1023 The eighth revised edition of the Nutrient Requirements of Beef Cattle: reproduction

The eighth Revised Edition includes updates to the calculation of body energy (BE) and protein reserves in beef females, empty BW (EBW, kg) change per BCS, and Mcal of BE change per BCS. Energy and protein requirements for the maintenance and growth of bulls, heifers, and cows and for milk production remain largely unchanged from the seventh Revised Edition. Replacement heifer target weights at the beginning of the breeding season are unchanged from the seventh Revised Edition (55% for dual purpose or dairy breeds, 60% for Bos taurus, and 65% for Bos indicus), but the eighth Revised Edition model allows the user to change this variable. A more complete description of BCS 1 through 9 has been created, and a BCS decision tree has been added. A discussion regarding the effect of cow nutrition on fetal and developmental programming has been added to the narrative. While the previous body reserves model assumed a variable BW change per BCS, the new body reserves model assumes a fixed BW change per BCS, and it is computed as 7.105% of the empty body weight at BCS 5. Within the model, the user can modify the 7.105% adjustment. For primiparous females, based on limited data, an adjustment factor of 1.6 × 7.105% is suggested for the EBW change needed to increase 1 BCS. Similarly, it is suggested that an adjustment factor of 0.4 × 7.105% be used for EBW change needed to lose 1 BCS for primiparous females. Energy content of 1 kg cow weight gain in the eighth Revised Edition has been changed from a constant of 5.826 Mcal/kg of SBW to a variable number ranging from 3.69 for a BCS 1 cow to 7.99 Mcal/kg of SBW for a BCS 9 cow. The estimated DMI calculation for cows in the model remains unchanged, but a DMI calculation based on neutral detergent fiber (NDF) intake, as a percentage of BW, has been added to the model output for user evaluation of DMI. It is often suggested that 1.1% be used for low- to medium-quality forages. Only minor adjustments have been made to the vitamin and mineral (Co) requirements for reproducing beef females.