Cold stress increases a cow’s energy requirement and can pull down her body condition. Thin cows can result in weak calves being born in the spring and/or poor breed up.
Winter storms have already swept across the Plains, giving indications this could be a long, cold winter for cows already thin due to summer drought stress.
The threshold at which cattle have to start using energy to maintain their body temperature is called the lower critical temperature. Cows in good condition (BCS 5.0 on a 1 to 9 scale) that have a heavy winter coat that is dry do not need to use energy to maintain body temperature until the wind chill index is below 19 degrees F. Providing wind protection can decrease energy needs by removing wind as a factor. If cows have protection from wind, then the ambient temperature can be used to determine energy needs. So, providing wind protection in the winter can be huge for reducing supplementation needs due to cold in the winter.
To figure out how much more energy a cow needs, you would take the cow’s LCT minus the wind chill index (if no windbreak is provided) or ambient temperature (if wind break is provided) and that would tell you the percent increase in energy requirement. There is a 1% increase in energy needs for every 1degrees below the LCT. For instance, if ambient temperature is 21degreesF and wind speed is 10, the wind chill index (WCI) is 11 degrees F. For a BCS 5 cow with a dry winter coat with a LCT of 19 degrees F, then 19 LCT- 11 WCI = 8% increase in energy needs. You can look up the energy needs of various classes of cattle here. A 1,200-pound dry cow in late gestation has an energy requirement of 13 pounds total digestible nutrients and the cold described above increased this need by an additional 8% which is equal to 1 pound of TDN for a total of 14 pounds per day.
Thin cows: A snowball effect
It is important that producers assess body condition score now, as many cows came through the fall thin (< BCS 4) and winter has had an early start in many regions. Body condition is a risk management strategy and affects the LCT. A thin cow with a BCS 4 with a dry winter coat has a LCT of 27 degrees F vs the 19 degrees F of a cow in BCS 5. Thus, if the wind chill index was 19 degrees F, a cow in BCS of 5 would not be using any energy to stay warm and a cow with a BCS 4 would be using 8% more energy to stay warm. So, getting cows into good condition early in the winter can be useful for managing risk of bad weather in that they have condition they can lose but also because cows with higher BCS will lose less with the same amount of energy intake than those with lower body condition. A 1,200 pounds gestating cow can only eat about 31 pounds of medium quality hay which is about 26 pounds DM. If you fed medium quality hay (53% TDN on DM basis) to a cow that was in a BCS of 4 when the windchill was 19 degrees F, she would not be able to eat enough hay to meet her energy needs and would be losing weight while the BCS 5 would be gaining a little condition. A practical management strategy may be to consider putting thin cows in a group with your first calf heifers as both have higher energy requirement in the winter, which can allow for strategic feeding of higher quality forage or supplementation when out grazing.
Wet hair coats result in huge energy loss
It is also important to understand that a wet hair coat ahead of a snowstorm exacerbates the situation. A wet coat increases the LCT of a cow in good condition to 53 degrees F. Thus, essentially anytime a cow’s coat is wet in the winter, they will be using energy to maintain body temperature. Therefore, in winters with more precipitation, especially freezing rain, we often see greater decreases in BCS.
Meeting cow’s energy needs during cold stress
It is not advisable to change rations daily but for extended cold and/or wet periods consider feeding more of the same ration, assuming cattle can eat more of the typical ration. If not, then providing a supplement is a good idea. When feeding lower quality hay, dormant range grazing or corn stalk grazing, additional feed will be needed. One option is to change to feeding a higher quality hay source, if available. Free choice high quality hay (58% to 60% TDN) can work down to temperatures of 34degreesF below the LCT of the cow (or -15 degrees F for cows in good condition with dry hair or 19 degrees F with wet hair). If cows are grazing, then supplementation with a high energy feed may be desirable. While corn can be used to provide more energy, it comes with risk. Feeding more than 2 to 3 pounds per head can decrease forage digestion, especially if the forage is lower in protein. Feeding corn with some alfalfa on low protein forges can mitigate this issue. With 3 lbs of corn one could make up the difference of about 15 degrees F between the LCT of the cow and the wind chill index temperature, which for a cow in BCS of 5 with a dry coat, corn supplementation would cover the increased energy requirement down to 5 degrees F or for a cow with a wet hair coat to only about 38 degrees F.
Distillers grains are another option. Distillers is a good source of energy. It has more energy than corn, and because it is high in protein, it does not cause as much of a substitution effect (will not decrease intake of the forage much). In the example above where the cow needed an extra 1.2 lbs of TDN, feeding 1.2 lbs (as-fed) of dry distillers would provide the extra energy needed. In the case of distillers and gestating cows, the pounds of TDN needed to account for energy used due to cold stress would be equal to the pounds of dry distillers that would need to be fed. Limitations on the amount of distillers that could be fed would be more based on budgetary concerns than digestive effects.
When wind chill temperatures are extremely cold or the cow has a wet hair coat, a lot of supplement would be needed to make up the greater energy needs and maintain body condition. For instance, if the wind chill was -10 degrees F and the cows had a wet hair coat, then 8.9 lbs of dry distillers would be needed to account for the increased energy requirement. However, feeding these levels is likely impractical. A better approach would be to provide a smaller amount of supplemental feed and to continue to feed the extra feed after the weather has moderated to allow cows to regain energy lost during the storm.
It is also important to remember that lactating cows have a much greater energy requirement than pregnant cows. Given this, the combination of cold stress and lactation can pull down BCS quickly. If lactating cows are also subjected to cold stress, increasing their energy intake prior to observing loss of condition is advisable.
