Basically, ketosis is a disease condition that occurs when the demand for energy needs by a fresh cow are not being met by the energy in what she is eating. Fresh cows are more at risk because they have high energy demands from milk production but they frequently aren't eating well. When this happens, the cow has to mobilize body fat to make up the difference in energy that she needs. The cow's liver can only process a limited amount of fat at a time and one byproduct of this fat conversion is ketones. When the demand for fat conversion is high, the levels of ketones in the cow's bloodstream can get so high that they make the cow feel ill.
The best prevention for ketosis is to have a good transition program for both pre and post-fresh cows that keeps them eating well. Additionally, treating fresh cow diseases such as sub-clinical milk fever, retained placentas and metritis can quickly keep cows eating and avoid the secondary ketosis that results from off-feed cows.