There are different methods to estimate the grazing activity energy cost (GAEC) of ruminants from heart rate (HR), regarding both treatment differences and contribution to total heat energy (HE). The primary objective of this experiment was to investigate differences between 2 such methods. Ten yearling Boer goat wethers (44.4 ± 0.95 kg) consuming fresh Sudangrass ad libitum while grazing a 0.8-ha pasture or confined in nearby 1.2 × 1.2 m pens were used in a crossover. Heart rate measured over 24 h in 5-min intervals and the ratio of HE to HR previously determined for each animal with a stationary calorimetry system for 24 h while consuming grass hay was used to estimate HE. A GPS collar and leg activity monitor were used when HR was measured to determine HE while resting-lying (L), resting-standing (S), grazing (G), and walking (W); behavior in confinement was L or S. The grazing activity method (GAM) was based on time spent in the different activities multiplied by corresponding HE values, with GAEC assumed the sum of differences between S, G, and W relative to L. The confinement method (COM) entailed subtracting total HE while confined from that when grazing. There were differences (P < 0.01) in percentage of the day spent in the 4 activities (33.8, 53.9, 11.4, and 0.9%; SE = 2.44) and the associated daily HE (241, 322, 75, and 6 kJ/kg BW^0.75; SE = 17.9), although HE per unit time only tended (P = 0.099) to vary (707, 598, 472, and 636 kJ/kg BW^0.75 on a daily basis for G, L, S, and W, respectively; SE = 65.8). Total daily HE (642 and 482 kJ/kg BW^0.75; SE = 17.2) and HE while lying (598 and 450 kJ/kg BW^0.75; SE = 18.1) were greater when grazing than confined (P < 0.01). Daily GAEC was considerably greater (P < 0.01) for the COM vs. GAM expressed in kJ/kg BW^0.75 (165 and 46; SE = 14.0) and relative to HE when confined for COM and of L on a daily basis for GAM (35 and 8%; SE = 3.5). In conclusion, method of estimation can have substantial effect on GAEC. Greater L HE per unit time when grazing than confined may contribute to lower GAEC for GAM than for COM, although factors such as dietary and environmental conditions will influence accuracy of COM.