Bitches are mono-oestrous animals, and the duration of a single oestrous cycle is long. The canine oestrous cycle is divided into four stages: pro-oestrus, oestrus, di-oestrus and anoestrus. The duration of canine pro-oestrus varies among individuals, ranging from 3 to 27 days (mean: 8.1 ± 2.9 days) (Tsutsui and Shimizu 1973). This period corresponds to follicular development, and male dogs are not accepted. Oestrus starts when females accept mating, and ovulation occurs on approximately day 3 of oestrus (Tsutsui 1973). Follicular development reaches the maximum when the cycle enters oestrus, and an LH surge is induced by positive feedback to the pituitary, inducing ovulation. The duration of oestrus also varies among animals, ranging from 5 to 20 days (mean: 10.4 ± 2.7 days), that is, acceptance of males continues after ovulation, and this is a characteristic of canine reproductive physiology not noted in other animal species. It is known that the time of ovulation after the onset of vulval bleeding at pro-oestrus markedly varies in bitches because of variation in the duration of pro-oestrus. However, generally, many dog breeders calculate the mating date from the onset of vulval bleeding at pro-oestrus and mate dogs 10–14 days after the onset. Although canine ova are ovulated in an immature state, they remain fertile for many days after ovulation and because sperm retain fertilizing capacity for approximately 5 days, the duration of the period when copulation may achieve conception is long: approximately 7 days, starting from 48 h before ovulation until 108 h after ovulation (Tsutsui 1989). Thus, the above mating schedule does frequently result in fertility. But, mating too early or late may reduce the fertilization rate (the rate of the number of implanted ova to the number of ovulated ova), and, of course, it is desirable to mate dogs in the optimum mating period, for which a method to accurately identify the ovulation day by measuring the plasma LH or progesterone (P4) concentrations has been established. However, simultaneously, it is interesting to know the characteristics of individual variation of the time of ovulation (days), that is, individual variation of the follicular phase. The variation of length from vulval bleeding to LH peak within bitches has been previously characterized by one report (England et al. 1989). They showed that the mean calculated day of LH peak was 10.8 ± 2.8 days after the onset of pro-oestrus, but there was great variation between bitches and between cycles in several of the bitches. But the individual variation of the time of ovulation after the onset of bleeding in dogs has not been well characterized, and there has been no previous report in which the ovulation day was investigated by the age.
In this study, we investigated the duration of the period between the onset of vulval bleeding at pro-oestrus and ovulation (ovulation day) in a large number of beagle bitches. The ovulation day was estimated from plasma P4 concentrations and the influence and association of individual animal variation, age and duration of the oestrous cycle were investigated. This information would be important for the purposes of making decisions on planned breeding at clinical practice and may be very valuable basic knowledge for research of canine reproductive physiology.