The estrus cycle and ovulation time of female dogs differ significantly from other domestic animals. In particular, the ovulation time during estrus varies greatly, often making it difficult for dog owners to determine the exact breeding time. Below is a brief introduction to the methods of determining the estrus cycle and ovulation time of female dogs.
-
Sexual Maturity and Appropriate Breeding Age
The sexual maturity of female dogs varies depending on breed, feeding and management conditions, and geographical environment. Generally speaking, small dogs reach sexual maturity earlier, while large dogs reach sexual maturity later. Sexual maturity usually occurs within the range of 6-13 months of age, and the breeding time should be later than the age of sexual maturity. In Germany, the breeding time of female dogs is determined by the local breeding association, usually during the second estrus after sexual maturity.
-
Estrus Cycle
Most female dogs have an estrus cycle lasting 6-8 months, while some dogs may have an estrus cycle lasting up to 12 months, which is considered physiological. The estrus cycle is divided into proestrus, estrus, metestrus, and anestrus (resting period).
The time from the start of vaginal bleeding to the acceptance of mating is known as proestrus, lasting 7-13 days. The time when female dogs accept mating is called estrus, lasting 3-21 days, with an average of around 9 days.
The luteal phase is generally considered as metestrus. Most female dogs still accept mating within 2(1-5) days of the metestrus phase. The duration of the metestrus phase is 9-12 weeks, by which time the corpus luteum has completely dissolved. However, many authors do not define the end of the metestrus phase as the dissolution of the corpus luteum, but as the complete cessation of uterine endocrine activity, which lasts for 18-20 weeks. The period between the end of metestrus and the beginning of proestrus is called anestrus. Although female dogs do not show signs of estrus during anestrus, it is found through examination that there are 3-4 follicular development and degeneration processes in the ovaries during this period, and there is also a mild elevation of estrogen levels in the blood, lasting for 1-3 months, ultimately entering the proestrus phase.
The follicular development phase lasts approximately 10-12 days, during which estrogen levels continuously rise. Estrogen reaches its target organs through the blood circulation, causing edema and hyperplasia of the vulva, vagina, uterus, and fallopian tubes. Blood flow to the mucosa increases, and blood cells from the uterine mucosa exude into the uterine cavity and mix with uterine secretions. The highest concentration of 17β-estradiol can be detected in the blood at the end of the follicular phase. Accompanied by the decreasing synthesis of estrogen in the follicles, a peak of LH secreted by the anterior pituitary gland appears in the blood, along with the release of follicle-stimulating hormone (FSH) (the regulatory mechanism of FSH secretion in the estrus cycle of dogs has not been fully explained). The appearance of the LH peak coincides with the formation of luteinization of follicular granulosa cells. Most female dogs begin to accept mating at this time. Ovulation occurs 2 days after the LH peak. The ova are fertile 2-3 days after ovulation, and the formation of the corpus luteum and the increasing production of progesterone begin with ovulation. Under the influence of progesterone, the swelling of the reproductive tract mucosa begins to subside, the blood vessel walls of the uterus begin to thicken, and the uterine endocrine phase begins.
-
Methods for Determining the Optimal Breeding Time
If female dogs can be bred during ovulation, the best pregnancy outcome can be achieved. However, it is regrettable that there is no reliable method to determine the ovulation time of female dogs in clinical practice. Nevertheless, comprehensive assessment using several methods can often yield good results. Indirect methods for determining ovulation time in female dogs include observing changes in the color of the vaginal mucosa, vaginal cytology examination, and progesterone measurement. If a good B-ultrasound diagnostic instrument is available, the ovulation time of the ovaries in female dogs can also be observed using B-ultrasound. It is worth noting that relying on any single method alone cannot definitively determine the exact ovulation time of female dogs. See Table 2 for details (omitted).
It is difficult to determine the ovulation time of female dogs based solely on the manifestations of proestrus and estrus because they vary widely. Observing the floe-like accumulation of vaginal mucosal cells and measuring progesterone levels
