Can cows self-select nutrients in just the right amounts to satisfy their needs? Having seen how they gorge themselves on barley, suffering acute acidosis and (in one case) death as a result, I would say not.
However, Mark Bader is a man who thinks they can, specifically when balancing their mineral requirements. Mark is President of "Free Choice Enterprises" and has carried out some fascinating work into the subject. His website can be found at http://www.freechoiceminerals.com/choose.php
Mark Bader in full flow, explaining his theory of self-selection minerals, among other things
The result of his hypothesis and subsequent investigations is that he likes to give cattle free access to fifteen different mineral formulations, plus salt. Greg Judy had one of Mark's self feeders in with his cows. Interestingly, Greg says it was noticeable and measurable that the cows changed the types and quantites of each mineral depending on the paddock being grazed. Put them into an old, stockpiled paddock and the ratio of the amounts of each mineral consumed would be different from if they were grazing, say, a lush green pasture with seedheads just forming.
Mark Bader-designed mineral self-feeder. The tray is covered with a heavy rubber lid, attached along the centreline of the feeder. The cattle soon learn to lift the rubber lid to gain access to the minerals underneath
The compartments below the rubber lid. There are eight compartments on each side of the feeder, each one holding a different mineral mixture or salt. The feeder is on skids and is dragged along by the ATV into each grazing paddock
Mark argues that an excess of one type of mineral has serious consequences as it almost invariably locks up some other vital trace element causing a 'deficiency'. He makes the point that providing a 'compound, balanced' mineral block might actually be giving the cows too much of something and may be doing the cattle more harm than good.
It's an interesting theory and one I would love to explore further. I did think about putting some of Mark's minerals into my suitcase to try out here in the UK, but wasn't sure what Customs and Excise would make of a carrier bag of white powder stuffed between my dirty laundry and my wash bag....