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A survival analysis was used to compare the culling rate of Icelandic horses due to the presence of radiographic and clinical signs of bone spavin. A follow-up study of 508 horses from a survey five years earlier was performed. In the original survey 46% of the horses had radiographic signs of bone spavin (RS) and/or lameness after flexion test of the tarsus. The horse owners were interviewed by telephone. The owners were asked if the horses were still used for riding and if not, they were regarded as culled. The owners were then asked when and why the horses were culled. During the 5 years, 98 horses had been culled, 151 had been withdrawn (sold or selected for breeding) and 259 were still used for riding. Hind limb lameness (HLL) was the most common reason for culling (n = 42). The rate of culling was low up to the age of 11 years, when it rose to 0.05 for horses with RS. The risk ratio for culling was twice as high for horses with RS compared with horses without RS and 5.5 times higher for culling because of HLL. The risk of culling (prognostic value) was highest for the combination of RS with lameness after flexion test, next highest for RS and lowest for lameness after flexion test as the only finding. It was concluded that bone spavin affects the duration of use of Icelandic horses and is the most common cause of culling due to disease of riding horses in the age range of 7–17 years.

Age-at-onset of bone spavin (BS) in horses was defined as a double-censored survival variable. A small material of binary responses to a radiographic examination of the distal tarsus in 439 Icelandic horses sired by 17 stallions was analysed. In order to test different models larger simulated data were generated according to an exponential proportional hazard model and subject to censoring. The resulting survivor functions were similar. The bias in survivor functions caused by double censoring in the material was reduced by use of the self-consistency (S-C) algorithm. Using a binary threshold model and the Weibull regression model, an analysis of age-at-onset of spavin resulted in severe underestimation of sire variance components in the simulated data. Survival analysis with model led to less biased estimates. Application of this method on the real data resulted in an effective heritability estimate of h2=0.33, which can be compared with an estimate of h2=0.1 based on an analysis of radiographic signs (RS) using the binary threshold model. These results indicate that the age-at-onset of RS, which reflect the predisposition of BS, is a trait with medium-high heritability. [Copyright &y& Elsevier]