Ionophores are products such as Rumensin, Corid, Deccox, etc., which are coccidiostats. These products are safe for ruminants, but they can be toxic to young calves (under 7 days of age), even at very low levels. There is also the "rumor" going around dairy circles that Deccox will aid in prevention of Cryptosporidiosis (Crypto). This has not really been proved in controlled testing, but some have felt there may be a beneficial effect in dealing with the clinical problem.
All of this has led some producers to feed Deccox to newborn calves to try to prevent Crypto. I would discourage this practice since it is putting these calves at risk of toxicity with very little evidence of real benefit in preventing Crypto.
One of the coccidiostats can and should be used once the calves are weaned and grouped together. The product chosen should be continued for the time directed on the label for the anti-coccidial effect. The calves are highly susceptible at that age and will certainly be exposed to coccidia, so it is reasonable to protect them.
If Crypto has been a problem on the dairy, forget the ionophores (such as Deccox) and concentrate on colostrum, sanitation and nutrition. Those in research, including here at USU, have looked at a whole host of medications and products and have found only a couple that are somewhat effective. These are used for human treatment, but are much too expensive for calves.
Colostrum is important for prevention of most diseases of newborn calves. It is especially important in the prevention of Crypto. Feed at least two quarts of colostrum within one to four hours of birth and repeat that within 12 hours. Some dairies are feeding a gallon of colostrum at the first feeding and find it works well. If problems continue, it may be necessary to sample some calves at 3-5 days of age to determine their absorption of colostrum.
Sanitation is important for Crypto as this organism is very resistant. But in a well done, recent study the main reservoir for Crypto was found to be in the outer two millimeters of plywood on the walls of the calf hutches. There was little in the soil and other sites sampled. Clean, wash, set the calf hutches idle in the sun and perhaps re-paint between use. Pay special attention to sanitation of esophogeal probes used for feeding colostrum or electrolytes, as well as nipples and buckets.
Regarding nutrition, monitor the body condition of the calves and be sure they are getting enough energy for their stage of growth. This is usually the greatest problem during the winter when their energy demands increase so much because of cold, wet, windy weather. Remember, it is still "the eye of the master that fatteneth the calf."©