The economic value of replacement breeding ewes attaining puberty within their first year of life on New Zealand sheep farms

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The economic value of increasing the reproductive performance of a breeding-ewe flock through selecting replacement ewes that attain puberty (AP) in their first year of life was quantified using bio-economic farm system modelling. In all of the scenarios modelled, the breeding-ewes were first presented for mating as yearlings (18–19 months of age) to enable them to start lambing at 2 years of age. For a New Zealand hill country sheep farm initially weaning 1.4 lambs per ewe mated, farm profit was improved by 6% when the percentage of breeding ewes that attained puberty in their first year of life was increased from 25% to 100%. However, if current sheep-industry target liveweight recommendations for rearing ewe lambs are met then between 70 and 95% of breeding-ewes should already attain puberty in their first year of life, and under these circumstances any further gains in farm profitability through specifically using this selection policy will be small (<2%). Countering the reproductive performance benefits of this selection policy was: (1) ewe liveweight increased with a higher AP%, which increased the individual feed requirements of the ewes; and (2) ewe mortality increased as multiple-bearing ewes became increasingly prevalent. The economic cost of using additional resources to further increase the farm's feed supply outweighed the benefits, resulting in the need to reduce farm breeding ewe numbers. The methodology and farm system model used for this study can be readily applied to other sheep farming systems. This can identify components of the reproductive process which should be targeted for further research to maximise on-farm benefits, and provide information on how a farm system will need to change for this to be successfully achieved.