Male dogs are generally capable of reproduction when their bodies develop to 85% of their adult weight at 6-12 months of age, but this also depends on the breed and physique of the dog, and is related to the environment, physique and nutrition. Breeding male dogs begin to produce sperm and store it in the epididymis. Semen is generally composed of sperm and a small amount of fluid secreted by the epididymis and prostatic fluid, so it is necessary to use veterinary B-ultrasound to observe the epididymis and prostate of male dogs. Male dog ejaculation is divided into 3 stages: the first stage is a clear liquid, mainly prostatic fluid, and contains some sperm. The second stage is a creamy white liquid rich in sperm, about 0.5-3mL, and the sperm concentration is different in different concentrations and colors.

Dogs are widely raised around the world. Therefore, all aspects of canine reproduction have always been the focus of research. However, due to the special reproduction mode of canines, most canines can only estrus twice a year, and the number of successful reproductions in a dog's lifetime is too small; when in estrus, instruments (such as veterinary B-ultrasound) are needed for observation. Thirdly, in order to ensure the normal growth and development of dogs and some other conditions, the breeding period of dogs is only 6-7 years; in addition, the breeding and management of dogs are mostly family-style and single-bred, and it is impossible to collect a large amount of data at the same time. These factors limit the information people get and hinder the study of the reproduction mode and law of this species. In fact, to summarize the reproduction mode and law of dogs, not only a large amount of data is needed, but also the long-term accumulation of staff, as well as the aggregation, classification, organization and analysis of data. Therefore, research on dog reproduction is more difficult than other livestock, and it takes more time, financial resources and energy. If a set of changes in reproduction indicators for dogs can be summarized from a large amount of data, it will not only have a certain guiding significance for the reproduction and management of dogs, but also provide a useful reference for the diagnosis of major obstetric diseases. In the process of summarizing data, veterinary B-ultrasound will play a great role.