Basically, it's just chicken feed and is mainly ground up corn. The 'mash' part refers to the consistancy itself, there is also 'crumbles' which is coarser/larger The mash gives the best results as it's pretty fine, just a bit coarser than corn meal.
I learned about it from bait worm farmers as the worms love it like junk food and it really gets them large and fat. The stuff added for the "laying" part (mainly Ca for shell development in eggs), seems to have the same effect on worms and cocoon production. Gets them fat AND horney. LOL It's also one of the main ingredients in Worm Chow.
The best part (and a drawback), is the cost. Here, a 50lb bag is a whopping $12. The drawback is finding anything smaller than a 50lb bag. The only place in town that sells smaller amounts charges $1/lb and telling me the whole time while measuring it out, that they have 50lb bags for $12. Every other place just offers the 50lb bags. One grocery store even carries it in the pet food section. It is certainly not hard to find.
Whatever. 50lbs is still a metric shit ton of worm food and airtight storage containers, even Homer buckets with lids will end up costing more than the feed itself. Plus, there are many more critters besides worms and chickens that love the stuff, especially mice, squirrels and chipmunks. Found that out 3 winters ago, when someone got into the garage to get warm and stayed for all the free food.
If you snag some (and I totally recc you do), especially with more than 1 bin going, the only thing you should never do, is bury it. It will sour and become toxic. Just sprinkle on the surface only and light enough where you can still see plenty of bedding. It will help absorb moisture and is usually gone in just a few days. The bait farmers usually feed again as soon as it's gone. If the bedding is drier, a very light misting kicks it into gear. You'll be able to tell pretty quick if it's sucking up moisture or not.
Give it a try, you'll be glad you did.