Pretty much every feed that is nationally distributed is milled in the state it is sold (or a nearby state). If you're buying Dumor in California, it's a different feed than people buying Dumor in Iowa, and that's a different feed than people buying it in the Carolinas.
If you're having issues with feeds crumbling, it's generally due to storage/handling issues - it is winter and you may be getting food that has been stored longer/etc. It may be worth trying a different store, or a different brand, but there's a better than average chance that the Purina is produced in one of the same mills the Dumor is, and sourcing ingredients from the same places.
So - try different things, but I wouldn't make any long term decisions either way. These things happen.
I feed Dumor, Purina, and Southern States mostly - depending on what's available, how far I feel like driving, what's on sale, etc - and I've had occasional problems with all of them - as well as with more expensive feeds. They all vary month to month based on grain availability/prices.