Calculating animal performance from limited liveweight
measurements of the population
G.C. WAKE AND A.B. PLEASANTS
Biomathematics Research Centre, Department of Mathematics and
Statistics, University of Canterbury, PB 4800, Christchurch
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NZSAP 2003 Abstract No. 42
Proceedings of the New Zealand Society of Animal Production
63: 173-175
We consider the problem of estimating the distribution of
carcass weights in a flock of animals from estimates made on a truncated
sample. This arises when a farmer chooses the heaviest lambs for
slaughter and then measurements are made by the meat processor. This
enables a farmer to answer two questions: what proportion of the animals
remaining exceed a nominated carcass weight, and/or what carcass weight
is exceeded by a nominated proportion of the population? Estimates of
these statistics and their uncertainties are derived and are exact if
the animal weights are normally distributed. These calculations can be
the basis of decisions about future feeding and drafting strategies,
important for farmers producing animals on contracts for future
delivery. An example is given based on 1000 lambs using a cut-off weight
of 15.5 kg with mean of this upper group of 16 kg. Using a realistic
estimate of a standard deviation (of the weighing scales) of 0.3 kg,
this gives an estimated mean of 14.6± 0.04 kg, with a standard
deviation of 0.94±0.044 kg, and that 75% of the lambs in the
population exceed 13.9±0.07 kg. The proportion of lambs that
exceed 14.5 kg is then between 51.3% and 55.7%.
Keywords: NZSAPAB;
truncation; normal; drafting; decision-support
Last Updated 8/07/2004
