Hollinshead FK, Hanlon DW. “Factors affecting the reproductive performance of bitches: A prospective cohort study involving 1203 inseminations with fresh and frozen semen.” Theriogenology 101 (2017): 62-72.
This study describes a variety of factors affecting the reproduction of female dogs using artificial insemination.
Researchers were interested in studying reproductive success in female dogs (bitches) after artificial insemination (AI). Artificial insemination refers to the deliberate introduction of semen into a female by a veterinarian. No natural breedings were included in this study.
Unlike many other studies about AI, this is a large-scale study using multivariate analysis (examining multiple variables instead of just one) to find effects on reproductive success. Although these studies are often time-consuming and costly, they are very powerful research tools.
Over nine years, researchers studied a total of 1146 bitches from 84 different breeds. They studied a total of 1203 artificial inseminations, about half from frozen-thawed semen and half from fresh (chilled) semen. The semen was from 1094 different males. When designing the study, the researchers controlled many factors to ensure that the artificial inseminations were done in the same way.
After the data was collected, scientists used powerful statistical methods to analyze the data and report findings. The findings from this study are varied and cover a wide variety of factors.
The average whelping rate (the chance of a bitch producing at least one pup, whether alive or dead) among all bitches was 74% and the average litter size was 5.8 pups per litter. It is important to note that with these findings, and all findings in this study, the researchers are examining averages. These averages are over a very large number of data points and varied widely with individual dogs.
The use of chilled fresh semen was, on average, more successful than frozen-thawed semen. The average whelping rate was 80% for bitches inseminated with fresh (chilled) semen, compared with only 71% for bitches inseminated with frozen-thawed semen. Additionally, bitches inseminated with fresh semen had significantly larger litter sizes (average 6.2 pups per litter) than those inseminated with frozen-thawed semen (average 5.4 pups per litter).
Researchers measured the motility (movement) of the semen and found that semen classified as having “poor” motility resulted in a significantly lower whelping rate than semen classified as having “good” or “excellent” motility. Semen with poor motility only had a 37% whelping rate.
Age appeared to significantly affect whelping rate and litter size. Researchers observed a steady, linear decline in whelping rate with increasing age. For each additional year of bitch age, litter size decreased by 0.13 pups per litter on average.
Greyhounds and Labrador Retrievers had a significantly higher whelping rate (meaning whether at least one puppy was born) compared with all other breeds. Greyhounds had a whelping rate of 88% and Labradors had a whelping rate of 94%. These breeds’ average whelping rates are much higher than the average of all other breeds (71%).
Researchers also found reproductive differences between different breed sizes. Smaller breeds had significantly smaller litters (average 4.4 pups) than medium (average 5.2 pups), large (average 5.9 pups), or giant (average 6.7 pups) breeds.
The average gestation length (length of pregnancy) was 65 days. Greyhounds had significantly longer pregnancy duration (average 68 days) than other breeds. Age also influenced gestation length. For each additional year of bitch age, gestation length increased by 0.11 days. For each additional pup per litter, gestation length was reduced by 0.08 days.
Scientists also recorded whelping outcomes. Of all bitches, 46% whelped normally without assistance, 28% had an elective (planned) C-section, 23% had an emergency C-section, and 3% required veterinary assistance for delivery. Smaller litters increased the chance of C-section. Bitches with litter sizes of one or two pups had a C-section rate of 83%, while bitches with litter sizes of three or more pups had a C-section rate of 43%.
Brachycephalic breeds (short-snouted breeds like Bulldogs and Pugs) were 11.3 times more likely to have a C-section compared to all other breeds. Most of the C-sections for brachycephalic breeds were elected by either the owners or the veterinarians.
The overall whelping rate after AI in this study (74%) was nearly identical to the whelping rate reported in previous studies involving large numbers of bitches. When different studies agree, it can strengthen the statistical power of the findings.
This study provides important information about whelping rates, litter size and the prediction of whelping in certain breeds for breeders and veterinarians.
These findings are only a short summary of this study. We encourage interested readers to read the “Discussion” section of the paper for a more in-depth look at these factors and comparisons to previous studies.