SummaryOver a three year period (1993, 1994, and 1995), a total of 350 Thoroughbred colts and 350 Thoroughbred fillies in Central Kentucky were weighed monthly on a portable electronic scale through 1B months of age. Wither height and cond tion score were also measured in about half of the foals. In order to estimate mature body size ln these horses, 472 brood mares were weighed 60 to 90 days after foaling. In addition, 25 Thoroughbred breeding stallions were also welghed. The average body weight of the brood mares equaled 570 kg and the average weight of the stallions equaled 580 kg. At 14 days of age, colts and fillies weighed an aveßge ol77.7 and 76.1 kg and had average heights of 107.3 and 106.3 cm, respectively. Colts were heavier and taller than fillies throughout the study and at 490 days averaged 9.9 kg heavier (427.8 kg vs 418.0 k9) and 1.6 cm taller {153.4 cm vs 151

Rations high in grain and formulated to exceed daily energy requirements may increase episodes of rhabdomyolysis in thoroughbred horses susceptible to RER.

The effect of dietary starch and fat content on serum creatine kinase (CK) activity and substrate availability was evaluated in 4 mares of Quarter Horse-related breeds with polysaccharide storage myopathy (PSSM). Four isocaloric diets ranging in digestible energy (DE) from 21.2% (diet A), 14.8% (B), 8.4% (C), to 3.9% (D) for starch, and 7.2% DE (diet A), 9.9% (B), to 12.7% DE (diet C and D) for fat were fed for 6-week periods (4 weeks with exercise) using a 4 4 Latin square design. Postprandial glucose and insulin responses were measured, and 4 hours postexercise, serum CK activity, glucose, insulin, free fatty acids (FFA), and-hydroxybutyrate (-HBA) were analyzed. Glycogen, glucose-6-phosphate, citrate synthase, 3-hydroxy-acyl-CoA dehydrogenase, lactate dehydrogenase as well as abnormal polysaccharide and lipid content were measured in middle gluteal muscle samples. Postprandial insulin and glucose response was higher for diet A versus D. Log CK activity was higher with diets A, B, and C versus D. Daily insulin was higher and FFA lower on diet A versus B, C, and D, whereas glucose varied only slightly with diet. Muscle oxidative capacity and lipid stores were low in PSSM horses and muscle glycogen and abnormal polysaccharide content high on both diets A and D. Individual variation occurred in the response of PSSM horses to diets differing in starch and fat content. However, for those horses with clinical manifestations of PSSM, a diet with 5% DE starch and 12% DE fat can reduce exertional rhabdomyolysis, potentially by increasing availability of FFA for muscle metabolism.

The incidence of developmental orthopedic disease on a commercial Thoroughbred farm was studied over a four year period. A total of 271 foals were monitored. DOD was diagnosed in 1070 of the foals. Fetlock OCDS tended to occur before 180 days of age whlle hock, shoulder and stifle OCDS occurred around 300 350 days of age. Foals that developed hock and stifle OCDS as yearlings tended to be large foals at birth that grew rapidly from 3 to 8 months. These foas were heavier than the average population as weanlings. Foals that developed fetlock OCDS before 6 months of age were born eary in the year (January, February or lvlarch). The results of this study suggest that groMh rate and management may affect the incidence of certain types of DOD.

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