Breeding Selection with Conflicting Objectives
A 400-ewe prime-lamb flock is selecting replacement rams. The stud offers two rams with Australian Sheep Breeding Values: Ram A has a post-weaning weight ASBV of +8 kg and a worm egg count ASBV of +40% (higher is worse); Ram B has +3 kg post-weaning weight and −25% worm egg count. The flock's main constraint is barber's pole worm, which has caused losses in wet years, and ewes currently average 1.4 lambs weaned per ewe. The owner wants faster growth to hit a heavier carcase target, but the vet has flagged rising drench resistance. Linkage to the stud's performance-recorded flock is strong for both rams.
1. Explain how you would weigh the two rams' breeding values against the flock's production and health constraints.
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The conflict: Ram A buys growth (+8 kg) at the cost of higher worm egg count (+40%), which increases pasture contamination and drench reliance - directly opposed to the vet's concern about resistance. Ram B gives less growth but improves worm resistance (−25%), which is a cumulative, permanent genetic gain.
Weighing the traits:
- Economic weight of growth: Faster growth helps hit carcase targets, but the benefit per lamb is bounded by the processor's weight grid; over-heavy lambs may be discounted.
- Cost of worms: With drench resistance rising, a ram that sheds more eggs imposes ongoing costs - more drenches, possible production loss, and accelerated resistance. These costs compound across the ram's many progeny.
- Accuracy: Both rams have strong linkage, so the ASBVs are reliable and the comparison is fair.
Judgement: The flock's binding constraint is worm control, not growth. Selecting Ram B protects the long-term viability of the flock, while growth can be improved over successive generations with rams that do not compromise worm resistance. Choosing Ram A would trade a short-term gain for a worsening health problem.
Caveat: The decision also depends on the ram's other traits (e.g. lambing ease, carcase) and whether a third ram combines moderate growth with worm resistance - the ideal is to find one that does both.
2. Recommend a selection decision and describe how you would monitor its effect over the next few years.
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Recommendation: Select Ram B, prioritising the worm egg count ASBV, provided its other ASBVs are acceptable. If a ram combining moderate growth with worm resistance is available, prefer it.
Reasoning: The flock's production is limited by barber's pole worm in wet years, and drench resistance is rising. Genetic resistance to worms is permanent and cumulative, whereas growth can be improved incrementally later without undermining health. The growth sacrifice (+3 vs +8 kg) is manageable because carcase targets can be met through nutrition and management, while worm susceptibility cannot be managed away once resistance spreads.
Monitoring plan:
- Worm egg counts - sample ewes and lambs each season, especially after wet periods, and track the flock average against the pre-selection baseline.
- Drench use - record drench frequency and monitor for resistance (e.g. faecal egg count reduction tests).
- Growth and carcase - record weaning weights and carcase weights at slaughter to confirm the growth target is still being met.
- Reproduction - track lambs weaned per ewe to ensure no unintended decline.
- Review ram selection annually - adjust the balance of traits as the flock's constraints change.
Justification: The monitoring plan tests the key assumption - that worm resistance improves without unacceptable growth loss - and allows the breeding objective to be revised if the trade-off proves worse than expected.