Effects of heat load on beef cattle activity budgets in a northern temperate grazing system.

Livestock production is increasingly at risk from rising temperatures under global warming. Despite this, how temperature increases impact the behavior of cattle on pasture is not fully understood. This research reports on patterns of beef cattle activity, including step counts and lying time, during the summer and fall grazing seasons of 2021, coincident with an unusual period of elevated temperatures and heat load within a northern temperate rangeland of Alberta, Canada. Beef heifers and first calf cows with calves at side were monitored using IceRobotics leg-mounted pedometers deployed from June 24 to August 26 in summer pasture, and August 27 to November 10 in fall pasture. Heat load conditions were quantified through the temperature-humidity index (THI), which has previously been reported to alter cattle behavior. Cattle exhibited marked diurnal shifts in activity patterns during elevated heat loading periods, increasing their activity (movement) at night and midday, with reduced activity in the morning and afternoon. While heifers had greater step counts in summer than cows, the reverse was true during fall, with cows being more active than heifers and having reduced lying times. A regression tree analysis indicated the specific thresholds for heat load (THI) that alter cattle activity were 54 (lower value) and 70 (upper value), below and above which, movement rates increased with greater heat loading. These results are useful for assessing cattle behavioral responses to weather conditions, including heat load. Further work is needed to understand how cattle activity under open-range grazing in temperate environments alter cattle behavior, stress and beef production outcomes, particularly in the face of escalating climate change.