% staggers or to give it it's proper name Hypomagnesaemia. This occurs when the intake of magnesium is exceeded by its output. The clinical signs can occur very quickly because the animal does not store magnesium, and is thus reliant on its daily dietary intake. Hypomagnesaemia is most commonly a disease of lactating cows at grass, because grass can be very low in magnesium and the output in milk is high. When the losses in the milk exceed the dietary intake, clinical hypomagnesaemia occurs.
The increase in disease in spring occurs because rapidl