Cows’ lower critical temperature helps determine increased energy requirements.

by Glenn Selk, Oklahoma State University Extension

Thus far, many parts of the country have experienced a relatively mild start to winter. Nonetheless, colder weather is likely to occur before springtime and green grass. The major effect of cold on nutrient requirements of cows is increased need for energy. To determine the magnitude of the cold’s effect, the lower critical temperature for beef cows must first be estimated.

For cows with a dry winter hair coat, the lower critical temperature is considered to be 32° F. In general, researchers use the rule of thumb that a cow’s energy requirement increases 1% for each degree the wind chill is below the 32° lower critical temperature. In this example, the TV weatherman has predicted that wind chills will average about 4° F. Therefore the calculation example for a cow with a winter dry hair coat would be:

  1. Step 1: Cow’s lower critical temperature is 32° F.
  2. Step 2: Establish expected wind chill from weather reports (4° wind chill in this example).
  3. Step 3: Calculate the magnitude of the cold as the difference between the lower critical temperature and the wind chill: 32°- 4° = 28°.
  4. Step 4: Adjust energy 1% for each degree magnitude of cold, or 28% in this example.
  5. Step 5: Feed cows 128% of daily energy amount. [If cow was to receive 16 pounds (lb.) of high-quality grass/legume hay, then feed 20.5 lb. of hay during the cold-weather event.]

Research has indicated that energy requirement for maintenance of beef cows with a wet hair coat is much greater. Cows that are exposed to falling precipitation and have wet hair coats are considered to have reached the lower critical temperature at 59° F. The fact that it is not feasible to feed a wet, very cold cow enough to maintain her current body condition underscores the need for cows to be in “good” body condition at the start of winter.

In addition, the requirements change twice as much for each degree change in wind chill factor. In other words, the energy requirement actually increases 2% for each degree below 59° F. To calculate the magnitude of the cold when the cow is wet would be the difference between 59° – 4° = 55°. True energy requirements to maintain a wet cow in this weather would be 2% x 55°, or 110%, increase in energy (which would mean more than twice the normal energy intake is needed.)

This amount of energy change is virtually impossible to accomplish with feedstuffs available on ranches. In addition, this amount of energy change in the diet of cows accustomed to a high-roughage diet must be made very gradually to avoid severe digestive disorders. Therefore, the more commonsense approach is a smaller increase in energy requirements during wet cold weather and extending the increase into more pleasant weather to help regain energy lost during the storm.

Cows that were consuming 16 lb. of grass hay per day and 5 lb. of 20% range cubes could be increased to 20 lb. of grass hay offered per day plus 6 to 7 lb. of range cubes during the severe weather event. This is not a doubling of the energy intake, but extending this amount for a couple of days after the storm may help overcome some of the energy loss during the storm. This is done in a manner that does not cause digestive disorders.

The fact that it is not feasible to feed a wet, very cold cow enough to maintain her current body condition underscores the need for cows to be in “good” body condition at the start of winter.

Editor’s note: Glenn Selk is an Oklahoma State University emeritus Extension animal scientist. This article is reprinted with permission from the Dec. 24, 2018, edition of the Oklahoma Cooperative Extension newsletter Cow-Calf Corner.