Derivation of the maintenance energy requirements in growing Jersey heifers

Maintenance energy requirements for growing dairy heifers have not been experimentally studied in great depth since the mid-1980s, and to our knowledge, no direct measurements of maintenance have ever been conducted on growing Jersey heifers. The maintenance energy requirement of growing animals is first fulfilled before energy use for gain. As a result, current estimates of maintenance are needed to effectively model gain. Therefore, the objectives of this study were first to use indirect calorimetry to estimate maintenance energy requirements and fasting heat production (FHP) of growing Jersey heifers and second to compare NEM via indirect methods of regression from the data associated with metabolizable energy for maintenance (MEmaint) versus that estimated using direct measures of FHP. Eight nonpregnant growing Jersey heifers (9.53 ± 0.114 mo, 239 ± 14.5 kg) were enrolled in the study and fed at maintenance estimated based on NASEM guidelines (NEM, Mcal/d = 0.10 × BW^0.75, where BW^0.75 represents metabolic body weight) for 24 d. Subsequently, energetic measures were made for 4 d, including indirect calorimetry, intake, and total feces and urine excretion. Heifers were then fasted for 96 h with measures once again taken over the final 2 h. Heat production for growing heifers at NASEM model-calculated maintenance intake averaged 9.55 ± 0.608 Mcal/d, and FHP was observed to be 6.56 ± 0.444 Mcal/d. This resulted in the following representation of maintenance energy requirements: MEmaint (Mcal/BW^0.75 ± SD) = 0.157 ± 0.0056 and NEM (Mcal/BW^0.75 ± SD) = 0.108 ± 0.0051 × BW^0.75. The efficiency of conversion of energy for maintenance averaged 0.684 ± 0.0410. Thus, these estimates of maintenance support those of NASEM (2021), although they are slightly higher.