Not all mineral supplements are created equal, not all pastures are created equal, and not all animals are created equal. The concentration of selenium provided within a block or lose feed is going to vary from 'recipe to recipe' used to create the supplement that is provided. The amount of/concentration of selenium found in the environment is going to be variable from location to location which affects how much additional selenium needs to be supplemented, which goes along with what
@Jeanne - Simme Valley said with "Almost every state in US is Selenium deficient." The amount of selenium required for each animal likely is somewhat variable as well, but can be ignored for all intents and purposes as the requirement variability is negligible. What is variable between animals is the amount of block or free mineral each will consume. That is going to vary from individual animal to individual animal based on preference and taste, as well as the proximity of the block/free mineral to other resources that are used repetitively and frequently. Namely water and shade sources. Don't put water, shade and mineral all in the same place in the pasture. It leads to several things that aren't particularly desirable. Luxury mineral consumption, over use of the pasture a the point where the resources are concentrated, creation of a nutrient sink where the cows poop and pee all the time rather than distribute it evenly across the pasture, lowered feed intake as the cows are being 'lazy' and aren't going to where the grass is, and under utilization of the pasture away from the concentrated resources.
Bo-Se sounds like a good idea. My understanding is selenium deficiencies can affect muscles, the heart is a muscle.
It might not be the problem, but it's "an ounce of prevention". Might I suggest regular, repeated treatments until you sell the calf as an attempt to prevent the sudden demise of the calf due to a selenium deficiency that otherwise might occur and goes unnoticed until it's too late.
Hopefully that solves the problem. And yes, get rid of the cow.
