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 (ADG_b−w). Weaning age explained 48% (p<0.001) of the variation in WW and 11% (p<0.001) of the variation in ADG_b−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 ADG_b−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.

KEY WORDS:

• Horse
• growth
• ADG
• Thoroughbred
• foal
• weaning
• pasture
• modelling

Acknowledgements

This work was financially supported by grants from the Hungarian Eötvös Scholarship for Á Bokor. Funding for the management of the stud farm during the time the growth data were collected was provided by the following organisations: New Zealand Foundation for Research, Science and Technology; the Global Equine Research Alliance (GERA); and, specifically, the Marilyn Simpson Trust (USA), the Horserace Betting Levy Board (UK), Utrecht University (The Netherlands), and the New Zealand Racing Board.

Notes

¹ RE Hickson, Massey University, Palmerston North, New Zealand