The relationship between milk yield and cystic ovarian disease in cattle.

Studies were conducted to understand the relationship, if any, between the amount and pattern of milk yield and the occurrence of cystic ovarian disease (COD) in dairy cattle. More than 70 cows with cysts were compared with herd mates of similar lactation number. Total milk yield in lactations when cysts occurred and in the preceding lactations were not different from the corresponding lactations in control cows. Patterns of monthly milk yield did not differ between the two groups of cows. However, weekly analysis in early lactation revealed differences. Both groups of cows achieved peak yields at a similar time (5.4 +/- 1.2 vs 5.7 +/- 1.9 weeks post partum in normal vs cows with COD). However, owing to an early decline in yield of cows with COD, their duration of peak yield was significantly shorter (P less than 0.05) than in the control cows (3.2 +/- 2.2 vs 5.6 +/- 2.4 weeks). Both the attainment and the decline of peak yield occurred 5.6 (range = 2-15) weeks before the cysts were diagnosed. It is concluded that COD is not particularly a disease of higher yielding cows and that COD has no discernible effect on the milk yield or its pattern. It is speculated, however, that some cows in early lactation, while trying to meet the requirements of sustained peak milk production, are more susceptible to minor environmental changes (such as new social interactions, or changes in diet). These events may lead to a drop in milk production and disruption of events controlling normal ovulation, resulting in the formation of ovarian cysts.