The agreement between the genetic logregression slopes, estimated from selection experiments, for intake rates or litter weaning mass on body mass and between species estimates gives confidence that these estimates may be generally applicable to most mammalian species. Main components of fitness do not show such agreement and may show or induce an intermediate optimum for body mass within populations. Evidence is presented and the conditions specified for the constancy of reproductive and growth feed conversions over species.The genetic logregression slopes for intake rates (MJ/d) may, at present, probably be regarded as constant at either 0,7 or 0,8, while periods of lactation or growth may be modifiable under managerial or nutritional improvement. This may allow a short-circuiting of the constancy of feed conversion for animals of different body sizes, as such constancy depends on longer duration of developmental times for larger animals.

C. Z Roux

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 55–60, 1986
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