good discussion! the noble foundation has an interesting article done several years ago to dispel some of the myths related to replacement heifers - - including the myth that they have to wean lighter calves.
http://www.noble.org/Ag/Livestock/Heife ... index.html
of course there are too many things that we don't know about this study for it to have value for everyone. we don't know about things like creep feeding or other differences in their management of first calf heifers. many larger operations run them separately so that they are able to provide the first calf pairs with superior nutrition. with the investment required to make a productive cow out of a heifer, that's a good idea if one can do it. but many smaller producers have to throw them in with the cowherd much sooner. many believe that it's best to throw them in with the cowherd after weaning - - "let them learn how to be a cow." i don't think either school of thought is necessarily wrong.
as to the argument that the first calf heifer is still growing, therefore putting more of the resources consumed into growth and less into lactation, that is very true. but it also has some room for variability. was she bred at 65% of mature weight or was she pushed harder to gain an extra 100+ pounds as a yearling? so much of this discussion on growth in the young cow relates to differences in heifer development pre-breeding and early in gestation. some producers push heifer calves harder after weaning or push them harder as yearlings in an attempt to negate some of the growth that would otherwise be required during their first lactation.
it would seem to me that pushing a heifer harder to achieve more of her mature weight at breeding or in the first few months following breeding could be offset with the money derived from expecting her to wean a heavier calf. having more of the growth done pre-breeding or shortly after breeding would also almost certainly have some impact on rebreeding as well. if a young cow with her first calf is still growing to the extent that she weans a lighter calf, wouldn't she also still be growing to the extent that she would have a harder time with rebreeding? for people who breed heifers at lighter weights, how many of them will skip a calf at some point early in their lives and what is the cost of that? some producers are perfectly willing to accept as fact that a young cow that calves early in life with minimal inputs into her development will skip a calf, while others are more demanding. once again, if something is working for you, it's not necessarily 'wrong' to be doing it. i'm a firm believer in people continuing to do what works for them - - just as long as they keep in an open mind to improvements.
but no matter whether it's fertility (breeding at the front end of bull turnout) or production (weaning a calf equal to her more mature herdmates), if we demand more from our replacement heifers instead of making excuses for them, we'll get where we want to be much faster.