The ultrasound examination of bovine ovaries is both fascinating and challenging. As the corpus luteum (CL) and follicles continue to grow and regress, explaining ovarian changes may be difficult. Especially, ovarian assessment can be performed at any given time point during the ovarian cycle. This article aims to provide an overview of the common structures and changes observed during ultrasound examination of bovine ovaries.

The estrus cycle of cows lasts for 18 to 24 days. During this period, there will be two to three follicular waves, leading to the emergence of dominant follicles. Although the corpus luteum maintains the production of progesterone, the primary follicle of the initial wave will regress due to atresia. Once luteal dissolution occurs and progesterone levels decrease, dominant follicles in the second wave of the cycle may continue to develop and ovulate.

Ovarian development during the estrus cycle of cattle

Animal B-ultrasound machine for detecting cow follicles

In ultrasound examination, fluid filled follicles appear as hypoechoic (black) areas within the ovarian stroma (Figure 2). As follicular waves continue to develop throughout the entire ovarian cycle, there are always larger follicles larger than 8 millimeters, except for the first few days of cycle 1. This makes identifying dominant follicles and predicting ovulation based on follicle size extremely challenging.

Ultrasound ovaries of cattle

Ultrasound image of mature follicles in cows

Cystic ovarian disease in cows is an important disease to consider, especially in herd management, as it can lead to abnormal cyclic activity and subsequent fertility decline. This condition is traditionally defined as the presence of fluid filled structures with a diameter greater than 25 millimeters on the ovary for more than 10 days without functional CL. The two types of ovarian cysts that cause reproductive/circulatory dysfunction are follicular cysts and corpus luteum cysts. The standard commonly used to define cyst types is:

1. Follicular cyst - smooth, thin-walled (less than 3 millimeters)

2. Luteal cyst - due to the presence of an inner layer of corpus luteum tissue and a thick wall (greater than 3 millimeters)

However, not all cystic structures strictly adhere to these standards, and distinguishing between luteal cysts and central fluid filled CLs may be challenging.

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