It has been well documented that pre-weaned calves raised during summer have reduced gains. The reasons are multifactorial and include pre- and postnatal heat stress, flies, and wet bedding result of rainfall. Heat stress seems to be the main culprit however, with mechanisms to increase cooling (i.e. respiratory and cutaneous evaporative cooling) triggered when ambient temperatures exceed 20 °C. These mechanisms use energy and result in more energy for maintenance and less energy available for growth.