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Studies were undertaken to assess low residual feed intake (RFI; efficient, EFF) and high RFI (inefficient, INE) cattle grazing behavior and performance on sagebrush steppe rangeland in Idaho. Global positioning system (GPS) and 3-axis accelerometers were used. As nonlactating 2-yr-old cattle grazed dormant forage, INE cattle lost more body weight (BW; P < 0.05) than EFF cattle. Subsequent research revealed that INE cattle tended (P = 0.07) to spend more time search-grazing than did EFF. Lactating cattle were evaluated during late spring and summer over 2 yr while grazing pastures with slopes approaching 60%, but mostly 5 to 25%. When temperatures were mild in late spring, INE cattle grazed 1.7 h/d more than EFF cattle (P < 0.05), starting earlier and grazing later into the morning. As summer temperatures increased, no differences in morning grazing occurred (P > 0.10), but INE cattle started grazing later in the afternoon (P < 0.05) than did EFF cattle. On days when cattle were in mild heat load (MHL), INE cows spent more time resting at lower elevations (P < 0.05). Inefficient cattle appeared to engage in more (P < 0.10) search-grazing than did EFF cattle. In a 4-d spring and summer forage intake study in 2016 with the same cows, no differences (P > 0.10) occurred for either forage intake or harvest rate. Treatment groups had similar (P > 0.10) milk production, BW and body condition. A genome-wide association study identified genetic markers associated with terrain use during MHL, but not RFI efficiency. In a 2021-2022 study evaluating 3- to 8-yr-old lactating EFF and INE cows and their calves, both INE cows and calves tended to rise earlier (P < 0.10) in the morning to start grazing when grazing flatter meadows in July 2022. This relationship did not hold true on steeper pastures, though INE cows tended (P = 0.08) to graze later into the morning. Daily activity was strongly correlated with MHL in July 2022. When the effects of MHL were removed by wind and cooler temperatures, INE cows walked 1.5 h/d more (P < 0.01) than EFF cows with their calves accompanying them. On days with increasing heat load, EFF cows tended to walk more (P < 0.10) than INE cows. On the day following the long travel day for INE cows, calves from INE cows remained a greater distance (P < 0.01) from their dams at lower elevations (- 290 m) than did calves from EFF cows (+ 0.1 m). Calves from EFF cows utilized more south facing slopes (P < 0.05) than did calves from INE cows in July of both years. The EFF cows and their calves appear to tolerate > MHL than do INE cows and calves. [ABSTRACT FROM AUTHOR]