Generally calves out of first-calf heifers are lighter at weaning but do eventually catch up to the calves out of mature cows if they are kept over as yearlings or to finished weights. The most obvious reason that the calves of heifers are lighter is the reduced milk production of two-year-old heifers; however, there are some other factors that may prevent heifer's calves from catching up completely to the mature cow's calves: 1) the colostrum of two-year-old heifers is not as good as that of mature cows; thus, the heifer's calves may have a somewhat compromised immune system that makes them more subject to disease challenges such as respiratory diseases, 2) the calf may have gone through a difficult birth which could have stressed the calf with lasting effects, thus preventing the calf from nursing colostrum fast enough for proper absorption to take place in the gut (only immunoglobulins in colostrum consumed in the first 4 to 8 hours are absorbed), 3) the two-year-old heifer is not as competitive for feed if fed with the mature cows; thus, two-year-old heifers should be sorted off and handled as a separate group, and 4) heifer's calves that are smaller are themselves less competitive in a large group of calves, many of which are bigger and more dominant at the feed bunk, so sorting the calves on size would help alleviate these dominance behaviors.

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