Are your calves ready for Ol' Man Winter?

Just like people, calves attempt to maintain a constant body temperature regardless of the outside temperature. Within a certain range of temperatures called the thermoneutral zone or TNZ, calves can maintain body temperature without needing extra energy. The boundaries of the TNZ are called the lower critical temperature and the upper critical temperature. These boundaries are not constant and are not determined by the outside temperature alone. The effective temperature experienced by the calf depends in part on wind, moisture, hair coat, sunlight, bedding and rumination.
During their first month, calves are most comfortable at temperatures between 55 and 70°F. Cold stress in these calves can occur when temperatures remain below 50°F. Between one month and weaning, the comfort zone widens to 46 to 80°F. At this age, cold stress is not likely until temperatures drop below 28°F. Small calves have a larger surface area relative to their weight than larger calves, which allows much more heat to be lost rapidly. Also, as calves reach one month of age they begin to eat starter. Fermentation of this grain in the rumen produces heat. The ruminant furnace can be very helpful in maintaining body temperature as calves grow older.
A clean, dry hair coat provides greater insulation from cold than a wet, matted coat, and calf blankets can be used to further insulate young calves. When using calf blankets, be sure that calves do not sweat under them during the day. The resulting wet hair can quickly chill calves when nighttime temperatures drop. This would obviously negate the positive effects of the blanket. Blankets are most useful for calves less than 3 weeks of age that are not yet eating grain. If calves must lie on a concrete, rock or sand surface, heat will be transferred from their body to the resting area; thick, dry straw or sawdust provides more insulation. In some situations, it may be beneficial to change bedding type with the season, moving to wood shavings and straw as temperatures begin to drop. Straw should be deep enough that a calf's legs are not visible when it is lying down. This nesting effect provides additional insulation. Protect calves from drafts, but be careful not to eliminate all ventilation; fresh air circulation is still required to remove bacteria and irritants that can contribute to respiratory disease.

For housing environment guidelines, see Gold Standards I (for calves from birth to 6 months). You'll also find a helpful wind chill chart in Gold Standards II (for heifers from 6 months of age to freshening).