The interplay of feeding and genetics on heifer rearing and first lactation milk yield: a review.

Weight gain of young dairy replacement heifers, even over several months, is highly variable, lowly heritable, and a poor indicator of either weight at calving or first lactation milk yield. Voluntary forage consumption is much less variable, more heritable, and a better predictor of weight at calving and milk yield. Weight at calving is highly heritable but not correlated genetically with milk yield. Heavier weight at first calving due to feeding and management does increase milk yield. When grain is fed according to milk yield, weight loss during the first month or two after calving and milk yield are under strong genetic control and tightly genetically correlated to each other and to energy intake. The feed to milk ratio is managementally determined. Better-grown heifers eat more forage and can tolerate more grain consumption without digestive or metabolic disorders. Good body condition at calving seems essential for high milk yield. Genetic ability to mobilize body condition to increase milk yield justifies more grain and results in greater total efficiency up to the point of causing digestive disorders. Good heifer rearing is essential to high milk yield and feed efficiency. Realizing genetic potential for high yield demands excellence of feeding and management during lactation.