As usual, I think it depends on your area and what you are trying to do. Spring calving in the south would be warmer, and possibly drier? Not so much here, so you have the calves born in either cold weather ( for us) or wet and possibly cold rain too ( later in the spring, Marchish), plus mud from freezing and thawing. When grass comes on here, with calves born in March or so, the grass gets a head start and they're not picking much that I can tell, until fescue starts tapering off due to heat usually, sometimes there is a shower to two to keep it going very well. Fall calves are born usually to very nice weather. Early calves might need to watch for maggots, but nights are cooling off, and usually very few cool rains until Nov., when I'm done calving. No snow that amounts to anything until Dec. Calves are good size and very capable of utilizing fall stockpiled fescue by Dec if it's available, and at spring greenup they are packing it on. I'm sure there is a cost to a lactating cow, but you also suffer a period of loss of gain in late summer that the dry weather is hurting the wsg and fescue hasn't taken off yet for the spring calvers. For me to sell the same weight calves, Id have to winter the spring calves or sell lighter calves when the price is lower. The same time all the spring calves are coming to town. As it is, I sell early fall also, but I'm in a different weight range. So, for here, pick your poison. You want to winter wet cows or you want to winter spring calves to sell as 800 lbers because they don't gain much in the heat and fescue. I have my first heifers in with bull now, so my first spring calvers of my own. I am not sure I will last, but I needed to try it due to some other issues arising with my current numbers.