Energetics of grazing cattle: Impacts of activity and climate1

Ruminants, because of their unique ability to convert forages and byproducts into high quality and nutrient dense foods for human consumption, are expected to fill increasingly significant and sustainable roles in meeting increasing world food demands. In ruminants, a high amount of metabolizable energy use ( > 50%) goes to maintain parent populations and over 70% of whole-herd energy expenditure in beef cattle can be associated with maintenance. Identifying and quantifying processes that consume energy associated with maintenance and productive functions create opportunities to enhance livestock management and food production. Our current understanding of animal energetics is built on relationships between metabolizable, retained, and heat energy (ME, RE, and HE, respectively; where ME = RE + HE). In extensive grazing systems, beef cattle are subject to environmental extremes that increase energetic requirements. Existing equations for estimating NEm for physical activity of grazing are likely better suited for cows during their acclimation to the grazing environment and do not accurately predict observed outcomes in cattle acclimated to extensive grazing conditions. Estimates of NEm expenditure for grazing activities of mature well-adapted cows appear substantially less than predicted by traditionally used equations. Impacts of climate on grazing energetics are associated with seasonal changes and temperature extremes. However, currently available equations do not accurately account for how animals modify their behavior to mitigate impacts of temperature extremes. Recent work suggests that cattle behavior, including positioning relative to slope, solar radiation, and wind can substantially reduce energy expenditure associated with cold extremes. Additional research is needed to more clearly define the impacts of physical activity and environmental extremes, particularly in extensive grazing situations. Data in this regard should provide a platform for more efficient beef cattle production and, consequently, contribute to meeting increasing world food demands.