Bar-Anan, R., Agricultural Research Organization, The Volcani Centre, Rehovot, Israel
Bowman, J.C., Department of Agriculture, The University, Reading, United Kingdom

The incidence, heritability and effects on lactation performance of twinning in 185 000 calvings in Israeli-Friesian dairy herds are presented. The incidence of twinning in heifer and cow calvings (parities 2 to 6 inclusive) was 0·7 and 5·4% respectively in the large Kibbutz herds and 0·7 and 3·5% in smallholders' herds. The heritability estimate for 144 sires each with over 200 calvings of daughters of parity 2 to 6 was 0·024 ± 0·002. The range of incidence of twinning in these parities among sire progeny groups each with more than 1000 calvings was 2·8 to 7·8 %, apart from one sire, whose daughters had 12·2% twins. His progeny excelled in five characters and as an explanation it is suggested that a rare gene is segregating in the breed with a major effect on the endocrine system and associated pleiotropic effects. © 1974, British Society of Animal Science. All rights reserved.