Research has shown that many components of the way we manage calves can have long lasting effects on the dairy animal. Calf health is one of the factors that can have long-term effects on the productivity and therefore profitability of the dairy animal.  There have been very few studies that took these factors and followed dairy calves through to calving, their first lactation, and their complete lifetime production.
Recently, we published the last of a 3-part series of papers in the Journal of Dairy Science where we studied calf nutrition, health, management and other variables to determine their short- and long-term effects on the animal. The study followed 795 calves from 21 farms in Susquehanna and Wyoming counties, Pennsylvania. We began by closely following calves as they were born; keeping track of all feed intake, health, housing, and management practices for their growing period. This was followed by calving data and then lactation records. Sixty eight different variables were followed on all calves and heifers until they began their lactation. Our statistical analysis allowed us to include this large amount of information while accounting for farm differences and at the same time reducing bias caused by missing data. The resulting analysis is more generalizable for outcome measures such as milk production. 
One of the interesting things to note from this analysis is the cost of sick calves and the benefit of treating those sick calves. To explain this in regards to production costs, all calf health incidences, whether respiratory illness, serious diarrhea, or other events, were grouped together. The final analysis of this data showed that for every day a pre-weaned calf was clinically sick, that animal as an adult cow produced 277 fewer pounds of milk in her first lactation (on a 305-d ME basis) than her counterparts that were healthy.

Calves that were treated for being sick had a positive response of 440 pounds of milk. To expand this finding to some common occurrences that were in the range found in the data set; a calf that was sick for 4 days and then treated for 2 days would have a net loss of 229 pounds of milk in the first lactation as opposed to a loss of 1109 if they had not been treated.
This long-term study clearly shows the impact of sick calves, of treating those sick calves, and how this impact is not only in immediate calf costs, but in long-term losses due to lower milk production two years later.  

For more information on the results of this study, see these previous Dairy Digest articles from February and June.

Source: Dr. Jud Heinrichs, professor, Penn State Department of Dairy and Animal Science