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Beyond the Bale : September 2012
39 ON-FARM September 2012 BEYOND THE BALE By the time a lamb is a hogget, there is no age-of-dam effect on its liveweight. OTHER CONSIDERATIONS Other considerations when retaining older ewes include animal welfare considerations such as ewe mortality. Limited research suggests while ewe mortality naturally increases with age, environmental conditions and pregnancy status will have a greater impact on ewe mortality than age. This again indicates the importance of nutrition and body condition when managing older ewes. The condition of a ewe's teeth, udder and feet can significantly impact on her productivity. Most sheep producers cull sheep based on the wear and loss of their permanent incisors. But the age incisor teeth are lost varies markedly between individual sheep and between breeds, so consider culling based on a physical inspection of the teeth, udder and feet rather than a blanket cull at a certain age. WHY DO IT? While there is no single strategy to deliver a large increase in reproductive performance, a combination of breeding and selection is a worthwhile investment, capable of increasing flock net reproduction rates (NRR) by 15 per cent over 10 years. A suitable within-flock selection strategy includes culling twice-dry ewes at three years of age, which begins delivering benefits after the second year and the full benefit of a 4 per cent increase in NRR by year five. Producers can make use of pregnancy scanning information to identify the double dries, and can wet and dry ewes at marking to identify those which have reared at least one lamb. Retaining the top 50 per cent performing ewes in a flock to six and seven years of age also leads to a 4 per cent increase in NRR after five years, increasing to 7 per cent after 10 years. To fast-track performance, select rams which have positive reproduction rates, represented by number of lambs weaned (NLW) in Australian Sheep Breeding Values (ASBVs). The genetic benefit of these rams begins two years after their first daughters are born and kept as replacement ewes. The ram genetic improvement alone will account for 4 per cent of the increase in NRR over 10 years, but the ram/ewe genetic improvement produces about a 4 per cent NRR increase annually. BENEFITS The potential flock reproduction rate gains over 10 years from a combination of strategies are shown in Figure 1. These strategies include culling poor performing, young ewes ('ewe culling), retaining productive ewes for an extra one or two years ('lifetime selection'), using rams with positive NLW ASBV figures ('ram genetics') and the subsequent improvement in replacement ewe genetics ('ewe genetics'). The flow-on benefits from the reproductive improvement and altered flock age structure include more, higher-value surplus hoggets and fewer surplus old animals for sale, fewer maiden replacements required and higher selection intensities possible on wool traits. WORKSHOP DEVELOPMENT From the NSW DPI and Sheep CRC research, a national extension package has been developed in the form of a workshop titled Identifying and keeping productive older ewes in the flock. Co-funded by NSW DPI and AWI, the workshop provides sheep producers with a step-by-step guide to identify, select and manage productive older ewes to improve reproductive performance. The workshop and manual covers five main components, including flock characteristics and age structure, the relationship between ewe age and productivity, identifying productive older ewes, managing productive older ewes and an economic evaluation of the effects of flock structure changes. It is hoped producers will be able to attend these workshops from the second half of 2012 onwards. Acknowledgments Kondinin; Greg Lee, Maryann Sladek, Sue Hatcher and Trudie Atkinson, NSW DPI. More information: Sue Hatcher, NSW DPI, (02) 6391 3861, firstname.lastname@example.org. AGE (YRS) SOUTH WEST SLOPES CENTRAL WEST PLAINS 3 1.22 1.24 4 1.36 1.42 5 1.40 1.43 6 1.58 1.58 Source: NSW DPI 16 14 12 10 8 6 4 2 01 2 3 4 5 6 7 8 9 10 Ewe genetics Lifetime selection Ewe culling Ram genetics Figure 1: Potential responses in reproduction rate over 10 years REPRODUCTION RATE GAINS Table 1: Effects of age on lambs conceived per ewe joined in flocks of Merino ewes on the South-West Slopes and Central-West Pains of NSW.