Bar-Anan, R., Ministry of Agriculture, Hakirya, Tel Aviv, Israel
Soller, M., Department of Genetics, Hebrew University, Jerusalem, Israel
Bowman, J.C., Department of Agriculture and Horticulture, University of Reading, Earley Gate, Reading RG6 2AT, United Kingdom

Records of 178 593 monoparous births between 1964 and 1970 in Israeli-Friesian dairy cattle were analysed for information on environmental and genetic factors affecting calving difficulties and perinatal mortality. The incidence of difficult calvings (DC) and peri-natal calf mortality (PM) were 6·9 and 9·1% in heifer calvings, and 1·6 and 4·1% in cow calvings. Calving characters were not related t o heifer age nor cow parity, but there was a seasonal trend in heifer calvings, with both DC and PM being high in winter and low in summer calvings. Heritability estimates for calf-genotype effects on PM were 0·042 in heifer calvings and 0·013 in cow calvings. Heritability estimates for dam-genotype effects on PM were 0·018 and 0·004 respectively. Heritability estimates for DC were similar t o those for PM. The genetic correlations between DC and PM in the same class of calvings ranged between 0·8 and 0·9, while the environmental correlations were considerably lower, about 0·3. The genetic correlations between the calf-genotype effects in heifer and cow calvings were about 0·5, indicating that different factors may affect heifer and cow calvings. It is proposed that sires already proven for milk yield be tested for their calf effects in heifer calvings, using contemporary comparisons within herds and seasons. Results will be available when the sires are 6 to 6½ yr old. Sires causing fewest calving difficulties would be nominated for heifer matings, so that most heifer inseminations would be by sires proven for both yield and calving characters. Some long-term progress might be obtained if sires with a high incidence of difficulties in their heifer mates were not used to produce young sires. © 1976, British Society of Animal Science. All rights reserved.