Analysis of factors affecting the pregnancy rate of mares after inseminations with cooled transported stallion semen

Artificial insemination (AI) with cooled stallion semen has increased markedly during the last decades in all countries, but fertility is often lower than when fresh semen or natural mating is used. The objective of this study was to examine field data (1634 ,cycles 523 Standardbred (SB) mares, 575 Finnhorse (FH) mares, and 90 stallions) using multivariable logistic regression for factors influencing the pregnancy rate (PR) after AI with cooled transported semen from SB and FH stallions. The PR per cycle for the material was 47%: Finnhorses 42% and Standardbreds 53%. When assessed with multivariable logistic regression analyses with a generalized linear mixed model, variables that affected the PR were breed, the number of inseminated estrus cycles, the percentage of progressively motile sperm (PMOT) in the ejaculate/AI dose at the time of shipment, and the number of progressively motile sperm in the AI dose at the time of insemination. In Standardbreds, variables that increased the per cycle PR were the number of AI per estrus cycle (multiple inseminations increasing the probability of pregnancy compared to only one insemination), the number of inseminated cycles, and PMOT in the AI dose at the time of insemination. In Finnhorses, the number of AI per estrus cycle (two and three inseminations increasing the probability of pregnancy compared to only one), the number of spermatozoa in the ejaculate and in the AI dose, and PMOT in the ejaculate/AI dose at the time of shipment increased the per cycle PR. Non-significant factors for the whole material included the type of artificial vagina (open-ended or closed), transport time, place of AI (stud farm or home stable), insemination done by veterinarian or technician, weekday, month, age of the mare (all age classes combined), age of the stallion, ejaculate parameters (sperm concentration, total number of sperm), and insemination dose parameters (volume proportion of seminal plasma, sperm concentration, PMOT, total number of sperm). In conclusion, breed, breeding opportunity in more than one cycle, more than one insemination/estrus, PMOT of the ejaculate/AI dose and the number of progressively motile sperm in the AI dose at the time of insemination are important for the outcome of inseminations with cooled semen.