Just like people, calves attempt to maintain a constant body temperature regardless of the outside temperature. Within a certain range of temperatures called the thermoneutral zone or TNZ, calves can maintain body temperature without needing extra energy. The boundaries of the TNZ are called the lower critical temperature and the upper critical temperature. But these boundaries are not constant and are not determined by the outside temperature alone. The effective temperature experienced by the calf depends on part on wind, moisture, hair coat, sunlight, bedding, and rumination.
When the temperature drops below the lower critical temperature, or LCT, calves must use energy to support basic bodily functions and maintain their body temperature. The LCT is affected by the age and size of calves. During their first month, calves are most comfortable at temperatures between 55 and 70°F. Cold stress in these calves can occur when temperatures remain below 50°F. Between one month and weaning, the comfort zone is much wider and includes temperatures from 46 to 80°F. At this age, cold stress is not likely until temperatures drop below 28°F. The biggest reasons for these differences are in the calf’s size and rumen function. Small calves have a larger surface area relative to their weight than larger calves, which allows much more heat to be lost rapidly. Also, as calves reach one month of age they are typically eating noticeable amounts of starter. Fermentation of this grain in the rumen produces heat. This can be extremely important to the calf as it becomes a ruminant.
The calf environment also affects the LCT. A clean, dry hair coat provides greater insulation from cold than a wet, matted coat, and calf blankets can be used to further insulate young calves. When using calf blankets, be sure that calves do not sweat under them during the day. The resulting wet hair can quickly chill calves when nighttime temperatures drop. This would clearly negate the positive effects of the blanket. Blankets are most useful for calves less than 3 weeks of age that are not yet eating grain. Radiant heat from sunlight also can increase body temperature. Radiant heat loss is another consideration. If calves must lie on a concrete, rock, or sand surface, heat will be transferred from their body to the resting area; thick, dry straw or sawdust provides more insulation. In some situations it may be beneficial to change bedding type change with the season, adding straw as temperatures begin to drop. In addition, drafts must be avoided because they encourage heat loss.
Most feeding programs are designed to limit the amount of milk or milk replacer fed to calves in order to encourage grain intake and rumen development. In addition, young dairy calves have very little stored fat they can use for warmth. As a result, cold weather can place extra demands on the calf. We can help the calf cope with cold stress by increasing her feed to provide extra energy. Remember, if calves are fed less energy than they need to meet their increased maintenance needs, they will lose weight. In addition, the stress of using body tissue to maintain energy levels causes the immune system to be depressed and less responsive to challenges. So we may need to feed extra to account for the amount of energy the calf spends keeping herself warm.
The 2001 Nutrient Requirements of Dairy Cattle (NRC) provides equations to estimate how much extra starter or milk replacer would be needed as temperatures drop. The LCT used by NRC are conservative; for calves less than 3 weeks of age the LCT is 59°F, and for calves over 3 weeks the LCT is 50°F. Although energy requirements may start to increase below these temperatures, it is likely that normal feeding practices prevent true cold stress until temperatures fall further.
The NRC equations were used to develop the table below, which shows the additional amount of feed (starter, milk replacer, or milk) that a calf would need to eat to compensate for extra energy used to keep warm during cold weather. The table assumes that all of the additional energy is provided from one feed source, not a combination. While increased energy can come from milk or grain, calves less than 3 weeks of age often do not eat enough starter to provide much extra energy. For these calves, the best way to provide extra energy is by increasing the amount of milk or milk replacer fed. With milk replacer, it is recommended that the volume fed be increased as well as the amount of powder. This enables you to maintain the same dry matter concentration. The additional milk can be offered in an extra feeding or added to the regular feedings. Based on the requirements calculated from NRC, one additional feeding (0.5 pounds of powder) of 20% protein, 20% fat milk replacer will meet the added maintenance energy needs of calves when the temperature drops to 20°F. At -20°F calves would need 2 additional feedings. Additional amounts would be similar for waste or whole milk containing 3.5% protein and 3.9% fat.
Of course, repeated changes in the calf’s diet to accommodate changing weather are not practical for you or desirable for the calf. Consider using calf blankets or additional bedding to get through times of temperature transition with frequent fluctuation. Feeding changes are more effective when cold weather sets in for at least a week and daily highs remain below freezing.
Increased Dry Matter Intake from starter, milk replacer, or milk needed to compensate for increased maintenance energy needs during cold weather.
|Temperature °F||Calf < 3 weeks old1
|Calf < 3 weeks old1
- Milk Replacer4lb/d
|Calf < 3 weeks old1
- Milk5 lb/d
|Calf > 3 weeks old2
|Calf >3 weeks old2
- Milk Replacer4lb/d
|Calf > 3 weeks old2
- Milk5 lb/d
1Calf < 3 wk: body weight 100 lb; LCT 59°F
2Calf > 3 wk: body weight 110 lb; LCT 50°F
3Starter containing 1.12 Mcal/lb of NEm, 18% protein
4Milk replacer containing 1.86 Mcal/lb NEm (20% protein, 20% fat)
5Milk containing 2.13 Mcal/lb NEm (28% protein, 31% fat on a dry matter basis; 3.5% protein, 3.9% fat as fed
Calves that are eating starter, especially those over 3 weeks of age have a lower LCT and can more easily cover their increased energy needs by voluntarily eating more grain. An additional 0.6 pound of a typical 18% protein calf starter will meet increased maintenance needs 20°F. If calves consume an extra pound, they can meet additional energy needs down to 0°F, and eating 1.4 pounds more will provide enough energy at -20°F. Notice that table values for both milk and grain intake are in addition to normal milk or starter intake.
The sooner calves start eating grain, the more benefit they will get in terms of generating heat. Anything we can do to encourage starter consumption will have a positive effect on calves’ ability to withstand cold temperatures. Offer small amounts of starter during the first week of life and be sure to have water available to all calves because drinking water stimulates starter intake. In cold weather, provide warm water three times per day for a minimum of 30 minutes each time to ensure calves have ample opportunity to drink. One of the most common challenges calves face in the winter is getting enough water—it’s hard to drink ice! Added benefits of relying on starter intake, rather than feeding extra milk, are improved consistency in the calf’s diet, lower feed costs, and better labor efficiency. In addition, reducing the temperature loss from the calf by means of dry adequate bedding that has insulation properties and reducing drafts on the calf will reduce the maintenance heat requirements of the calf.
In conclusion, the first step to maintaining calf health and growth during the winter is checking the calf housing to ensure that it provides a comfortable environment that reduces heat loss as much as possible. Consider calf blankets, particularly for young calves housed outside. The next step is to evaluate the calf feeding program and ensure calves have early access to grain and water.