To prevent pasty butt, you want to check your brooder temps. This is the typical reason for developing this issue, although sometimes there are digestive reasons. If they are too warm or too cold, they can get this problem. You can usually see ones standing around like little sargeants or huddled if they're chilly. You will usually see them trying to get away from the light or sleeping a lot or even panting if they're too warm. If they're active and showing no signs of temperature issues, then it could be the food. I recommend only feeding baby chicks a formulated chick feed for proper growth and development and until they're digestive systems are working well. There are some natural remedies, as mentioned (apple cider vinegar, yogurt and dry milk) that can help. Other treats can cause more problems. I hope this helps.