Optimizing pregnancy rates (Proceedings)

The goals of canine breeding programs are usually to optimize both pregnancy rates and litter size. The most important factors affecting those goals are the health of the bitch, the quality of the semen, the timing of the breeding and the method of insemination. Fertility is best in bitches between 2 and 5 years of age. After 6 years of age, pregnancy rate and litter size decrease and neonatal losses increase even when all other factors remain constant. The health of the reproductive tract, body condition, diet, stress, medications, and systemic illness will all affect fecundity. Although pregnancies can occur in females that are not in prime condition, reproductive performance will be best when only normal, healthy bitches are included in breeding programs. Routine wellness programs, such as vaccinations and de-worming, should be completed prior to breeding.

Even seemingly trivial dietary deficiencies and excesses can diminish reproduction by affecting conception rates, litter size, the number of stillbirths, birth weight and lactation. Current recommendations are that pregnant and lactating bitches be feed a high-protein diet that contains 27-34% protein (29-32% are preferred). The protein source should be primarily animal-based. The diet should contain at least 18% fat with a balance of both omega-6 and omega-3 fatty acids. Only 20-30% of the diet's calories should be from carbohydrates because diets with 40% or more carbohydrate calories may be at the expense of protein and fat. Vitamin and mineral levels in commercial diets must meet American Feed Control Officials (AAFCO) requirements to be sufficient for reproduction in diets labeled as such. However, breeders commonly supplement diets, particularly for calcium. The important thing to remember about supplementing a balanced diet is that the "balance" will be changed by the supplement.

The average length of proestrus is 9 days and the average length of estrus is 9 days. Therefore, the average season or heat is 18 days, with ranges from 3-38 days reported for normal, fertile bitches. Ovulation occurs about 48 hours after the LH surge. Bitches ovulate a primary oocyte that must undergo one more division before it can be fertilized. This takes about 2 days. Therefore, the usual recommendation is to breed 3-5 days after the LH surge. This has been referred to as the fertile period. The egg has a fertilizable life span of about 2-3 days. There are no clinical or behavioral signs that indicate when during a season ovulation will occur, although it usually occurs during the first few days of estrus. A common practice for natural service is to begin breeding on a predetermined day of heat, often day 10 or 12, and to breed every other day for as long as the bitch is receptive. This method of breeding management works well for bitches with a typical cycle because with proestrus of average length, they would be in standing heat on day 10. Given the longevity of sperm in the female tract, breeding every other day would insure that viable sperm are available for fertilization whenever ovulation occurs. Vaginal cytology can be used to determine when a bitch is actually in estrus because the characteristic changes in the vaginal epithelium are a reflection of the hormone estrogen. Vaginal cytology is especially useful for bitches that do not show strong behavioral signs of estrus.

The LH surge can be determined by measuring serum concentrations of LH itself, or by measuring serum concentrations of progesterone. This practice is referred to as ovulation timing. In bitches there is a preovulatory increase in progesterone above basal anestrous levels. This initial rise in progesterone coincides with the LH surge. Because progesterone assays are readily available and because the LH surge lasts for only about 24 hours, which necessitates daily sampling, progesterone is often determined instead of LH. Knowing the LH surge, one can make the necessary arrangements to breed in 3-5 days, which is the estimated fertile period. There is a sharp increase in serum progesterone concentration after ovulation. Studies from AI programs have shown that pregnancy rates are best when insemination is performed on days when progesterone concentrations are between 30-80 nmol/L (about 10-26 ng/ml). The recommendation is to breed twice, 48 hours apart. If progesterone concentration was already nearly 60 nmol/L (19 ng/ml) at the first breeding, the second is performed in 24 hours. Pregnancy rates and litter size are greater with 2 breedings during the fertile period than with 1. Three breedings during the fertile period do not improve pregnancy rates above 2 breedings, but there is a trend toward larger litters.

The semen factors thought to be most important to fertility are the number, the morphology and the motility of the spermatozoa. The total number of sperm in the ejaculate should be 300 million to 2 billion. At least 70% of the sperm normally have rapid, steady forward progression. At least 80% of the sperm should have normal morphology. A minimum of 200 million motile sperm per insemination is usually recommended. Freshly ejaculated sperm remain viable and capable of fertilization for at least 4-6 days in the estrous female tract. Frozen-thawed semen has a life span of about 24 hours and it does not penetrate cervical mucus very well. Therefore intrauterine, rather than intravaginal, insemination of frozen-thawed semen is critical to success. Pregnancy rates achieved with vaginal insemination using frozen-thawed semen of good quality have been about 30%, whereas pregnancy rates of 50% to 80% have been achieved when intrauterine insemination has been performed. It has been shown that intrauterine artificial insemination with fresh and chilled semen also improves pregnancy rates over intravaginal artificial insemination. Overall, the best results for pregnancy rates and litter size are achieved when bitches ≤ 6 years old are inseminated twice during the fertile period with semen of good quality from dogs ≤ 8 years old.

From: Johnson CA, Reproductive System Disorders. In: Nelson RW and Couto CG (eds) Small Animal Internal Medicine 4th edition. St. Louis, Elsevier.