I have heard of grass tetany before but not familiar with exactly what it is. I found this pretty straightforward explanation:

Grass Tetany

Thank you for the heads up on this. Especially important in that it seems to affect cows nursing younger calves especially. The article says it often affects "the best cows in the herd".

Since nitrogen fertilization of pastures increases the chance of grass tetany this may be another reason that pastures should be fertilized in June, after the first flush of grass, not ahead of it in early spring.

Jim at Dawn