A deterministic computer model was used to predict effects of genetic improvement in nine performance traits on flock feed cost per unit of empty body weight (TDN/EBW) or of carcass lean (TDN/CLN) equivalent output value from lambs, ewes and wool, when nutrient requirements for normal fleshing are supplied. Relative values used for EBW or CLN were 1.0 for market lambs, .33 for cull ewes vs 2.04 or 1.0 for clean wool. Under optimal annual lambing, a 10% trait improvement was expected to reduce TDN/EBW or CLN most (-15 to -20%) for lamb viability, -6 to -8% for litter size born, -5 to -6% for ewe fertility, -1 to -3% for mature size or growth rate and for wool growth, -1 to -2% for milk production and for earlier sexual maturity. A 10% greater leanness reduced TDN/CLN less when lambing rate was low than high (-1 vs -3%), but increased TDN/EBW by 3%. Effect of genetic change diminished at higher levels, especially for lambs born and milk. A 10% increase in length of breeding season reduced TDN/EBW only under shortened 7 or 8 mo lambing intervals and more when other reproductive traits were low (-9 to -14%) than high (-2 to -3%). The 8- and 7- mo intervals reduced TDN/EBW more for long-season sheep with low vs high reproductive rate (-35 to -45% vs -15 to -20%), but would increase costs for short season sheep by 35 to 45% vs 3 to 2%. Increased protein requirements or greater increases in non-feed than in feed cost for higher performance would mean slightly less reduction in costs than shown. Greater than US values of wool vs meat or costs of feed vs other costs would change importance of traits
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XV. Beef cattle, sheep and pig genetics and breeding, fibre, fur and meat quality., , 65–68, 1990
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