"How are you controlling coccidia?" That was my question to the dairyman. The answer was, "I have X coccidiostat in the calf starter grain." Calves are housed individually in hutches, grain is replaced daily for younger calves, for older calves the pail contents are checked daily and more grain added is needed so the calves do no run out of grain before the next feeding. Calves have free-choice water. The dairyman said that the pre-weaned calves did not show clinical signs of coccidiosis although they did have some scours among calves three to four weeks old.
With a bit of calculation we determined the volume of calf starter grain the calves needed to consume in order to get protection from coccidia. The only calves eating enough calf stater grain to provide protection were those whose milk replacer ration had been reduced as the first step in weaning. They were about 35 to 40 days of age.
Thus, while the coccidiostat was present the weak link in the protection protocol was the factor of consumption. Given this one route of administration the calves were left unprotected for the first five weeks of life.
Was it valid on this dairy to assume that exposure to coccidia was low enough that none or very few animals would be exposed to the parasite? Without going too many details, let's just say that while a reasonably good job of sanitation was being done in the prefresh housing, calving areas and calf housing my opinion is that many of the calves are receiving an infective dose of both coccidia and cryptosporidia by the time they are a week old. 
We discussed alternative routes of administering the available coccidia-control medications in addition to the calf starter grain. Since the calves are being fed milk replacer the most cost-effective method in this situation appeared to be the addition of a coccidiostat to the commercial milk replacer - a phone call confirmed that their supplier had the product available. 
The lesson here was that just because a coccidia-control medication is present the effective dose may not be available to the population at risk. Don't be lured into thinking that your coccida control program is effective for all the calves at risk just because you have medication in the concentrate or calf stater grain.