THE SYNCHRONIZATION OF THESE HORMONAL CHANGES AND THE PHYSIOLOGICAL STATE OF THE FEMALE DOG’S REPRODUCTIVE TRACT PLAY A PIVOTAL ROLE IN DETERMINING THE BEST BREEDING WINDOW.

Female dogs in estrus refer to the periodic changes in physiology and behavior caused by hormonal secretion in dogs under the control of hypothalamus-pituitary-gonad axis after dogs reach sexual maturity. The estrus cycle of dogs can be artificially divided into four stages: pre-estrus, estrus, post-estrus and anestrus. Generally, depending on the breed of dogs, the pre-estrus lasts about 6~11 days, the estrus lasts 5~9 days, the post-estrus lasts about 56~60 days, and the anestrus can last for several months. During the inter-estrus period, the serum estrogen level of female dogs maintains a low basal level. As the follicles develop and mature, the level of estrogen in the body gradually rises to a plateau, causing a series of physiological and behavioral changes in female dogs. In the initial days of pre-estrus, follicular cells begin to secrete progesterone as a precursor hormone for estrogen secretion. In the 1~2 days before entering estrus, the serum estrogen concentration in female dogs reaches a peak. After the follicles develop and mature, the estrogen concentration decreases and enters estrus. With the luteinization of the follicles, a large amount of progesterone is secreted, and the estrogen will gradually decrease to the basal level over the next 5~20 days. The gradually increasing progesterone concentration and decreasing estrogen level in the serum of dogs during pre-estrus lead to the change of female dogs’ behavior from refusing mating to active courtship behavior. In addition, changes in hormone levels have a strong positive feedback effect on the hypothalamus and pituitary, causing the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH).

Progesterone is a steroidal hormone secreted by the corpus luteum, which plays a crucial role in various reproductive activities of mammalian females, such as ovulation, maintaining pregnancy, initiating childbirth, and mammary gland development. The serum progesterone concentration in dogs is low in the last 48-72h before the end of pre-estrus, with a basal value below 0.5ng/ml. At the end of pre-estrus and the beginning of estrus, accompanied by the decrease of estrogen concentration, the serum progesterone concentration in female dogs will gradually increase to above 1.0ng/ml. On the other hand, the function of follicular cells to synthesize and secrete progesterone precedes the development of corpus luteum. The progesterone secreted by these cells is the reason for the increase in serum progesterone concentration at the end of pre-estrus and the special physiological and behavioral changes in female dogs. As the follicles ovulate and luteinize, the serum progesterone concentration gradually increases. Concannon et al. studied that the peak of LH secretion occurs 24~48 hours before ovulation after the formation of corpus luteum, and reaches a peak and enters a plateau period after ovulation. Zhou Tiansheng monitored the development of reproductive organs and serum hormone levels in dogs and found that the serum progesterone concentration reached a peak when beagle dogs developed to 12 months of age. During the estrus cycle, the serum progesterone level generally showed an upward trend before the 9th day and reached a peak on the 9th day, followed by a gradual decrease, which is consistent with the statistical results of this study.

He Xingliang et al. observed the morphological changes of cells in different stages of estrus in female dogs using the “vaginal smear method” and compared the acceptance of mating, pregnancy rate, and litter size of female dogs when the vaginal epithelial keratinocyte rate was 70%~80%, 80%~90%, and 90%~100%, respectively. The results showed that when the degree of vaginal epithelial keratinization was low, the mating success rate, pregnancy rate, and litter size of female dogs were all reduced, and the best mating timing was when the keratinocyte rate reached 90%~100%. The cytological changes of vaginal epithelium are related to the changes of hormone levels in female dogs. Xu Yongzhi et al. explored the changes of vaginal keratinized epithelial cells in naturally estrus female dogs and found that most naturally estrus female dogs had a keratinized epithelial cell proportion exceeding 80% and obvious ice-like accumulation of vaginal mucosa after the appearance of bloody secretions in the vagina for 9~13 days. This study found that most Tibetan mastiffs and German shepherds had serum progesterone concentrations that rose to suitable breeding ranges after 9~15 days of estrus, and some dogs had keratinocyte rates in vaginal smears of over 80%, which is consistent with the research results of Ma Wenzhi et al. Hori et al. found that beagle dogs ovulate about 11 days after the appearance of bloody secretions in the vagina.