Many things can impact litter size in the bitch.
Many things can impact litter size in the bitch. These include number of eggs released from the ovary during a given heat cycle; presence of adequate numbers of normal spermatozoa to fertilize those eggs; presence of a normal reproductive tract in the bitch to permit implantation of developing embryos, absence of disease; and persistent secretion of progesterone. This paper will not include information about pathologic causes of lack of conception, such as uterine infection or poor semen quality, or causes of pregnancy loss, such as canine brucellosis or premature decline in progesterone production. This paper also will not include information about methods for enhancing number of eggs ovulated, or superovulation. That technique works best in animals that ovulate few eggs with each heat cycle, such as cows and ewes, and has not been proven to work in species that ovulate numerous eggs with each cycle, including bitches. Instead, this paper will focus on proven causes for decline in litter size in bitches, to help you and your veterinarian best determine when a pathologic cause for decreasing litter size may be present.
Dogs ovulate eggs (ova) that are immature, incapable of being fertilized. It takes, on average, 48 hours for the majority of those ova to mature to the point where they can be fertilized. After that period, ova survive for a variable number of days, with presumed decline in viability as they age. Therefore, optimal breeding day in dogs, for natural service or insemination with fresh or chilled semen, is two days after ovulation. Optimal breeding day with frozen semen is three to four days after ovulation; once thawed, spermatozoa that had been frozen live only a matter of hours so insemination must be performed when all ova are known to have matured completely. It has been well demonstrated that litter size is optimized by breeding on optimal breeding day in bitches. Determination of optimal breeding day is best done by measurement of progesterone in blood. Litter size can be maximized by inseminating twice during a given heat cycle; most often breedings are accomplished two and four days after ovulation with natural service, fresh or chilled semen, and three and four days after ovulation with frozen-thawed semen.
According to records from the American Kennel Club (AKC), natural service produced larger litters than did artificial insemination, with a 15% decrease in those litters produced from fresh or chilled semen and a 25% decrease in those litters produced from frozen-thawed semen. Scandinavian researchers reported a 30.5% decrease in litter size in bitches inseminated with frozen-thawed semen compared to bitches inseminated with fresh semen.
In general, litter size is increased with any type of semen if it is deposited directly into the uterus; in a Scandinavian study, litter size averaged 4.0 pups for vaginal insemination compared to 5.4 pups with intrauterine insemination. Intrauterine insemination is particularly important when using frozen-thawed semen; in one study, transcervical intrauterine insemination (TCI) yielded average litters of 6.0 pups while vaginal insemination yielded litters of 4.0 pups.
Intrauterine insemination can be accomplished surgically or by TCI. No good studies directly comparing these two techniques have been published. Advantages of surgical insemination are that the reproductive tract can be observed directly and the technique is successful in all attempts. Disadvantages of surgical insemination are the need for general anesthesia and only one opportunity for performance of the technique during a given heat cycle. Reported average litter size with surgical insemination of German Shepherd dogs was 7.0 pups in one study. Advantages of TCI include lack of anesthesia and ability to perform the procedure multiple times during a given heat. The primary disadvantage is that the technique is not possible in all dogs. Dr. Marian Wilson, who developed the TCI technique, reported an average litter size of 5.0 pups from TCI of 46 bitches of various breeds.
Breed variation in litter size has been well documented in the veterinary literature. Average litter sizes reported for small, medium, large and giant breeds are 3.9 pups, 5.7 pups, 5.9 pups, and 6.1 pups, respectively. A review of a large number of litters for the 15 most popular breeds registered in the AKC demonstrated this, as shown in the following table and graph.
Very young and very old bitches tend to produce smaller litters than do "middle-aged" bitches. Age at which reproductive success peaks varies by breed, with a longer breeding life in those breeds that are longer-lived. For a given breed, an "apparent critical age" can be determined, defined as the age at which average litter size has decreased by at least 15% from average litter size for that breed.
Season of the year may affect fertility in bitches. A study in Scandinavia demonstrated increased whelping rate from artificial inseminations performed in the spring, with lowest whelping rate from inseminations performed in July. A study of German Shepherd dogs in Kenya did not demonstrate an effect of season on litter size.
Variation in apparent critical age with body weight in bitches
Two nutritional components that have been demonstrated to be associated with reproductive efficiency in dogs are protein and fat. Feeding of suboptimal percentages of protein is associated with decrease litter size in dogs. The minimum recommended percentage of protein in canine pregnancy diets by the American Association of Feed Control Officials (AAFCO) is 22%. Most commercial canine diets considered appropriate for use during pregnancy contain 29 to 32% protein.
Essential fatty acids also play a role in canine reproduction, with larger litters born to bitches fed 20% fat compared to those fed 16% in one study. The type of fatty acids present also is important, with a recommended ratio of omega-6 to omega-3 fatty acids of 5:1.
It may be that some lines of dogs ovulate fewer eggs than do other lines of dogs. There certainly can be a genetic component to litter size; that is well known from work in sows, where genetic selection for large litter size has been performed successfully for decades, if not centuries. Just as we can select for large litter size, we may inadvertently select for small litter size. Dog breeders often select breeding stock for traits that are not associated with reproductive success, including good hip conformation, normal bite, and good temperament. Unfortunately, good breeding candidates for these traits may carry "bad" genes for reproductive traits. Breeders can work away from this by monitoring reproductive success and including those factors in their decision of whom to breed.
Inbreeding coefficient is a numerical measure of how inbred a given individual is. Inbreeding coefficient varies from zero (not at all inbred) to 1.0 (completely inbred). Studies in laboratory species have demonstrated a decrease in reproductive efficiency with increase in inbreeding coefficient. Two studies have evaluated this effect in dogs. In a study evaluating 42,855 litters of dachshunds in Germany, it was determined that increasing inbreeding coefficient was associated with decreasing litter size and with increased number of stillborn pups. Dr. Joellen Gregory, in unpublished work completed while a student at the University of Minnesota, evaluated 72 Otterhound litters and also demonstrated a decline in average litter size with increasing inbreeding coefficient, as demonstrated graphically below.
Variation in average litter size with body weight in bitches
In conclusion, as a breeder it is important to consider all of the above when trying to determine cause of low litter size in a given bitch or in your kennel. Regular testing of bitches and stud dogs for brucellosis, maintenance of all animals in good body condition, and close observation for signs of reproductive tract disease also are imperative for you to optimize reproductive efficiency in your dogs.