Changes in milk production, fertility and calf mortality associated with retained placentae or the birth of twins.

The milk yield and fertility records of cows which at any time either gave birth to twin calves or retained their placenta have been examined. Milk yield was higher in the lactation preceding the birth of twins than in any other lactation and the lactation following the birth of twins tended to be longer. Milk yields of cows which retained their placenta were not significantly different from those of cows which cleansed normally. Many cows which gave birth to twins or which retained their placenta were subsequently barren. There was significantly greater mortality among twin calves and after the birth of twins placentae were more often retained.