The impacts of heat stress on calf performance are sometimes overlooked in the dairy industry. It is believed that calves are less susceptible to heat stress than lactating dairy cows due to their larger surface area relative to body weight, and smaller amount of metabolic heat produced.
However, calves are greatly impacted by the increased ambient temperature and humidity during summer. During hot weather, calves have increased body temperature, especially at night, because they cannot dissipate all the heat accumulated during the day. When ambient temperature reaches 68°F, the calf starts losing water through panting. By 75°F, water loss through sweating has increased considerably. Combined with the reduced grain intake, heat stress results in lower body weight gains and compromised immune systems of pre-weaned calves.
Changes in management should be made to reduce the negative impacts of heat stress on calves. Providing fresh water and shade over hutches; improving airflow; and maintaining clean, dry bedding all are recommended. Additionally, nutritional strategies should be considered to increase the energy consumption of calves under heat stress.
During extreme temperatures -- either cold or hot -- calves utilize extra energy to maintain their normal body temperature. For example, during cold environment, calves increase starter intake to compensate for the extra energy utilized and to maintain normal growth. However, in hot conditions, calf starter intake is depressed and the energy cost to maintain normal body temperature is increased; thus the energy available for growth and development is reduced. Feeding more milk could be a solution to increase energy intake during summer, as it is very unlikely a healthy calf will refuse to drink milk.
Such a strategy to reduce the impact of heat stress has not been widely studied; therefore there are no recommendations of the type and amount of milk to be fed as well as feeding frequency. A recent study completed at the University of Georgia-Tifton Dairy during the summer 2016 examined how much -- and what formulation of -- calf milk replacer optimized calf performance in heat stress. Details included:
- Calves were fed 1.25 lbs./day of traditional, 20:20 (Fat:Protein) milk replacer, OR 1.5 or 1.75 lbs./day of 16:27 milk replacer twice daily.
- Those fed the 16:27 formulation at both feeding levels had improved body weight at weaning than those fed 1.25 lbs./day of a traditional 20:20 milk replacer.
- Feeding 1.75 pounds of solids per day of the 16:27 milk replacer did not show any improvements in body weight or average daily gain compared with 1.5 lbs./day of the same formulation.
In this study, grain intake was similar regardless of the amount or type of milk replacer offered. Unexpectedly, during the first two weeks of life, especially during the second week, calves from all treatments consumed less milk than the amounts offered. When calves reached three weeks of age their intake increased, but it wasn’t until week four that calves drank all the milk offered. Possibly, milk intake was depressed in response to heat stress.
It is important to mention that even though weaning body weight was higher for calves fed 1.5 and 1.75 lbs./day compared to 1.25 lbs./day, they did not double their birth weights. The reduced milk replacer intake during the first weeks of life may have affected overall performance until weaning. In addition, other factors -- such as energy used to maintain normal body temperature and a functional immune system -- may have contributed to the low calf performance. It is vital to consider that some health problems can occur when feeding large amounts of milk twice daily during summer. At the beginning of this study, there was a fourth treatment where calves were fed 2.0 lbs./day of 16:27 milk replacer twice daily. This treatment had to be stopped shortly after several incidences of bloating occurred.
Certainly, more research needs to be conducted to determine the optimal feeding program to minimize the effects of heat stress on dairy calves during summer. Data from this study indicate that feeding up to 1.5 pounds of solid/day of a good-quality milk replacer twice daily improves weaning body weight and average daily gain compared with feeding the traditional 1.0 or 1.25 lbs./day. However, feeding 1.75 lbs./day has no further improvement on body growth and feeding more than 1.75 lbs./day twice daily can compromise calves’ health.
The Georgia researchers are conducting additional research to evaluate the reasons for the lack of response to feeding more nutrients, and the effects of heat stress on calf growth and development.