Selection for productive traits on back fat depth in ewe lambs

J.C. McEwan, P.F. Fennessy, J.N. Clarke, S.M. Hickey, M.A. Knowler

Woodlands Research Station, Ministry of Agriculture and Fisheries, Invercargill; Invermay Agricultural Centre, Ministry of Agriculture and Fisheries, Mosgiel; Ruakura Animal Research Station, Ministry of Agriculture and Fisheries, Hamilton

Proceedings of the New Zealand Society of Animal Production. 1984, 44: 249-252

A sheep breeding experiment at Woodlands Research Station consists of 5 self-contained Romney selection lines (closed in 1973). Four lines are selected respectively for dam's number of lambs born (dam NLB), 100-day weight, hogget fleece weight, and a production index in which all 3 traits are combined, while the fifth line is a randomly selected control.

Back fat thickness was measured in the 1982 born ewe lamb progeny at approximately 8 and 14 months of age. Relative to the controls (back fat 6.3 mm) the dam NLB (5.0 mm) and the production index (4.8) lines had 20% lower back fat depths (P<0.01) in May after variance adjustment for live-weight. The differences were slightly lower when measured in November. The fat thickness of the 100-day weight and hogget fleece weight line progeny were similar to the controls. The heritability of back fat depth adjusted for live weight by half-sib analysis was 0.28±0.13 (s.e) in May.

The implication is that there are no major genetic antagonisms between leanness and the production traits recorded under Sheeplan and the evidence suggests that those using the Sheeplan index may be indirectly selecting for leaner progeny.

Keywords: NZSAPAB; Ultrasonic; sheep; back fat depth; production traits; heritability; genetic correlation; selection

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