Identification of factors contributing to the improved lamb survival in Marshall Romney sheep

Abstract

Abstract In each of three years (1979, 1980, 1981) 260 – 280 mixed age Marshall Romney (MR) and 300 ‐ 400 mixed age control Romney (CR) ewes were allocated to eight groups with ewes of each strain present in each group. Four groups were single‐sire‐mated with four MR rams and the other four groups with four CR rams. A new selection of rams was used each year. The haemoglobin type of all ewes and rams was determined. Ewes were weighed 3 – 4 weeks before the beginning of lambing and their respective weights adjusted to conceptus‐free weights. At lambing, the lambs born to each ewe were identified and birth weight, pelvic width, and head and chest circumferences were measured. Each day dead lambs were picked up and autopsied. In the last 2 years this included looking for cranial and spinal haemorrhages. Lambs present at weaning were weighed and recorded. The conjugate and transverse diameters and area of the pelvic opening were measured on the pelvises of 47 MR and 37 CR ewes after slaughter in 1980 and 1981. The length of gestation was recorded for all ewes in 1980. Lamb survival was 8.5010 higher for lambs born to MR ewes and 5.5% higher for lambs sired by MR rams. The strain of ewe by strain of ram interaction was not significant and the improvement in lamb survival for MR ewes and rams occurred for both single‐ and twin‐born lambs. The improved survival of lambs born to MR ewes was associated with a lower incidence of dystocia (2.3 v . 5.7%) and starvation (2.9 v . 5.1%). The main effect of MR rams was a lower incidence of parturient deaths (0.1 v . 1.0%). Overall incidence of cranial and spinal haemorrhages was 24% and 41 % respectively. Most of the cranial haemorrhages occurred in lambs dying from dystocia (41 %) and hypoxia (33 %) and most of the spinal haemorrhages were found in lambs dying from starvation (41 %) and dystocia (61 %). There was no effect of ewe or ram strain on the occurrence of central nervous system haemorrhages. The conceptus‐free pre‐lambing liveweight of MR ewes was 56.9 kg compared to 50.4 kg for CR ewes. Despite the large difference in liveweight between the two strains of ewes, the mean birth weight of the lambs was only 0.08 kg heavier for lambs born to MR ewes (4.64 kg) compared with CR ewes (4.56 kg). This small increase in birth weight of lambs born to MR ewes was accompanied by a 0.7 cm (1.9%) increase in chest circumference of the lambs. Lambs sired by MR rams were 0.1 kg lighter than lambs sired by CR rams (4.55 v . 4.65 kg). This was accompanied by a 0.09 cm (1.0%) narrower pelvic width for lambs sired by MR rams. There were quadratic relationships between lamb survival and birth weight, chest circumference, and head circumference, and a linear relationship between lamb survival and pelvic width. MR ewes had pelvic openings with larger conjugate (10.9 v . 10.6 cm) and transverse (9.5 v . 9.0 cm) diameters than CR ewes. The area of the pelvic opening was 8.0;0 larger for MR than CR ewes (104.1 v . 95.3 cm). There was evidence of improved maternal behaviour of MR ewes compared with CR ewes. MR ewes had a 0.5‐day shorter gestation length than CR ewes but this was offset by a 0.5‐day longer gestation length for ewes with lambs sired by MR rams compared with lambs sired by CR rams. There were no effects of haemoglobin type of the ewe or ram on any of the lamb survival traits. Single‐born lambs had a higher survival than twinborn lambs (85.3 v . 80.7%) and this was associated with a lower incidence of infection (1.8 v . 4.2%), starvation (3.0 v . 5.5%), and of lambs missing at weaning (1.7 v . 4.0%). The birth weight of single lambs was heavier than twin lambs (4.98 v . 4.05 kg) but birth weight difference did not account for all the difference in survival between single‐ and twin‐born lambs.