Invited review: Unlocking growth and development potential in dairy calves through precision protein feeding

Optimizing protein nutrition is pivotal for enhancing growth, metabolic efficiency, and long-term productivity of dairy calves. This review summarizes current research on precision protein feeding based on the NASEM (2021) model, which aligns MP and ME with calf developmental stages to optimize nutrient utilization. In the preweaning phase, the CP requirement of Holstein calves (<125 kg BW) increases with increasing ADG, with a 50-kg calf requiring 102 g of CP (18.2% DMI) for 0.2 kg/d ADG and 315 g of CP (25.5% DMI) for 1.0 kg/d ADG, with ME intake increasing from 2.56 to 5.66 Mcal/d. The review underscores the importance of adjusting the CP-to-ME ratio; adequate supply of limiting AA such as Lys, Met, and Thr; and a balanced ratio of RDP and RUP to optimize growth and metabolism. Amino acids are essential for muscle development, immune response, and enzyme function. Protein intake in the preweaning phase improves growth performance, weight gain, and feed efficiency, although excess protein in the diet can reduce overall growth efficiency. The balance between RDP and RUP is critical for microbial protein synthesis and nitrogen retention, with RDP becoming the dominant protein source after weaning due to increased reliance on microbial protein. Furthermore, macronutrient-specific adjustments in milk replacers, such as the addition of protein, fat, or lactose, lead to different metabolic responses, with protein primarily supporting lean tissue building, whereas fat and lactose promote energy storage. These findings emphasize the need for precise nutritional strategies that consider protein quality and AA availability to ensure optimal growth and metabolic efficiency. Furthermore, protein levels in calf starter diets, ideally between 20% and 23% CP (DM basis), lead to optimal growth, especially when combined with moderate to high milk feeding, although higher protein levels are not always growth promoting, highlighting the need for precise nutritional adjustments according to the developmental needs of the calf. For growing heifers, protein nutrition after weaning continues to be a critical factor in optimizing growth, efficiency, and later lactation performance. Stage-specific adjustments to the balance between RDP and RUP are important to support microbial protein synthesis and nitrogen retention, with RDP becoming the dominant protein source as heifers mature and transition to a more energy-driven fat deposition phase. These findings are critical for refining protein and energy utilization strategies, optimizing calf development, and supporting long-term lactation performance. In summary, this review provides insights into the optimal protein feeding strategies to improve calf growth and metabolic efficiency in the pre- and postweaning phases.