REVIEW: New information on the protein requirements and diet formulation for dairy calves and heifers since the Dairy NRC 20011

Research published since publication of the 2001 NRC publication Nutrient Requirements for Dairy Cattle relating to protein needs of calves and heifers was reviewed and compared with requirements from NRC (2001). Experiments used varied intakes or concentrations of CP or varied fractions of CP in the diet relative to energy. Animal requirements were reviewed in 4 categories to identify advances in understanding of nutritional requirements since publication of NRC (2001). Categories included 1) preweaned calves less than 2 mo of age fed milk or milk replacer and starter, 2) calves transitioning through weaning to approximately 4 mo of age fed starter with limited forage, 3) heifers from 4 mo to breeding, and 4) postbreeding age heifers. For calves in category 1, new data estimating optimal ratios of amino acids were identified. For calves in categories 1 and 2, new data estimating optimum ratios of CP to ME were identified but were limited. For heifers in category 3, optimum diet CP:ME appeared similar to NRC (2001). There were no experiments that tested the 70% RDP of CP recommendation for calves in category 3; however, approximately 65% RDP supported more typical dairy heifer ADG than did lower amounts. Few differences from NRC (2001) were found for heifers in category 4. Precision or limit feeding versus more conventional ad libitum feeding programs appears to offer utility to save costs and reduce nutrient and fecal outputs with dietary adjustments to maintain protein intake relative to energy and DMI.