In this part of the world, there are usually three or four significant cold fronts during January. One of the most common questions we get during this time is whether or not to temporarily increase feed levels for the cow herd, and if so, by how much.
First of all, that's the right question to ask. In winter, keeping warm is the largest part of a cow' s maintenance requirement, and she' ll use available nutrients for maintenance before fulfilling any other needs. Cattle perform best in a fairly wide zone of temperatures. For a cow in adequate body condition with a normal, dry winter coat, the lower critical temperature (LCT) of her comfort range is about 32 degrees F. Below that temperature, energy requirements begin to increase. If her hair coat is wet, maintenance requirements begin to increase below 60 F.
How much more energy is needed in the face of a cold weather event depends mainly on wind chill and whether the front is wet or dry. According to Glenn Selk, OSU Extension animal reproduction specialist, a pretty good rule of thumb is this: for each degree the wind chill temperature is below the LCT, the energy needs of a cow with a dry winter coat increase one percent two percent if her coat is wet. The following steps are recommended to determine how much additional energy is needed:
- Determine wind chill levels from your local weather report. The National Oceanic and Atmospheric Administration' s (NOAA) Web page (http://www.crh.noaa.gov/jkl/windchill.html) has a wind chill calculation function if you know the predicted air temperature and wind speed.
- Subtract the reported or calculated wind chill from the LCT. (32 F with a dry winter coat; 59 F if wet.)
- Energy requirements will increase one percent for each degree below the LCT if the hair is dry; two percent if the hair is wet. For example, a cold front is bringing 25-degree F temperatures and 7 mph winds. The wind chill, then, is about 17 F, 15 degrees below the LCT of 32 F for a cow with a dry winter coat. If her ration is balanced for her normal comfort range, she'll need 15 percent more of it during the cold snap to hold her own. Now, what if that same front brings precipitation?
Instead of one percent increase in energy needs, cows with wet winter coats need a two percent increase. In the same scenario, the wind chill of 17 F is 42 degrees below the LCT of 59 F. She needs 84 percent more energy than she normally would need. In many instances, it is impractical, impossible or inadvisable to feed her that much additional energy. Digestive disorders can occur if energy levels are increased suddenly and drastically.
Common sense and practical animal husbandry can go a long way in dealing with inclement weather. Windbreaks and shelter can change the calculation factors significantly for the better. Increasing energy intake before, during and after a cold front can help even out excessive deficiencies. Managing body condition to have some additional stored energy prior to the onset of winter can also provide a valuable "cushion" for times of increased energy needs. Also, some research supports the assumption that the lower critical temperature for a cow with a heavy winter coat is 19 F. This factor might fit many English and Continental breeds and crosses in our area in the coldest part of the winter and could change the energy deficiency calculations considerably.