The choice of stallion is the decision that most shapes a foal before it is even born. A good cross is not the result of intuition or name recognition — it is the product of technical criteria applied with method. The stallion contributes 50% of the foal’s genetic material, but influences 100% of the breeding operation’s market reputation.

This article presents the objective criteria that guide stallion selection: genetic merit, fertility evaluation, semen quality, and compatibility with the mare.

What the Stallion Passes On

Before defining selection criteria, it helps to understand what a stallion actually transmits. Genetically, a foal receives half its genes from the sire — including genes that influence conformation, gaits, temperament, athletic ability, disease resistance, and hoof quality.

But genetic inheritance is not simple addition: genes interact with each other and with the environment. A stallion with excellent conformation may produce irregular offspring if the mare has poor conformation — and vice versa. Good cross planning considers both the individual qualities of each parent and the complementarity between the two.

Beyond genetics, certain diseases can be transmitted through semen — which is why the stallion’s health status is a non-negotiable criterion.

Genetic Merit and Proven Performance

Genetic merit is the first dimension to evaluate. Depending on the breed and the goal of the cross, metrics vary:

Sport horse breeds (show jumping, dressage, eventing): the major European and American associations publish estimated breeding values (EBVs) for traits such as gait quality, jumping ability, temperament, and conformation. Stallions performance-tested under saddle whose offspring have competed successfully have proven merit.

Stock and working breeds (Quarter Horse, Criollo, Paint, Appaloosa): competition records (working events, reining, rodeo) and the average performance scores of a stallion’s offspring are the primary indicators.

Purebred breeds with morphological focus (Lusitano, PRE, Arabian, Mangalarga Marchador): results at breed shows, grading exam scores, and selection evaluation scores of the stallion’s offspring are the dominant criteria.

In any breed, stallions with a large evaluated crop are more predictable in terms of what they transmit than young stallions with no evaluated offspring — the difference between a “potential” stallion and a “proven” one. Predictability has a price: proven stallions often command higher stud fees or have waiting lists.

Compatibility with the Mare: Corrective Crossing

One of the most important principles of cross planning is that the stallion should be selected to correct or complement the mare’s characteristics — not simply to reproduce the stallion’s own qualities.

A mare with slightly closed hind limbs benefits from a stallion whose offspring have open, well-angled hindquarters. A mare with a short, high-set neck benefits from a stallion that passes a long, well-attached neck.

Equally, serious defects should not be “crossed against more defects” — in genetics, two faults don’t cancel out; they tend to compound or produce unpredictable results.

Comparing the detailed conformational scorecards of both mare and stallion — ideally with the guidance of a breed improvement specialist — is the safest path to this decision.

The Andrology Exam: What It Is and Why It Matters

The andrology exam is the technical assessment of a stallion’s reproductive capacity. It should be performed annually before the breeding season, or whenever fertility concerns arise.

The exam includes:

Physical examination of the reproductive system: testicles, epididymides, vas deferens, accessory sex glands, and penis are evaluated for conformation, symmetry, consistency, and lesions. Asymmetric or abnormally textured testicles are warning signs.

Testicular biometry: testicular volume — measured by ultrasound or calipers — correlates with daily sperm production. Stallions with testicles small for their breed tend to produce less semen.

Semen evaluation: the most important parameter of the andrology exam. It includes:

  • Total and progressive motility: percentage of sperm in motion and in forward motion. Progressive motility below 40% is considered compromised.
  • Morphology: classification of sperm defects. Sperm with major defects (head, mid-piece) or a total defect rate above 30% indicate a quality problem.
  • Concentration: number of sperm per mL. Determines how many doses can be produced per collection.
  • Ejaculate volume: normally 1 to 3.5 fl oz (30 to 100 mL), with variable concentration.
  • pH: normal semen has a pH between 7.2 and 7.8.

For stallions intended for frozen semen use, a freezability trial — collection, processing, thawing, and re-evaluation — is essential before scaling that stallion’s use. Not all stallions with good fresh semen quality freeze well.

Health Status: A Non-Negotiable Requirement

Breeding and artificial insemination are pathways for disease transmission between animals. Stallions must have current health clearances before any reproductive use. Requirements vary by country and breed registry, but generally include:

Equine infectious anemia (EIA): the Coggins test (agar gel immunodiffusion) is mandatory for horse movement in many countries. EIA-positive stallions are excluded from breeding.

Equine viral arteritis (EVA): caused by equine arteritis virus, which can be shed in semen. Seropositive shedding stallions should be vaccinated before reproductive use, per local regulations.

Contagious equine metritis (CEM): caused by Taylorella equigenitalis, a bacterium that can be carried on the stallion’s penis and prepuce without clinical signs. Transmitted to the mare at breeding or through semen contamination. Requires bacteriological culture swabs of the genitalia.

Brucellosis and other disease testing: depending on region and local regulations.

In addition to these tests, up-to-date vaccination and strategic deworming of the stallion are management practices that protect the reproductive horse and indirectly protect the mares he breeds.

The Stallion’s Historical Fertility

The stallion’s conception rate with previously bred mares is one of the most direct indicators of his real-world fertility. Reputable breeding operations and AI centers record and publish this data.

For stallions with a large breeding record, per-cycle pregnancy rate (percentage of mares that conceived on the first breeding cycle) and seasonal pregnancy rate (percentage that conceived by the end of the breeding season) are the most relevant parameters.

Stallions with a consistent per-cycle rate above 65% are considered fertile; below 50% per cycle (with quality chilled semen and a correct protocol) suggests a problem. These numbers must be interpreted carefully — fertility depends equally on the quality of the mares bred (young, healthy mares vs. older mares with uterine issues).

Cost, Availability, and Logistics

The genetically best stallion isn’t always the smartest choice. The logistics of the breeding must also be considered:

Semen availability: is the stallion active? Does he have frozen doses in inventory? What is the lead time for chilled semen delivery?

Stud fee: high fees can compromise the financial return of the cross, especially for mares of intermediate commercial value.

Breed registry requirements: some registries have specific rules about the reproduction method (prohibition of AI, for example) or about the stallion (he must be registered with a certain grade or approval).

Choosing a stallion is an exercise in balancing genetic quality, compatibility with the mare, proven fertility, health status, and economic viability. Breeders who approach this process methodically — with records, technical consultation, and patience — progressively raise the genetic standard of their herd.