Review: The paradigm shift in calf nutrition: Redefining energy and protein nutrition in the dairy calf

Calf nutrition research has undergone a paradigm shift over the past two decades, moving from practices focused on cost efficiency and early weaning toward a deeper appreciation of the long-term biological and economic value of early-life feeding. This review synthesizes advances in energy and protein nutrition of dairy calves, with particular attention to how milk allowance, liquid feed composition, weaning strategies, and postweaning diets shape growth, health, and lifetime productivity. Evidence consistently demonstrates that higher planes of whole milk or milk replacer feeding improve growth, welfare, and gastrointestinal development. However, milk replacer formulations often diverge from whole milk in fat, protein, and carbohydrate profiles, raising concerns about metabolic imbalances and morbidity risk. In this review, we highlight how macronutrient quality, fat composition and structure, and protein quality and composition can influence digestion, metabolism, and health. Weaning outcomes are shown to depend not only on the timing and amount of milk but also on the quality and formulation of the entire diet (including starter feed) across preweaning, weaning, and postweaning phases. Starter feed composition, including starch source, effective fiber, protein-to-energy balance, and rumen undegradable protein fraction, plays a central role in sustaining growth and ensuring a smooth transition. Postweaning and peri-pubertal nutrition influence mammary gland development, age at puberty, and body composition, underscoring the importance of aligning heifer targets with lifetime productivity goals. We conclude that calf nutrition should be engineered as a continuum from birth to breeding, with each period's diet designed to prepare for the next. The integration of precision feeding technologies, improved nutrient requirement models, and longitudinal studies that link early-life strategies to lactation and longevity will be critical to refine recommendations. This review positions calf nutrition not simply as a management tool for short-term performance but as a biological lever with far-reaching impacts on lifelong animal health, efficiency, and sustainability.