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According to EquinEdge, in racehorse terminology, particularly for Thoroughbreds, the term "half-siblings" refers to two or more horses that share the same dam (mother) but come from different sires (stallions).

In other words, offspring that share the same sire are not considered half-siblings. This is because a single stallion can produce hundreds of offspring over his lifetime, whereas a dam produces far fewer foals. Therefore, the half-sibling relationship is considered more significant through the maternal line.

A publication titled "Horse Genetics 4.0: Evolution, Breeds, Breeding Strategies, and Inbreeding" notes that, genetically, half-siblings typically share about 25 percent of their DNA, according to the principle of the coefficient of relationship.

So, what is the difference between full-siblings? The same source explains that full-siblings are horses born from the same sire and dam. Genetically, full-siblings share roughly 50 percent of their DNA.

This similarity often translates to consistent physical traits, temperament, and even racing ability, especially if both the sire and dam have outstanding performance records. 

Full-siblings of successful racehorses are generally valued higher in the racing market because their performance is more predictable, thanks to the shared bloodline.

Meanwhile, half-siblings still hold value, particularly if their dam has a strong track record. However, their potential performance is generally more difficult to predict.

Several studies support this. Kim, Mitchell, & Wang (2019), in their study Hedonic Pricing and the Role of Stud Fees in the Market for Thoroughbred Yearlings in Australia, found that horses sharing the same dam or sire could command higher prices in the yearling auction market.

Similarly, Mouncey et al. (2024), in Determinants of Thoroughbred Yearling Sales Price in the UK, showed that siblings of successful mares also influenced the market value of young horses in the UK.

These findings emphasize that lineage and family performance are key considerations for buyers and breeders in assessing a horse’s potential and long-term investment value.

Understanding the difference between full-siblings and half-siblings is crucial because it directly relates to the racehorse registry, known as the Stud Book.

In Indonesia, the Stud Book Indonesia (SBI) records every foal or young horse in detail, including information about its sire, dam, and registration status. With this data, breeders and buyers can better evaluate a horse’s quality and potential while designing more strategic breeding plans.

This is different from mating a mare directly with her offspring, which is extremely risky.

Breeding between relatives, including half-siblings, can cause inbreeding depression, which affects performance, health, fertility, and even durability. This occurs because deleterious genes that are usually hidden may become active.

A study titled Inbreeding Depression and the Probability of Racing in the Thoroughbred Horse, published by The University of Edinburgh, explains that in Thoroughbreds, inbreeding reduces the likelihood of a horse racing and increases the risk of miscarriage in mares.

The same source notes that Thoroughbred racehorses have a limited genetic pool, which makes inbreeding a serious concern.

For this reason, many studies advise that mating management should carefully consider the genetic relationship between horses to maintain diversity and prevent long-term negative effects.

Based on this principle, PORDASI developed a breeding management system through the Stud Book Indonesia regulations, which require that both stallions and mares be registered with the Bureau of Horse Registration or Stud Book Indonesia.

From the discussion above, half-sibling racehorses can indeed be mated biologically, as they share part of the dam’s genes. However, this practice remains risky due to the potential for inbreeding depression, which can lower performance, affect health, and reduce fertility.

Therefore, it is essential for breeders to understand the difference between full-siblings and half-siblings and to utilize data from the Stud Book Indonesia.

By using complete information about the sire, dam, and registration status, breeders can design safer breeding strategies, maintain genetic diversity, and minimize the long-term negative effects of inbreeding.

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