[Ratio between lactating and dry periods in cows differing in milk yield and duration of the period between calving]

A study was carried out on the duration of the lactation period at different milk yield values and variable duration of the calving interval. It was found that the time period of lactation was influenced by the calving interval and the amounts of milk obtained. When the calving period was more than 365 days there was an unproportional lengthening of the lactation period which led to the respective lengthening of the dry period of cows. One day lengthening of the calving period brought about an increase of the dry period by 0.3 to 0.4 days. In the groups where milk yields amounted up to 4000 1 per cow the relative share of animals whose lactation period was below the standard was higher as compared to the groups with milk yields amounting over 4000 1 per cow. The higher the milk yield the higher the number of cows whose lactation period was within the norm. However, what is commonly observed is that with most of the cows the lactation period is below the standard, and this substantiates the fact that considerable amounts of milk are not really obtained.