So... last year we tested a few of our cows for selenium and all came back deficient. I offer a "mineral mix" free choice, but clearly it didn't do the job then and I don't see any reason it would have changed over the course of the last year. But I have finally gotten around to addressing the problem by building a cafeteria style mineral feeder. Supposedly cows will readily address their various deficiencies if given access to an array of necessary minerals and vitamins, eating only what they need and leaving the rest. Mineral availability and ratios in pasture plants change through the seasons and according to some sources cows will balance out the feed if something is lacking in a given pasture or hay bale.
Cafeteria style mineral programs are also supposedly less expensive long term since the animals will only eat what they need, not everything at once in an attempt to get their lowest stave higher on the side of the proverbial leaking barrel. Did you follow that metaphor? I thought not. Sorry.
I'm 90% sold on the concept. I've done a small trial run with my bull and two other companions in with him. They clearly have hit the selenium and manganese harder than the other minerals. This is convincing to me because in soils manganese and iron have an antagonistic relationship of sorts. High iron requires higher manganese to get proper uptake into plants. And I have high iron soils without a whole lot of manganese in it. The salesman who sold the minerals claimed that selenium similarly competes with iron and that he sells a lot of selenium into upstate New York because there are many areas with high iron soils and almost everywhere in New York is low in selenium.
The next step is to move the box over to the main herd and see which compartments empty out the quickest, not just show evidence of consumption.
Free Choice Enterprises in Wisconsin is source of the minerals I purchased. I might just try the free choice thing on my pigs. I'm not sure whether they'll be more or less discerning than cows.