| Phillip Lancaster, MS, PhD Ruminant nutritionist Beef Cattle Institute Kansas State University palancaster@vet.k-state.edu |
Cattle have a thermal neutral zone of 60 to 75 °F with a summer hair coat meaning that outside this range (below or above) the animal must use additional calories to maintain it’s body temperature. This increases the maintenance energy requirements of the animal. As fall weather cools off cattle develop a thicker hair coat and the lower critical temperature drops to 20 °F as long as they are dry and out of the wind. Thus, through most of the winter in a lot of areas of the country, cattle maintenance energy requirements do not need to be adjusted.
But, with the single digit and subzero temperatures we have had lately, cattle maintenance energy requirements can increase 50 to 150% depending upon temperature, wind speed, and wet vs. dry hair coat (Table 1). But how do you know how much supplemental feed is needed, if any, to meet the energy requirements of the cow when environmental conditions dictate? Using an example of a 1,400-lb cow in late gestation (Figure 1), we can see the calculations to meet her energy requirements for maintenance and pregnancy at different maintenance energy requirement multipliers.
At 1.0X NEm, the cow is required to consume 22.7 lb of hay (57% TDN) to meet her energy requirements and is expected to consume 28 lb of hay. Thus, she can easily meet her energy requirements at this level of maintenance as total NEm intake is greater than NEm Req. However, at 1.5X NEm, hay alone is not able to meet her energy requirements Hay Req is greater than Exp. Hay DMI. Initial calculations indicate that 2.25 lb of supplement (80% TDN) would meet the deficiency in energy requirements which balances Total NEm Intake with NEm Req. However, the cow cannot just consume all the hay + supplement as there is only so much rumen capacity. Assuming 1 lb of supplement replaces 0.75 lb of hay in the diet, we can see that the cow needs 4.2 lb of Adjusted Supplement Intake plus 24.8 lb of Adjusted Hay Intake to meet her energy needs. This level of supplementation is within normal range of supplementing beef cows during the winter.
At greater levels of maintenance energy requirement multipliers (2.0X and 2.5X NEm), the ability to meet energy requirements at economical amounts of supplement becomes compromised. The Hay Req drastically exceeds the Exp. Hay DMI by 12 and 20 lb/day. The Adjusted Supplement DMI to meet energy requirements is 14.6 and 25.0 lb/day with 17.1 and 9.3 lb/day of Adjusted Hay DMI, respectively. These diet proportions are tantamount to starter and growing diets in the feedyard.
When environmental conditions are expected to increase maintenance energy requirements greater than 50%, especially for extended periods of time, measures other than or in addition to supplemental feed should be used to minimize the increase in maintenance energy requirements. The first priority, although somewhat difficult to achieve, is to keep cattle dry, which means protecting them from rain and snow both from above and below. Constructing some type of shelter such as a temporary shed or roof could be used, clusters of evergreen trees can also keep snow off cattle, and provide bedding so cattle are not laying in mud or snow. The second has a smaller effect but is easier to achieve in most cases than a dry hair coat and that is to get cattle out of the wind. Constructing stacks of large round hay bales, providing rows of evergreen trees, moving cattle to pastures with low lying areas, or constructing permanent man-made wind breaks are all possibilities to decrease the wind chill on cattle.
