Farm Table says:
What is the problem?
The profitability of southern Australian sheep production systems depends on the optimisation of stocking rates by meeting the nutritional demands of the breeding ewe while effectively utilizing grown pasture.
The aim of the study was to evaluate the genetic variation in live weight and body condition of Merino ewes across their breeding life within a wool-based enterprise.
What did the research involve?
• The study used data from the multiple-bloodline D flock managed at the Agricultural Research Centre, Trangie, from 1975 to 1989
• The ewes were maintained as 15 distinct flocks but were managed as a single group for key management practices
• The flocks were sampled from across the Merino strains of importance at the time (fine-wool Saxon, medium-wool non-Peppin, medium-wool Peppin, South Australian strong-wool, medium-wool fertility Peppin; Table 1).
• The size of each individual flock was stabilized at 100 breeding ewes (apart from a fertility line stabilized at 200 breeding ewes), with replacement ewes bred from within each flock (Table 1). The original base flocks were established from 100 flock ewes (2–3 years old) sourced from the stud and commercial flocks through random sampling.
• Weaner ewes were managed as separate mobs from the breeding flocks until their hogget shearing in October when replacements were selected in time for the next joining in February. Replacement ewes were first joined at 19 months of age, with the average ewe allowed five mating opportunities in a spring-lambing system
• The Merino ewes were weighed and their body condition was scored (score 1–5 for fat coverage of the anterior lumbar vertebrate, with 1 being lean and 5 being fat) at four measurement times across the production cycles.
- The first measurement on the ewes was taken 1 week before natural mating in single-sire groups spanning a 5-week period.
- Ewes were measured again at mid-pregnancy (6 weeks before lambing),
- pre-lambing (1 week before lambing)
- Weaning (Lambs were weaned from their ewes at 4–5 months of age)
• Ewes were weighed and their body condition was scored for an average of five parties including their first joining as hoggets.
What were the key findings?
The results were consistent with findings in crossbred ewes and showed that the genetic component of weight and body condition remained constant across the production cycle and age. The overall additive genetic effect accounted for 92% of the genetic variation in weight of Merino ewes bred across five production cycles. A genetic correlation of 0.85 suggested that ewes that were superior at maintaining their condition when rearing a single lamb would maintain this superiority when rearing multiple lambs.
The majority (92%) of within-flock genetic variation in weight of adult Merino ewes is constant across measurement times, similar to findings for crossbred. Ewes that are superior for maintaining condition when rearing a single lamb will also maintain this superiority when rearing multiple lambs. To genetically improve weight and condition at tough times, selection should be focused on having more genetic weight and condition at all times. Ideally, the focus should remain on output traits (reproduction), because the condition is really a proxy (character) of maternal performance and will work as insurance against poor management or unforeseeably tough periods.