North Dakota State University has done quite a lot of work in this area. Generally, creep feeding does not seem to pay. There may be a couple of exceptions. 1) in drought conditions when cows are not producing enough milk (but NDSU also highly favors early weaning in those times) 2) for fall calving herds studies have shown that creep feeding may pay in certain years if the calves are being sold in the spring and are feedlot bound. However, the result indicate that creep feeding fall calving herds does not pay if the calves are being kept over the summer on grass. Compensatory gains -- the calves make up the difference faster on grass if they have not had grain. My bottom line -- I don't like to creep grain, but do provide creep hay for fall calves in the winter. This is so they don't have to compete with mama around the bale feeder. Also the university in at least one study showed that creep feeding fall calves show less winter weight loss in the cow herd (likely because the calves left more hay for the cows -- forage competition works both ways).