| Abstract: | To investigate the breeding structure in the Japanese Thoroughbred population, we applieda demographic analysis to the populations of foals produced from 1978 to 2005. Themigration rate estimated from the proportion of foals produced by imported breeding horseswas around 40% over the investigated period. After early 1990s, the migration rate throughstallions imported from USA sharply increased. The average generation interval was withinrange of 10.5–11.5 years. The longer generation interval of Thoroughbred was considered tobe a reflection of the fact that Thoroughbred horses begin breeding only after completingtheir performance in races. After the peak of 729 in 1993, the number of sires of foalsprogressively declined to 358 in 2005. Although the coefficient of variation of theprogeny number of sires was within range of 1.0–1.2 until early 1990s, it graduallyincreased and reached the value of 1.6–1.7 in recent years. The effective number of siresconsistently decreased after the peak of 302.6 in 1992, and reached 120–130 in recentyears, which is 25–30% of the actual number of sires. In parallel, the demographicestimate of the effective population size declined after early 1990s. The main cause ofthe observed change in the breeding structure was inferred to be the intensive use of alimited number of stallions for breeding. |
