During calving seasons, lots of calves will develop scours – diarrhea - and immediate treatment is critical to calf survival in many cases and treating the scours is not the major problem. The major problem is dehydration so the first item to check is hydration status and the quickest method to check this is the amount of skin tenting and how sunken the eyeballs are into the skull. You can measure skin test on the neck, but I believe the shoulder skin gives you a more accurate reading. It is a good idea to check this on normal calves so you will know when a calf is dehydrated but basically it takes longer for the skin tent to go back to normal in dehydrated calves. These dehydrated calves also become acidotic in which the pH of their blood decreases which leads to increased weakness and inability to nurse. Calves that are minimally dehydrated and not acidotic and not depressed can be rehydrated with oral fluids with an esophageal feeder. For oral fluids, sodium should be about 100 mmols/liter, chloride at about 60 mmols/liter and potassium should be added at about 20 mmols/liter. Also, glucose can be added at the same amount as sodium to provide energy and it aids in sodium absorption. Acetate and propionate are the best ingredients to treat the acidosis and all can be added to the milk replacer if the calf will nurse.
If the calf is down and cannot stand, depressed and has no suckle reflex, intravenous fluid will likely be required to save the calf and your veterinarian will determine electrolytes to add in the iv depending on blood work from your calf. Most vets will add sodium bicarbonate to the mixture to treat the acidosis and as soon as possible, get these calves back on milk replacer for energy.