wanted to add fyi. salt is combined with minerals for animals because there have been no proven studies showing that animals can self regulate anything but salt. They crave salt when they need it but they do not crave, for instance, magnesium or pottasium or copper rich foods or supplements when they need it. So it is very important that your salt mineral supplement have balanced ratios of all the minerals
in comparison to the salt
and each other. Other factors to consider are enviromental conditions. If your
water has a high iron content or your animals are drinking out of old zinc plated troughs the excess of those minerals in the rest of their diet can interfere with the absorption of others in the mineral or
feed. you can for instance be using a good supplemental copper around here but but still have deficient animals because the iron in the water binds to it and prevents its absorption. here is an interesting chart showing some mineral interactions.
http://www.evergreenjuices.com/html/mic.htm