(1) During the estrus cycle of cows, the ovaries also undergo corresponding cyclic changes, known as the ovarian cycle. This is the basis of physiological changes during the estrus cycle. The ovarian cycle can be divided into two stages: luteal phase and follicular phase.

Observing the luteal phase of imported cattle using B-ultrasound: from the formation of the corpus luteum after ovulation to the degeneration of the corpus luteum, it is equivalent to the late estrus and estrus periods. During this period, luteal activity was predominant. After ovulation, the corpus luteum forms, begins to grow, and secretes progesterone. It reaches a large volume (diameter 20-25 millimeters) 9-10 days after ovulation, and its secretion function also reaches its peak. If a cow is not pregnant, the corpus luteum maintains a large volume and function for only a few days, and begins to regress 14-15 days after ovulation. The concentration of progesterone in the blood drops sharply, and the volume of the corpus luteum rapidly shrinks. The luteal phase of imported cattle is observed by B-ultrasound for 16-17 days. During the luteal phase, the body is controlled by high concentrations of progesterone. Although larger and developing follicles may appear on the ovaries, mature ovulation and estrus are not possible, accompanied by partial follicle degeneration and atresia

Follicular period: The transition from luteal phase degeneration in the previous cycle to follicular maturation and ovulation in this cycle is equivalent to the pre estrus and estrus periods. After the degeneration of the corpus luteum in the previous cycle, 1-2 follicles rapidly grow on the ovaries and mature within 2-3 days. Cows show estrus and then ovulate. The follicular phase usually lasts for 5-6 days, and mature follicles have a diameter of 12-19 millimeters. Imported cows are observed by B-ultrasound during the follicular phase. During the maturation process, the follicles secrete a large amount of estrogen, and the body enters estrus under estrogen control. The follicles rapidly mature and ovulate

(2) Changes in the concentrations of several major reproductive hormones in the blood of cows during their estrus cycle.

Observation of progesterone in imported cattle using B-ultrasound: During the luteal phase, progesterone levels in the blood remain at their peak; After corpus luteum degeneration, progesterone rapidly decreases to baseline levels. High levels of progesterone suppress the secretion of hypothalamic gonadotropin-releasing hormone (GnRH), pituitary luteinizing hormone (LH), and follicle stimulating hormone (FSSH) through negative feedback. Resulting in the inability of luteal phase follicles to mature and ovulate, as well as the absence of estrus. During the follicular phase, the progesterone level is * * * low.

Observation of estrogen in imported cattle using ultrasound: During the follicular phase, there is a peak in estrogen levels; During the luteal phase, estrogen also experiences small fluctuations. During the follicular phase, due to the decrease in progesterone and the elimination of negative feedback inhibition, the secretion of hypothalamic gonadotropin-releasing hormone (GnRH), pituitary luteinizing hormone (LH), and follicle stimulating hormone (FSH) is enhanced, and follicles rapidly develop and mature, while secreting a large amount of estrogen, forming an estrogen peak. Before ovulation in cows, high concentrations of estrogen have a positive feedback effect on the hypothalamus and pituitary gland, promoting the massive release of luteinizing hormone (LH) and follicle stimulating hormone (FSH), leading to the formation of LH-FSH ovulation peak.

(3) Imported cows are observed by B-ultrasound for luteinizing hormone (IH) and follicle stimulating hormone (FSH), which form peaks in the blood after the estrogen peak, known as LHFSH ovulation peak, causing ovulation in cows. Cows typically ovulate 24 hours after the formation of the LH-FSH peak. At other times, luteinizing hormone (LH) and follicle stimulating hormone (FSH) are at baseline levels

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