This is considerably different than the bacterial population found on a high concentrate diet typical of early lactation. It may take ten days to two weeks to change the population of bacteria in the rumen following a diet change. Therefore, if the diet is changed rather quickly to one of higher concentrate, the production of lactic acid can become quite significant before the population of bacteria that metabolize lactic acid can increase sufficiently to prevent a large buildup of lactic acid in the rumen. Therefore the lactic acid that is produced cannot be absorbed as fast as normal and would tend to build up more in the rumen during early lactation. This explains why transition cows are more susceptible to acidosis than later lactation animals.
Cows eat to satisfy their energy demand.
At the time of calving cows are not consuming maximally because there is not the demand for energy that there is when milk production is large. This physical change within the body cavity at calving is likely a significant factor in the incidence of displacement of abomasums early in lactation. In the first few weeks following calving, the high producing cow may loose between twenty and fifty kilograms of body fat, or from one-half to one full body condition score or possibly even more. These reserves can then be replaced later in lactation when demand for nutrients is less intense. If this loss of body fat depots is too rapid or extensive it can predispose the animal to various metabolic disturbances, but in particular ketosis.