AKC eNewsletter

Winter 2007
Timing Is Everything: Breeding Strategies and the Estrous Cycle

Part One: Understanding the Cycle's Stages
By Arliss Paddock

The reproductive potential of the bitch is the cornerstone of any breeding program. If you as a breeder wish to optimize that potential, it’s important to have an accurate understanding of your bitch’s heat cycle. And as new technology makes increasingly available advanced reproductive techniques such as artificial insemination and the use of chilled or frozen semen, that knowledge becomes even more vital.

Border Terrier
Photo by Isabelle Francais
Border Terrier

Not All Bitches Are “Average”
Misconceptions among breeders about aspects of the heat cycle are surprisingly common. “The most common cause of apparent infertility is improper timing of breeding, because of the [misguided] assumption that all dogs ovulate about 11 days after first signs of heat, and so will be fertile when bred at 9, 11, and 13 days,” says Dr. Margaret Root Kustritz, associate professor of small-animal reproduction at the University of Minnesota College of Veterinary Medicine. “The most common misconception by far is that all bitches are average bitches; in fact, many normal bitches are not average.”

Veterinarian Phyllis Giroux, who breeds Labrador Retrievers and Norwich Terriers, encounters similar misunderstanding among her clients regarding cycles. She notes, “I still hear, on a daily basis, ‘She’s not ready to breed, it’s only day 5,’ or ‘We don’t need to test yet, it’s not day 12.’ ”

Blindly playing a generic numbers game isn’t an effective breeding strategy. Of course it is helpful to know what is average, as a point of reference, but a more complete understanding of the heat cycle and its variations is the key to success.

An Overview of the Basics
For starters, the heat cycle is more properly referred to as the estrous cycle. (It can be a little confusing that estrous is identical in sound to estrus, the name of one of the cycle’s stages.) The estrous cycle is the ongoing, repeating sequence of events which, throughout the bitch’s life (or until she is spayed), produces eggs ready to be fertilized, and prepares and maintains her reproductive organs to provide for the union of sperm with the eggs and the subsequent nurturing of offspring.

A bitch’s first cycle may occur at anywhere from 6 to 24 months of age; the average is between 10 and 12 months. As with several aspects of the estrous cycle, there is much variation in age at puberty (first heat), both among individuals of a single breed and from breed to breed.

Small breeds generally reach puberty earlier than larger breeds, although there are exceptions to this: Several small breeds, including the English Toy Spaniel, Brussels Griffon, Schipperke, and Lakeland Terrier, may not cycle until age 12 months or later.

Certain breeds tend to have especially late puberty. Clumber Spaniels, Samoyeds, and many sighthounds may not have their first heat until they are 18 to 24 months old.

The duration of the overall cycle—the time from one “heat” to the next—also varies greatly among both breeds and individuals. (An individual bitch’s own timing will generally be consistent from one cycle to the next, however.) Although the average overall duration is seven months, some breeds, such as German Shepherd Dogs, Chow Chows, and Rottweilers, may have cycles as short as four or five months. And a few breeds, including the Basenji and the Tibetan Mastiff, cycle only once a year.

The Four Stages: A Primer
But whatever the variation in timing, the internal sequence of events, intricately controlled by hormones, is exactly the same for all bitches. The cycle is continuous, but for the purpose of understanding the processes it is commonly divided into four stages: proestrus, estrus, diestrus, and anestrus. Let’s take at look at each individually.

Proestrus is the beginning of what we generically refer to as “heat.” This stage begins with swelling of the vulva and the first appearance of blood-tinged discharge.

Proestrus averages 9 days, with a range of from 1 to 17 days. The male will be interested in the bitch during this time, but she won’t allow breeding—she may tuck her tail, sit, or even snap if he tries to mount.
It is important to note that in this as well as other stages, outward signs and behavior can vary greatly in duration and intensity among individual bitches.

During proestrus, escalating hormonal activity stimulates the bitch’s ovaries to prepare for ovulation, and begins to prepare the uterus for pregnancy. Estrogen levels will peak just prior to the end of this stage.

Your veterinarian’s microscopic examination of vaginal smears taken over the course of proestrus will show a gradual shift of the epithelial cells (cells of the vaginal lining) to predominantly cornified (converted to a harder type of tissue and having a distinctive shape), from predominantly noncornified. Complete cornification occurs about two days before estrogen peaks, and about four days before the bitch will allow breeding.

Thus, microscopic exam of vaginal smears to assess the degree of cornification is one way to keep tabs on the approach of the next stage.

Estrus, also known as “standing heat,” is the period during which the bitch will accept the male to be bred. This stage begins when she will stand still and raise her tail up and to the side (called “flagging”) on his approach. With the onset of estrus, the vulvar discharge may lose much of its red tinge and shift to more of a straw color.

Estrus averages 9 days, ranging from 3 to 21 days. Much hormonal activity continues to move the breeding cycle forward: As estrus begins, estrogen levels fall. Rising progesterone causes the bitch to be receptive to mating. The release of a surge of luteinizing hormone (LH) from the pituitary gland on about the first day of estrus stimulates ovulation—the release of eggs ready to be fertilized—approximately two days later. This happens on the third day of estrus, on average, but with great variation among individual bitches.

Following the release of each egg is the formation of a progesterone-producing structure in the ovary called the corpus luteum (CL). The CL will remain in the ovary for about 60 days, continuing to produce progesterone sufficient to support a pregnancy.

Since progesterone levels rise at ovulation, it can be helpful to have blood tests performed every few days during estrus to detect this sudden increase in progesterone that will pinpoint when ovulation has occurred—which, in turn, will allow accurate prediction of the whelping date, 63 days from ovulation.

Diestrus is the period of about two months following estrus. The bitch’s first refusal of the male is generally considered to signify the beginning of this stage. Another, more clear-cut signifier of the onset of diestrus is when microscopic exam of vaginal cells shows an abrupt return to complete non-cornification.

In a bitch that has been successfully bred, diestrus is the period of pregnancy. Progesterone levels during this time are the same, however, whether or not the bitch was bred or became pregnant. A result is that the nonpregnant bitch will sometimes develop pregnancy-related signs and behavior, a phenomenon known as “false pregnancy.”

Anestrus is the long resting phase of the cycle, averaging 60 days, but ranging from 50 to 80 days. During anestrus the bitch is completely uninterested in breeding. This period of reproductive inactivity, characteristic of wild as well as domestic dogs, differentiates canines from many other species—including cows, mice, and humans—which have back-to-back ovulation cycles every two to four weeks or so throughout the year.
Once you’ve gained a detailed understanding of the basic aspects of the bitch’s reproductive process, you’re ready to employ this knowledge in making decisions that will give you the best chance of breeding success.

Coming Up in Part Two
Tests, Tips, and Breeding Options

References and Further Reading
The Dog Breeder’s Guide to Successful Breeding and Health Management, Margaret V. Root Kustritz, DVM, Ph.D. Saunders, 2005.

Canine Reproduction: The Breeder’s Guide, Phyllis A. Holst, MS, DVM. Alpine, 2000 (second edition).

Successful Dog Breeding, Chris Walkowicz and Bonnie Wilcox, DVM. Howell Book House, 1994 (second edition).

Book of the Bitch: A Complete Guide to Understanding and Caring for Bitches, J.M. Evans and Kay White. Interpet (UK), 2002.

Arliss Paddock breeds and shows English Cocker Spaniels and is former managing editor of the AKC Gazette.

Ronald N. Rella, Director, Breeder Services
Email: AKCbreeder@akc.org
Customer Service | Phone: 919-233-9767 | Email: info@akc.org

© The American Kennel Club 2007