By Merry Fitzgerald, DVM
How do I determine the best day to breed my bitch?
OVULATION TIMING Is a valuable tool to insure a successful breeding. It will determine when to breed a bitch to optimize the chance for pregnancy, as well as achieve conception in bitches that have a history of infertility. Ovulation timing enables you to breed precisely during the period of peak fertility, resulting in higher conception rotes and litter sizes, as well as minimizing the number of breedings required.
Faulty timing results in more missed breedings than any other cause. Chilling or freezing semen shortens its life span,
therefore accurate ovulation timing is crucial for success with chilled semen and essential when using frozen semen. Another benefit to ovulation timing is the ability to accurately calculate the whelping date.
The three important hormones in a bitch's estrous cycle are estrogen, luteinizing hormone ( LH), and progesterone. There are several ways to monitor hormones in a cycling bitch, including vaginal cytology (smears), blood serum LH levels, and blood serum progesterone levels. With each of these methods, proper interpretation of the results is necessary to properly time the insemination.
Estrogen is the hormone that causes the bitch to act receptive. Rising estrogen levels result in swelling of the vulva, bloody vaginal discharge, and changes in the vaginal wall. Vaginal smears are made using a cotton tipped applicator to gently remove cells from the bitch's vag inal vault. These cells are then applied to a glass slide by rolling the swab across the surface. The slide is dried, stained, and examined under a microscope for structural changes in the cells. As the estrogen levels rise in the female, the vaginal walls thicken, producing "cornification" (flatter cells with sharper edges) as the vagina prepares for copulation.
In the average bitch, estrogen rises anywhere from 4 to 20 days before ovulation. Because of this very w ide range,estrogen levels are only a rough estimate of when the bitch w ill be fertile. Estrogen plays no actua l role in ovulation. It is easy and inexpensive to make smea rs and evaluate vaginal cytology daily or every other day until approximately 70% of the vaginal cells are cornified. At this point, direct hormonal assays of LH or progesterone can be done to determine the exact fertile period.
Luteinizing hormone ( LH) is the hormone that actually causes ovulation. It is normally at low levels in the blood, then suddenly surges and drops back to near zero again. Two days later,ovulation occurs. The eggs must mature for 2 more days before they are capable of being fertilized. The eggs are receptive for 2-3 days, after which they deteriorate and die if they have not been fertilized. If you count the LH peak to be day zero,the eggs can be fertilized 4 to 7 days later.
LH testing is the most accurate method of ovulation timing. Daily blood testing is required since LH is only elevated for a short time,possibly as little as 18 hours. Serum samples can be drawn, frozen,and run in batches if daily laboratory access is not available. By identifying the LH peak and counting it as day zero, you can also calculate the whe lping date. Plus or minus 1 day,the due date will be 65 days from the LH peak. A planned Caesarian-section can safely be done on days 63, 64, or 65.
Progesterone is a hormone that is also normally at low levels. As the bitch's heat cycle progresses, progesterone levels rise suddenly. Progesterone levels remain elevated for 8-9 weeks (the duration of a pregnancy) whether or not the bitch was bred or is pregnant. At the end of the pregnancy, or 8-9 weeks later,the progesterone level drops abruptly.This sudden drop causes labor to start and is also responsible for the symptoms of false pregnancy.
Progesterone concentration can be tested by a qualified laboratory with a small blood sample drown from the bitch every 1-2
days as she is nearing ovulation. The first rise of progesterone above 2 ng/ ml usually correlates with the day of the LH surge.
Assuming that correlation, a bitch con begin breeding 2-4 days after the progesterone rises over 2 ng/ mL. The level w ill reach about 5.0 ng/ml the day of ovulation and should be in the 8.0- 12 .0 ng/ml range when the eggs are fully mature. Because progesterone continues to rise, as compared to a quick surge, testing can be done every 2 to 3 days in most cases.