Characteristics of the Bovine Estrous Cycle

- [Andrew] Hello, this is Andrew Sandeen and I'd like to take a few brief moments to explain some key characteristics of the estrous cycle in cattle.

Let's start at the ovary, a very busy place in a mature non-pregnant heifer or cow.

There are two ovaries and on them are many follicles of various sizes and stages of development.

The two ovaries were loaded with follicles prior to birth, and each follicle houses a single oocyte, also commonly known as an egg.

One or more of these follicles can grow to become a prominent structure on an ovary and secrete estrogen into the circulation.

The other prominent structure often on ovaries, the corpus luteum or CL, produces high concentrations of progesterone, the hormone known for maintaining a pregnancy.

Heifers and cows have an estrous cycle that is 21 days in length on average during which time follicles grow in a wave-like pattern with two or three waves of growth during each cycle.

A cohort of healthy follicles, indicated here with yellow circles, grow in size and compete for dominance until most have regressed, indicated in red.

In a high progesterone environment represented by the shaded orange curve, all of the developed follicles eventually fade away and the new wave begins.

However at the end of the cycle when progesterone has declined, a follicle is able to continue developing to the point of ovulation.

Towards the end of the estrus cycle around day 17, if a cow is not pregnant, prostaglandin F2 alpha is released from the uterus and causes regression of any functional CL on the ovaries.

As a result of CL regression, progesterone production ceases.

In the absence of high progesterone, a developing follicle can continue to develop which also causes estrogen concentrations to rise and eventually trigger estrus.

High estrogen from a preovulatory follicle causes significant changes in cow behavior.

It is usually quite obvious when she becomes restless and allows other animals regardless of gender to mount her.

Around this same time estrogen also triggers a surge of luteinizing hormone to be released from the pituitary gland which has direct impact on the preovulatory follicle and initiates events that lead toward ovulation.

Ovulation generally occurs about one day after the surge of luteinizing hormone.

It also happens to be one day after the onset of standing estrus, our best visual predictor of when ovulation will take place.

At ovulation, the oocyte is released from the fluid-filled follicle into the oviduct where it can be fertilized.

Meanwhile the follicular wall collapses in on itself and is rapidly transformed into a functional corpus luteum producing progesterone.

So back to the diagram.

We have a 21-day estrous cycle on average with follicles developing and regressing throughout.

Progesterone represented with orange, prepares the uterus for pregnancy and limits development of follicles on the ovaries for most of the cycle.

But when the CL regresses and progesterone declines, that is the moment when a follicle can really take off producing more and more estrogen, represented by the brown line, which causes estrus.

A surge of luteinizing hormone from the pituitary at the end of the cycle triggers ovulation to occur a day or so later.

If the ovulated oocyte is fertilized and pregnancy is established, the CL will not regress during the next cycle and progesterone production will be sustained helping maintain pregnancy.