title: The association of age at first start with career length in the Australian Thoroughbred racehorse population.
abstract: BACKGROUND
  Studies of Thoroughbred racing populations have provided evidence of a positive effect on racing careers for horses that commence racing as 2-year-olds. Currently, research investigating the presence of this effect in the Australian Thoroughbred racing population is limited.
  
  
  OBJECTIVE
  To investigate the association between age at first start and career length in the Australian Thoroughbred population and estimate the risk of racing retirement for horses racing in Australia based on age at first start, career earnings, number of starts as a 2-year-old and distance raced.
  
  
  METHODS
  Data were collected for Thoroughbreds, born on or after 1 January 1998, that had raced between 1 August 2000 and 22 February 2011 in Australia. A Kaplan-Meier survival curve, stratified by age group, was produced for career length. A Cox proportional hazard model was fitted to assess factors influencing the risk of retirement from racing. The model included sex, age at first start, career earnings, number of starts as a 2-year-old, distance raced and appropriate interaction terms.
  
  
  RESULTS
  The study population included 117,088 horses. Geldings had significantly (P<0.001) longer careers than females and intact males, and females had significantly (P<0.001) longer careers than intact males. Risk of retirement from racing decreased with a younger age at first start, a higher number of starts as a 2-year-old, and a longer average distance raced. For intact males, the risk of retirement from racing increased as earnings increased, while for females and geldings the risk of retirement from racing decreased as earnings increased.
  
  
  CONCLUSIONS
  The introduction of young Thoroughbreds to racing appears to have no apparent adverse effects on these horses racing in Australia. The impact of some risk factors associated with retirement from racing varied between sexes and should be considered when evaluating career outcomes.
authors.affiliation: Faculty of Veterinary Science, University of Sydney, Sydney, New South Wales, Australia. brandon.velie@sydney.edu.au
authors.forename: B D
authors.initials: BD
authors.lastname: Velie
authors.forename: P K
authors.initials: PK
authors.lastname: Knight
authors.forename: P C
authors.initials: PC
authors.lastname: Thomson
authors.forename: C M
authors.initials: CM
authors.lastname: Wade
authors.forename: N A
authors.initials: NA
authors.lastname: Hamilton
mesh_terms: D000818:Animals; D001315:Australia; D005260:Female; D006736:Horses / Q000502:physiology*; D008297:Male; D010805:Physical Conditioning, Animal; D016016:Proportional Hazards Models; D012420:Running; D013177:Sports