Heat stress is a crucial concern when it comes to lactating dairy cattle, both from a welfare and economic standpoint on our dairy farms. Much research has been conducted to elucidate the effects of heat stress on lactating cows and to develop effective heat abatement strategies.
Many of you will be familiar with a color-coded chart that combines the factors of ambient temperature and relative humidity (RH) to determine levels (mild, moderate, severe, fatal) of heat stress a cow may experience. Knowing the conditions allows for initiation of heat stress alleviation measures for cows. Is it time for calves to have a similar temperature-humidity index (THI) chart just for them?
This question came to mind when a customer asked point blank if I had one to give her that she could post for her calf-feeding team. It would have been inappropriate to tell her to just use the cow chart, for a couple of reasons. Calves have a larger surface area compared to bodyweight than cows, so they can dissipate heat load more readily. They also do not have an active large furnace, aka the rumen, inside of them generating additional heat load.
However, calves may be more at risk for death in very hot conditions, as they do not have the energy reserves of cows and do not have as developed of an immune system to protect them. Since panting is one of the primary mechanisms for heat exchange in cattle, calves with underdeveloped lungs are not as effective at this as cows. They may use much of their energy in the process and are at risk of metabolic acidosis as they blow off carbon dioxide and lose bicarbonate in saliva. For these reasons, I set off to evaluate the literature and data available for heat stress in calves to create a chart that would be more useful when making management decisions in this age group.
Most articles cite a thermoneutral zone for a calf that is between 50ºF and 78ºF (10ºC to 25ºC). This is a very small range; however, the upper limit is roughly 10 degrees higher than that of a lactating cow due to the reasons discussed above. Several studies look at respiration rates to determine the level of heat stress a calf may be experiencing. Without wearable technology containing accelerometers, respiratory rates would be difficult for lay staff to measure.
It is much easier to measure ambient temperature, and there are many cost-effective devices available that will combine temperature with humidity to give a THI number. Across many reference sources, 78ºF seems to be a consistent point at which calves begin to feel heat stress. A temperature of 78ºF combined with RH of 60% (RH is considered comfortable between the range of 30% and 60%) equals a THI of 74. For this reason, anything less than 74 would be green on my chart (Table 1).
One particular research paper mentioned risk for death in calves when temperature reaches 108ºF at RH of 60%. This equates to a THI of 98 and is very similar to the risk for death found on cow THI charts. On my calf chart, anything greater than 98 would be purple. The three zones in between of yellow (mild), orange (moderate) and red (severe) are more difficult to delineate based on the literature that exists.
Perhaps there only needs to be one checkpoint in the middle of the chart, as this is a tool to be used for management decisions. In adult dairy cattle, several checkpoints can be beneficial to make management decisions. For example, fans come on in the first zone, sprinklers in the second zone, and electrolytes are added to the water or feed in zone three. In calves, I might suggest making the facility changes of hutch orientation, shade, air movement (natural or fan), switching from organic to inorganic bedding and adding a daily electrolyte in the first zone (THI at 74 or more).
More aggressive heat abatement actions could be implemented in the second zone (THI at 86 or more). This approach would require fewer zones on a decision-making chart. However, a farm with multiple progressive heat abatement strategies could use a chart, similar to the adult, with the mild (THI 74 to 82), moderate (THI 83 to 90) and severe (THI 91 to 98) zones described above.
Now that we know the conditions, here are some management changes which may be considered when calves fall into the different ranges on the chart.
Avoid sunlight/provide shade
During the winter months, orienting calf hutches to a southern exposure can provide warmth. Keep in mind that direct solar radiation can be responsible for up to 30% of the effects of heat stress animals experience. In the summer months, hutches should be oriented away from direct sunlight shining in. Wood hutches stay cooler than plastic hutches; however, there are many factors to consider when making hutch material choice. Shade cloth over hutches and reflective blankets over individual hutches are other strategies. I have also seen farms move calves into tree lines where they can get natural shade during the summer months. Calf barns can have sunny sides, and addition of shade cloth on that side can prevent direct exposure.
Increase ventilation
Consider prevailing winds when positioning hutches and allow plenty of space between them (4 feet apart, 10 feet between rows). Open all available vents and think about raising the rear of the hutch with 8-inch blocks. In calf barns, sidewall curtains and fans should be managed based on temperature and desired air flow. Automatic controls take the guesswork out, and “greenhouse” type structures will require more attention than solid, opaque structures.
Fresh water/electrolytes
The customer I mentioned above wanted a chart so her calf manager and employees would know when to start offering an electrolyte to the calves on a daily basis. Calves attempt to dissipate heat load by increasing respiration rate or panting. They lose the critical electrolytes of sodium, potassium and chloride during this process. That is why it is so important to offer fresh water continuously and provide electrolytes when it is very hot.
Other thoughts
Consider feeding more milk or milk replacer, either volume or more energy-dense, as calves that pant to dissipate heat will require more energy, and they will tend to eat less starter grain when they are hot. Of course, this becomes a delicate balancing act as price, total solids content and continuing to encourage starter intake are all important factors. Keep starter grain fresh. Use an inorganic source of bedding such as sand that will trap less heat. Do any work on the calves during the cooler morning hours. This may include vaccination, dehorning, pen moves and other management changes.
Yes, calves do deserve their own THI chart. At a minimum, it would draw our attention to the fact that heat stress causes decreased appetite and immune suppression, which will lead to lower average daily gain (ADG) and increased health events in pre-weaned calves. Such a chart would allow calf managers and staff to make critical heat abatement decisions that will protect the health and well-being of their calves.
