Genetic effects on return intervals in cows

Abstract The genetic effects on the length of post-insemination return cycles (RC) were studied by means of 241,808 RC records from Israeli Friesians and 190,989 RC records from Bavarian Fleckvieh. No effect was found of the mating sire on the frequency distribution in various parts of the RC. As for the effect of the sire of the cow, heritability values of 6—15% were found in the relative frequency of returns between 22 and 31 days post insemination. There were indications that the delayed returns were preceded by early embryonic death. It was suggested that the effect of the mating sire was on the rate of fertilization, especially in inseminations at an early stage of the estrus, and that, on the other hand, the genetic effect of the cow and of the sire of the cow was on the rate of implementation of the conceptus in the second week post fertilization. A plan for further investigations was presented.