Dog Owner's Home Veterinary Handbook.pdf - Mr. Walnuts

440 •DOG OWNER’S HOME VETERINARY HANDBOOK
elevated for 8 to 10 weeks during diestrus, and returns to baseline levels during
anestrus. Progesterone is essential for keeping the uterine lining receptive
to the implantation and growth of embryos. Removal of the ovaries during
early pregnancy, or failure of the corpora luteal cysts to produce progesterone,
results in spontaneous abortion.
The hormonal effects of the estrous cycle have implications for preventing
pregnancy (see Contraceptive Drugs, page 457). If progesterone is given during
the first three days of proestrus, it blocks the release of pituitary FSH and aborts
the heat cycle. Testosterone works by blocking the release of LH from the pituitary
gland. It must be given 30 days before estrous to abort the heat cycle.
WHEN TO BREED
The most common cause of an unsuccessful mating is bad timing. Many dog
owners do nothing more than count the days in a cycle. They attempt to
breed on the 10th through the 14th day of the heat cycle. But, as noted earlier,
each female is an individual with her own breeding timetable—a
timetable that can vary according to the lengths of proestrus and estrus.
Ovulation cannot always be predicted just by counting the days of the heat
cycle. Furthermore, counting will not be accurate if you miss the early signs of
heat or if your bitch shows very little evidence of being in heat.
Fortunately, nature provides a safety factor: Fresh sperm are able to survive
for up to seven days in the female oviducts. An offsetting factor is that eggs
must mature in the oviducts for three days before they can be fertilized. This
still leaves a window of several days before and after ovulation for successful
breeding. In fact, the peak of female fertility occurs three days after ovulation.
There are reports of bitches being bred as early as the fourth day and as late
as the 21st day of the heat cycle—and still conceiving a litter. When the heat
cycle is this atypical, a combination of vaginal cytology and progesterone measurements
can be used to determine estrus and predict the time of ovulation.
Multiple breedings are more likely than a single breeding to produce pregnancy,
and may even have an influence on the size of the litter. Accordingly,
most veterinarians recommend breeding a bitch every other day (or every
third day) for as long as she remains in standing heat. If, for any reason, a
bitch cannot be bred more than once, consider timing that breeding using
progesterone assays to predict the exact time of ovulation. Using hormonal
assays to determine the best time to breed is especially important if you are
using fresh-shipped semen or frozen semen.
THE TIE
Male dogs differ from humans in that they do not have seminal vesicles in
which to store sperm. Sperm flows directly into the urethra from the spermatic