Growth curves from birth to weaning for Thoroughbred foals raised on pasture

Abstract AIM: To mathematically describe the growth of a population of Thoroughbred foals reared on pasture in New Zealand from birth to weaning. METHODS: Twice-monthly liveweight data (including birthweight; BW) from 218 foals (98 colts and 120 fillies) born over a 6-year period, and reared on four different pasture types, were available for analysis. Data truncated to the time of weaning (107–217 days old) provided 3,200 data points. After testing for non-linearity of the relationship of age and liveweight (LW), five non-linear regression equations were tested for goodness of fit. RESULTS: Colts and fillies had similar BW, weaning ages, weaning weights (WW) and average daily gain (ADG) between birth and weaning (ADGb−w). Weaning age explained 48% (p<0.001) of the variation in WW and 11% (p<0.001) of the variation in ADGb−w. After correction for weaning age, BW explained 16% (p<0.001) of the variation in WW and 6% (p<0.001) of the variation in ADGb−w. The best fit of age and LW was the polynomial equations based on Akaike's information criteria (AIC), residual variance (RV), residual standard deviation (RSD), and coefficient of determination (R2). The most accurate, biologically sound representation of growth was provided by Brody's type equation: , and an equation including , that was developed in this study (b0 is an intercept, b1 and b2 are regression coefficients, and e is the base of natural logarithms). The advantage of the Brody's equation is that ADG at a given LW can simply be calculated as: . CONCLUSIONS AND CLINICAL RELEVANCE: The similarity of the present data with growth data from the Northern Hemisphere provides further recognition that foals reared on pasture can grow as well as foals reared in grain-based systems in the Northern Hemisphere. BW provides a logical set point for the explanation of growth of the foal up to weaning, rather than the use of polynomial equations, which do not fully describe the biology of growth, even though they provide an accurate mathematical description.

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