A Sheeplan supplementary lean lamb option has been developed to assist in the selection of sheep for meat production. Output information includes a breeding value for fat-depth measurement "C" over the 'eye muscle' and a lean growth index (LGI), combing leanness with growth performance. Performance records, including live animal ultrasonic fat depth measurements and subjective assessments of fatness by lamb drafters were analysed on 1970 ram hoggets from 25 ram breeding flocks. The average mob live weight at assessment was 47.9 kg (range 34.2 to 60.3) associated with an average fat depth C of 3.3 mm (range 1.7 to 5.6). The subjective assessment of live-animal fat depth has the advantages of speed and simplicity but further work is required to develop the most appropriate calibration techniques. The extent of within-flock variability in the animal records was examined and comparisons made between 2 methods of adjusting fat depth for live weight within each flock. The calculation of within-flock fat- weight relationships for this adjustment was adopted for use in larger flocks, but a constant exponent method is preferred when flock size is small. The wide variation in the LGI within flocks suggests considerable opportunity exists to make worthwhile genetic progress in lean growth rates.
Proceedings of the New Zealand Society of Animal Production, Volume 45, , 67-72, 1985
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