ABSTRACT.- Feed and energy efficiency have been topics of extensive research in recent years, with residual feed intake (RFI) being the preferred index to determine feed efficiency in beef cattle. Nevertheless, the biggest downside of RFI is that its measurement requires accurately determining individual feed intake, which leads to the need to carry out RFI trials in confinement conditions. In this context, residual heat production (RHP), defined as the difference between observed and predicted heat production, presents an alternative to determine efficiency in grazing conditions as it is independent of feed intake. This work aimed to evaluate the repeatability of RHP and study its relationship with metabolic, endocrine, and reproductive variables. Seventy-four Hereford heifers were used, and at the beginning of the experimental period, heifers were 18 ± 0.7-month-old and weighed on average 278 ± 26 kg. Heat production (HP) was determined at the beginning (March to May; fall) and at the end (September to November; spring) of the experiment using the heart rate-O2 pulse technique to classify heifers according with the residual heat production (RHP). Efficient heifers showed lower HP (P