Milk production and left displaced abomasum: cause and effect relationships.

In a Danish dairy herd affected with a number of cases of left displaced abomasum the milk production during the lactation periods of affected cows was retrospectively compared to that in two groups of non-affected cows. Group A contained all cows calving at the same time as the cases, and group B had one cow for each of the cases of similar age and with similar preceding production level. The average 305-days milk production was 610 kg lower among the cases than among cows in group B, as a best estimate of the amount of milk lost for each case of left displaced abomasum. The difference between mean production in groups B and A was 183 kg, indicating a higher than average production potential among cases, since matching on preceding production level had been carried out between the cases and the cows in group B, but not in group A. It may therefore be concluded that high yielding cows are at excess risk of left displaced abomasum, which in turn leads to a considerable loss of milk among affected cows.