The Reproductive Efficiency of Dairy Cattle

Abstract

High reproductive efficiency in livestock is economically a very desirable characteristic. The frequency of calving determines to a great extent the amount of milk a dairy cow produces during her lifetime. A review of lit- erature reveals different methods of measuring the reproductive powers of dairy cattle. Williams (1) recognized the economic and scientific importance of the application of some intelligible standard of breeding efficiency. He adopted two years as the ideal age at which a heifer should calve, which necessitates conception at fifteen months. Every calendar month after the fifteenth was designated as a "breeding month." Twelve months was considered as an ideal calving interval. He determined the average number of breeding months required to produce a calf by dividing the total breeding months by the number of calves born. The percentage reproductive efficiency was ob- tained by dividing twelve, the ideal number of breeding months for the pro- duction of a calf, by the determined average number of breeding months per calf. Williams (2) reported on a Guernsey herd which produced a calf for each 28.7 breeding months, or 41.8 per cent of ideal efficiency. K~b (3) studied the breeding records of 1475 cows, 136 bulls and 7104 calves of the Yellow Franconian breed of dairy cattle. "An analysis of the fertility of the daughters of 22 bulls revealed considerable variation between the various families which indicated a genetic basis. Through several gen- erations 35 families showed high fertility and 11 families low fertility." The number of services per pregnancy and percentage of conceptions are frequently employed when measuring the breeding ability of dairy cattle (4) (5). These measures do not take into consideration the fact that many cows fail to show oestrus regularly, thereby lowering their reproductive effi- ciency, but not necessarily affecting the services per pregnancy. In such a calculation heifers that never calve are naturally omitted from the records. A mediocre individual might be disposed of as sterile after three or four services, whereas a more valuable animal might be given more consideration and conceive to a later service. PLAN OF STUDY AND PROCEDURE The study to be reported in this paper was undertaken to determine the reproductive efficiency of groups of dairy cows descended from different foundation cows. The data available for the study consist of the individual