Heat stress starts to harm dairy calves even at about 22 C with a relative humidity of at least 60 per cent. At this humidex level, I have seen many calves become restless, reduce feed intake, and significantly increase water consumption. As temperatures rise (coupled with more humidity), most sooner or later breathe rapidly and then exhibit open-mouth panting, since they don’t sweat. In addition, research demonstrates that their immune system literally shuts down. All the while, their general metabolism shifts away from growth to solely maintaining a normal body temperature. Therefore, it’s our job to implement good calf management, whether they are housed outside or raised in a calf barn, so they remain healthy and keep on growing.
It's rare to see pre-weaned calves die from heat stress-related diseases or extreme fever. What kills many of these baby calves is severe dehydration caused by a loss of more than 10 per cent of their bodyweight in bodily fluids. This heavy loss of liquid bodyweight is primarily made up of cellular/tissue water and dissolved electrolytes. We often see this type of dehydration in sick calves dying of pathogenic or nutritional scours, yet the considerable fluid loss in heat-stressed calves doesn’t come from their guts or intestines, but rather from their lungs and urine.
On my last calf barn tour, I put my hand in front of a panting calf with sunken eyes (a sign of severe dehydration) and my hand got wet within a couple of minutes. That’s because a great amount of water as vapour is exhaled by the calf into the air.
This pulmonary water loss underlies one of the many reasons why calves double or triple their water intake during heat stress and then have frequent urination, which further leads to a dangerous loss of vital electrolytes — sodium, chlorine and potassium. It’s this loss of electrolytes, which are ironically involved in the retention or discharge of cellular/tissue water in pre-weaned calves, regardless of heat stress status.
Electrolyte treatments have been frequently used to save the lives of many heat-stressed calves, yet it’s very important to make sure that their water requirements (10 litres —normal, 20 litres — hot weather) are first met simply by providing extra water in addition to the amount of whole milk or milk replacer being fed. That’s because pre-weaned calves have a natural requirement for water which is usually higher than what is provided by the majority of milk-based feeding programs in normal or hot weather conditions.
Research proves the point
A case in point: A calf research trial (1999) by nutritional supplier APC Inc. measured water consumption in 120 newborn Holstein bull calves. The researchers offered 4.0 litres of “extra water” to each calf on a daily basis in addition to a 6.0-litre milk replacer diet. Their results showed that calf water consumption significantly increased when outside air temperatures rose from 20 to 25 C, which was comparably more than when the temperature rose from 5 to 10 C.
I know of many dairy producers who follow such “extra water” recommendations that parallel this research. At this point, I also see many of them add a packet or scoop of commercial “electrolyte supplement” to the same pails of “extra water.” These salts are typically less concentrated than the “treatment” electrolytes, which the former are fed as an aid to prevent a subtle loss of electrolytes (as mentioned above) that contributes to heat stress dehydration.
Aside from providing enough water and a good electrolyte supplement to any group of heat-stressed baby calves, there isn’t much I would do to change their current whole milk/milk replacer and calf starter feeding programs. Afterwards, I would turn my attention to keeping these dairy calves as comfortable as possible.
For example, if the calves are housed in outside hutches, I would allow an outside area where they can leave the hutch and catch a breeze as well as provide shade to keep them out of direct sunlight. If they are housed in a calf barn, each pen should have good airflow ventilation. Finally, it is important to keep all pre-weaned calves in a comfortable environment; frequently cleaned, appropriately bedded and dry (reduces heat stress). A good fly-control program is also recommended.
Much of these calf heat stress recommendations are similar to those successfully used to combat heat stress in the lactation barn. Therefore, it is important to implement good calf management to help reduce heat stress in order to keep young calves healthy and growing at an optimum level. Even though our summers can be unbearable at times, dairy calves should have a chance to enjoy them.