Factors influencing pre-race serum concentration of total carbon dioxide in Thoroughbred horses racing in California

Reasons for performing study: Many racing jurisdictions monitor pre-race serum concentration of total carbon dioxide (TCO₂) among racing horses. To our knowledge, factors influencing concentration of TCO₂ among horses participating in racing have not been systematically evaluated and reported. Objectives: To determine if characteristics of horses and racing conditions routinely recorded were significantly associated with pre-race concentration of TCO₂, while accounting for and estimating effects of trainer and horse. Methods: Pre-race serum TCO₂ concentrations from 5028 starts made by 2349 horses trained by 287 trainers at 2 racetracks in California during 2005 were examined. Data regarding characteristics of starters and race conditions obtained from a commercial database were recorded for each start. Data were analysed using mixed-effects, with TCO₂ concentration as the dependent variable, and trainer and horse nested within trainer as random effects. Results: Sex, class and distance of race, frusemide administration and cloudy weather conditions were significantly (P<0.001) associated with pre-race TCO₂ concentration. Horses that finished in the top 3 positions had values that were slightly (0.2 mmol/l) but significantly (P<0.001) greater than horses not finishing in the top 3. There were significant effects of trainer on pre-race TCO₂ concentration. Conclusions: A variety of factors may influence pre-race TCO2 concentration in horses. Horses with better performance tend to have higher pre-race TCO ₂ concentrations. Potential relevance: TCO₂ concentration is associated with improved performance although the magnitude of effect was quite small. Regulatory programmes based on monitoring should consider the influence of other factors on TCO₂ concentration.