THE many studies conducted on the genetic aspects of breeding efficiency, or the effectiveness of selection for it, in dairy cattle are distinguished by their lack of agreement. Eriksson (1943, Koch (1938), Kab (1937), Spielman and Jones (1939), as well as many others, have indicated that differences in cow families and daughters of individual bulls are largely due to heredity. Espe (1946), reports that lactation tends to suppress the normal functioning of the bovine ovary, while Clapp (1937), and Dickinson (1942), believe that high production may inhibit the estrus cycle rather than prevent conception following mating. Olds and Seath (1953), found that the time from calving to first estrus was 32.1±18.6 days. Their repeatability for this measure of breeding efficiency was 0.29, with a heritability of 0.27, and a correlation between production and time from parturition to first estrus of 0.09. The average time from parturition to first estrus is generally between 40 and 50 days.