It is a win-win situation, once cows reach their maturity, you don’t pay “growth tax” anymore, and also, the feed efficiency of a mature cow is better compared to a heifer. It also allows you to use part of the herd to breed hybrid bulls. Raising a heifer costs 2,000 USD, but the price of a heifer on the market is 1,300 USD only. Also, you spread the environmental impact of calf rearing over more lactations, which lowers the methane emission per kg of milk produced during the lifetime of a cow.