If a cow has good winter hair, she does fine until temperatures drop below 20 to 30 degrees F. Below that, she compensates for heat loss by increasing energy intake; she must increase heat production to maintain body temperature. Healthy cows, in average body condition and acclimated to cold weather have a “lower critical temperature point” (point at which maintenance requirements increase and you need to feed them more) of about 20 degrees F. Lower critical temperature is defined as the lower limit of the “comfort zone” (below which the animal must increase its rate of heat production; it’s also the temperature at which performance begins to decline as temperatures become colder).
For example, a 1100 pound pregnant cow needs 11.2 lbs. of TDN per day when temperatures are above freezing. If temperature drops 20 degrees below her lower critical temperature, she needs 20 percent more MN or 2.2 more lbs. of digestible nutrients. To supply that, you can feed her 3 lbs. of grain, or 5 lbs. of hay containing 50 percent TDN.
Wind or moisture makes effective temperature (the temperature felt by the body) lower than the temperature on the thermometer. You must figure the wind chill factor when arriving at amount of degrees below a cow’s critical temperature point. For example, a 10 mile per hour wind at 20 degrees has the same effect as a temperature of 9 degrees with no wind. If the temperature drops to zero (or equivalent of zero, with wind chill) energy requirement of a cow increases between 20 percent and 30 percent – about one percent for each degree of coldness below her critical temperature. Cattle can’t eat enough extra feed to compensate for heat production loss at minus 50 degrees F with wind chill; they need windbreaks under these conditions to reduce heat loss during winter storms. During severely cold weather, cattle also need bedding to insulate them from the frozen ground, which will help conserve their body heat.
Cows with normal winter hair coats need about one third more feed when exposed to wind chill temperatures at or near zero. Critical temperature for any cow or calf will vary according to hair coat, moisture conditions, age, size of animal, fatness (fat under the satin is good insulation against cold), length of time exposed to adverse conditions, and amount of wind. A rough rule of thumb to compensate for cold is to increase the amount of feed (energy source) by one percent for each two degrees F of cold stress. For thin cows with poor hair coats, or in wet conditions (wet hair coat) figure a one percent increase for each degree of temperature drop. A wet storm is worse than dry cold. Wet hair loses insulating quality; the cow will chill sooner. When hair coat is wet, the critical temperature is about 59 degrees F.