The nature of many beef producers is the morning of every new winter storm to rush out and feed the cows something extra.
In truth, this may be backward thinking, says Glenn Selk, Oklahoma State University emeritus extension animal scientist. He notes the major effect of cold on the nutrient requirement of cows is an increased need for energy. When cattle face extreme cold or even worse -- cold and wet conditions -- they really cannot keep up with these demands for energy. Therefore, Selk says, they should be fed better on either side of the storm to help overcome the condition losses they will surely suffer.
Here's his explanation: To determine effects of cold weather, 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 degrees. Researchers generally use the rule of thumb that cow energy requirements increase 1% for each degree the cold or wind chill is below the 32 degree lower critical temperature.
In this example, the predicted wind chills will average about 4 degrees F. Therefore the calculation example for a cow with a dry winter hair coat would be done this way.
Step 1: Cow's lower critical temperature is 32 degrees F.
Step 2: Expected wind-chill from weather reports will be 4 degrees with wind chill.
Step 3: Calculate the magnitude of the cold as the difference between the lower critical temperature and the wind chill: 32 - 4 = 28 degrees
Step 4: Energy adjustment is 1% for each degree magnitude of cold or 28%.
Step 5: Feed cows 128% of daily energy amount. This says if a cow was to receive 16 pounds of high-quality grass-legume hay; she would need 20.5 pounds of hay during the cold weather event.
Further, research says energy requirement for maintenance of beef cows with a wet hair coat is much greater. Cows with wet hair coats are considered to have reached their lower critical temperature at 59 degrees. Even worse, their requirements change twice as much for each degree of change in wind-chill factor, meaning their energy requirement actually increases 2% for each degree below 59 degrees. To calculate the magnitude of the cold when the cow is wet you would use the above process, but use 59 degrees instead of 32, and your cow energy requirements would be multiplied by 2% instead of 1%. This means those cows facing a 4-degrees wind chill with wet hair coats would need a 110% increase in energy, which Selk notes is more than twice their normal energy intake.
"This amount of energy change is virtually impossible to accomplish with feedstuffs available on ranches," Selks says. "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 common-sense 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."
He adds these calculations show us it is not feasible to feed a wet, very cold cow enough to maintain her current body condition, and that underscores the need for cows to be in good body condition at the start of winter.