The influence of maternal handicap, inbreeding, and ewe's body weight at 15–16 months of age on reproduction rate in Australian Merinos

Abstract

The influence on number of lambs born per ewe joined of the ewe's age, type of birth, body weight at 15–16 months, and inbreeding level, and the inbreeding of the foetus, has been investigated in a flock of medium Peppin Merinos run at Cunnamulla, Queensland.

The influence on survival rate to weaning of these variables (except dam's birth type), plus gestation and lamb's birth weight, has been examined for single-born lambs.

Age of ewe had a marked influence on the number of lambs born, but not on survival rate. Number of lambs born increased with age of ewe and then declined.

Ewes born in multiple births produced 4.0 more lambs per 100 ewes joined than those born as singles.

The ewe's body weight at 15–16 months had a marked influence on the number of lambs born and a small influence on the survival rate, but the effect was linear throughout the range with no evidence for a "critical" weight. Each 10 lb increase in body weight produced 8 more lambs born per 100 ewes joined, whilst survival rate increased by 5 per 100 lambs born for rams and 2 for ewes.

Inbreeding had a marked deleterious effect on reproduction rate. Inbreeding of the dam was nine times as important as the inbreeding of the foetus on the number of lambs born. This effect was reversed for survival rate.

Survival rate showed a curvilinear relationship with both birth weight and gestation period, the greatest survival rate occurring at or slightly above the mean birth weight, but the birth weight effect was highly significant while that for gestation period was not. The effect of high birth weight was not as marked as that for low birth weight in reducing survival.