Breeding management 102 — It's all in the timing (Proceedings)
Canine breeding management centers hinges on ovulation timing. Ovulation timing is important because it is critical to have viable spermatozoa in the bitch's oviducts at the time the eggs are mature and ready to be fertilized. In many bitches, the period of sexual receptivity (standing and flagging for the male) does not coincide with the bitch's most fertile period. Breeding the bitch based on sexual receptivity, when it does not coincide with the fertile period will result in non-pregnancy; or alternatively if the period of receptivity overlaps somewhat (either at the beginning or at the end) with the fertile period, conception may result but litter size will be reduced. Optimal breeding management is designed to time the breeding precisely during the period of peak fertility. Not only does this result in maximal pregnancy rates and litter size but it also minimizes the number of breedings required, thus reducing the potential for overuse of the dog.
Great advances in canine assisted reproduction techniques have been achieved during the past 20 years. In additional to natural breeding, artificial inseminations with fresh "neat" (unextended) semen, fresh-chilled extended semen and frozen semen are now viable breeding options. With each technological advance, so too come more manipulations by human hands and an increased risk for human error in semen processing and insemination.When good quality, fresh semen is used for breeding with either natural mating or "side by side" artificial inseminations, there is much forgiveness with respect to breeding timing. This is because fresh semen remains highly viable and "fertilizable" for 5 to 6 days once deposited in utero. Live spermatozoa have been recovered from the uterus up to 11 days post-breeding. In contrast, survival of fresh, "neat" semen in extrauterine environments is poor.
Advances in preserving extrauterine longevity of spermatozoa include both reducing energy consumption and increasing energy sources by and for the sperm cells. Lowering the temperature of sperm cells slows metabolism and reduces their energy consumption. Replacing seminal plasma with extender bathes sperm cells in an energy-rich environment. However, even optimally extended and cooled spermatozoa fail to survive nearly as long as their fresh, intrauterine counterparts unless the extender is replaced with a new aliquot every 48 to 72 hours.
Fresh-chilled sperm cells have a relatively short life span. Fresh-chilled sperm cells, once warmed to body temperature and in utero, lives about 24 to 72 hours. Properly frozen sperm cells can be stored indefinitely in liquid nitrogen at -322°F (-180°C). Once thawed, however, sperm cells have an ultra-short life span. Frozen-thawed spermatozoa live an average of 12 hours, 24 hours maximum! Curtailed sperm cell longevity necessitates precise ovulation timing and breeding.
There is an art to the science of ovulation timing (OVT) in the canine. Unlike other species in which ovulation occurs in an estrogen environment, ovulation in the canine occurs only after progesterone levels have risen to >4ng/ml. Another unique feature is that the bitch ovulates ova in the primary oocyte stage. These primary oocytes must undergo another meiotic division and further maturation before they become secondary oocytes and can be fertilized. Unlike most species in which breeding/insemination is timed to coincide with ovulation, insemination in the bitch is performed 2 to 4 days after ovulation. It is important to remember that, in the bitch, the critical timing events occur 4 to 6 days before the optimal breeding dates.
A number of tools are available for ovulation timing. These include LH (luteinizing hormone) assays, qualitative and quantitative progesterone assays, vaginal epithelial cytology, vaginoscopy, examination of external genetalia and behavioral assessment. Success in breeding is optimized when as many tools as possible are utilized to time ovulation.
Increased estrogen causes an increased turnover rate of vaginal epithelial cells, resulting in the progressive cornification seen on vaginal cytology, and thickening of the vaginal wall in preparation for breeding. Also seen is progressive edema of the vaginal mucosa, which can be visualized with endoscopic examination. Estrogen assays are performed by radioimmunoassay (RIA) by many commercial laboratories. However, the information given is of little value for ovulation timing, since peak estrogen levels are variable from bitch to bitch, and even relative changes do not correlate to ovulation or the fertile period. Estrogen is best assessed by serial vaginal cytology evaluation and vaginoscopy. Estrogen levels do not indicate the fertile period since ovulation is triggered by the LH surge, not an estrogen peak.
Estrogen levels rise, peak and begin to decline during proestrus. Under the influence of estrogen, the vulva and vagina become edematous, a serosanguinous vulvar discharge (of uterine origin) is present, the vaginal epithelium becomes thickened and cornified and the bitch exhibits behavioral signs of estrus. However, blood estrogen levels are highly variable, do not accurately predict ovulation and are not useful as a tool for timing ovulation.