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Adjustment of lamb birth and weaning weights for continuous effects of ewe age

Published online by Cambridge University Press:  09 March 2007

D. R. Notter ,
R. C. Borg  and
L. A. Kuehn
D. R. Notter*
Affiliation:
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061, USA
R. C. Borg
Affiliation:
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061, USA
L. A. Kuehn
Affiliation:
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061, USA
*
†E-mail: drnotter@vt.edu
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Abstract

Procedures for continuous adjustment of lamb birth and weaning weights for effects of ewe age were developed using 18 747 birth and 13 139 weaning weight records of Polypay sheep enrolled in the US National Sheep Improvement Program. Changes in birth and weaning weights across ewe age groups were modelled using hybrid curves that combined asymptotic regression models to describe initial increases in lamb weight as ewes moved into adulthood with secondorder polynomials to describe declines in lamb weights in older ewes. Lamb birth and weaning weights were highest (and the asymptotic and polynomial forms comprising the hybrid curves intersected) at ewe ages of 76 and 52 months, respectively. Across all ewe ages, hybrid curves were superior to second- and third-order polynomials in goodness of fit, producing a parabolic form with a flat top and different decay rates on either side of the ewe ages corresponding to maximum lamb weights. Fourth- and fifth-degree polynomials were equivalent to hybrid curves in goodness of fit, but generally did not produce reasonable predictions for the oldest ewes. Adjustment factors derived from the hybrid curve predicted that lamb birth weight would increase from 76% of maximum in 11-month-old ewes to 90 and 96% of maximum in 24- and 36-month-old ewes, respectively, and then decline to 97% of maximum at 105 months. For weaning weight, 83, 95, 99, and 93% of maximum lamb weight were attained at ewe ages of 11, 24, 36, and 105 months, respectively. Resulting multiplicative adjustment factors avoid discontinuities at boundaries between ewe age categories and are particularly useful in accelerated or other multiple-season lambing systems.

Keywords

adjustment factorsage groupsbirth weightsheepweaning weight
Type
Research Article
Information
Animal Science , Volume 80 , Issue 3 , June 2005 , pp. 241 - 248
Copyright
Copyright © British Society of Animal Science 2005

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