A FIELD TRIAL - BONE-MINERAL IN YOUNG THOROUGHBRED HORSES IS AFFECTEDBY TRAINING

The objective of this study was to observe bone mineral content (BMC) of the third metacarpal (McIII) of young Thoroughbred horses in respon se to race training under field conditions. Long yearlings, 13 fillies and 16 geldings, were fed the same diet. They were divided into two g roups: Group A began training at 85 +/- 1.1 weeks of age, Group A at 9 3 +/- 1.7 weeks. Every 56 days, BMC was estimated by computer-assisted image analysis of radiographs of the left McIII and an aluminum step wedge. Jugular blood samples were takes for assay of serum Ca concentr ation. Horses weighed 482 +/- 8 and 456 +/- 9 kg in Groups A and B, re spectively, at the start of training, but weight was not correlated wi th age (P = 0.59). Mean BMC values were 21.6 and 19.5 g/cm(2) in Group s A and B, respectively, at the start of training. It decreased (P = 0 .016) during training in Group A from 85 to 93 weeks of age, but incre ased during training in Group A from 97 to 105 weeks (P < 0.01) and in Group B from 93 to 101 weeks (P = 0.008). Serum Ca remained unchanged during training in Group A from 85 to 93 weeks but decreased during t raining in Group A from 97 to 105 weeks (P < 0.001) and in Group B fro m 93 to 106 weeks (P < 0.001). No differences were found between filli es and geldings. The responses of BMC and serum Ca were consistent wit h Ca removal from bone when training began at 85 weeks of age, and wit h Ca deposition when it bean at 93 or 97 weeks. These results suggest that the difference in age between groups was critical in the bone res ponse to the initiation of race training. We recommend that individual s should be evaluated for skeletal maturity before starting training.