The 8th Revised Edition of the Nutrient Requirements of Beef Cattle: Maintenance and growth

Abstract Text: Objectives of this review are to discuss updates to maintenance and growth components of the 8^th revised edition of the Nutrient Requirements of Beef Cattle. From an energy supply standpoint, the traditionally held relationship of DE to ME (ME = DE × 0.82) needs to be reassessed. Maintenance requirements are computed by adjusting the base NEm requirement for breed, lactation, and heat loss vs. heat production (HE), which is ME intake minus retained energy (RE). Adjustments for animal insulation and environmental conditions are considered. The NEm requirement is computed based on the basal metabolism coefficient (a1) and adjustment factors for previous temperature (a2), breed (BE), lactation(L), gender, and previous plane of nutrition (COMP) as follows: NEm = SBW^0.75 × (a1 × BE × L × COMP × SEX + a2), where a1 = 0.077 and is the basal metabolism coefficient in Mcal/kg^0.75 daily; BE is breed factor; L is lactation factor;  COMP = 0.8 + (BCS – 1) × 0.05 and is the NEm adjustment for previous nutrition; SEX is gender effect (1.15 bulls vs. 1 for others); a2 = 0.0007 × (20 – Tp), with a2 being the acclimatization factor in Mcal/kg^0.75 daily, and Tp the previous temperature in °C. The 7^th revised edition of the Beef NRC adjusted the a1 coefficient by 10% for all Bos indicus cattle; in the revised edition, this adjustment is removed for Nellore cattle. Previous adjustments to NEm for cold or heat stress are retained in the revised version, but users are cautioned on applying current equations, and this is an area of research need. Previous adjustments for the physical activity of grazing have been removed in the 8^th revised edition, and additional research is needed on energetic costs of physical activity. Methods to estimate MP for maintenance remain unchanged. Growth equations from the previous NRC were retained. Additional data were used to evaluate equations for predicting retained energy and protein, with resulting satisfactory accuracy for predicting RE, but improvements are needed for predicting retained protein. Serial slaughter data measuring body composition in modern cattle with and without growth technologies are needed. Problems and barriers associated with accurately predicting NE and protein requirements for growth were delineated and discussed.

Keywords:  energy, maintenance, protein