Progesterone: A Brief Overview

What is progesterone?

The sea urchin is an unexpected place to find hormones!
The sea urchin is an unexpected place to find hormones!

While perhaps not as well-known a female sex hormone as oestrogen, progesterone is one of the most important hormones in the female reproductive system. Although most commonly associated with mammals, this hormone is produced by many different animals, from chickens and turtles to fish and sea urchins.

As a steroidal sex hormone, it is primarily involved in the development and maturation of reproductive systems. However, there is a growing body of research on the many and varied roles of progesterone in other parts of the body. Progesterone also promotes brain development and can protect against brain damage. It also complements both the immune system and insulin regulation. As research continues into this fascinating subject, perhaps we will find more ways in which this hormone contributes to the complexity of bodily functions.

How is it produced?

In mammals, progesterone is produced in varying amounts throughout both the menstrual and oestrous cycles of animals. Aside from human and our closest relatives, only a few other species menstruate. The vast majority of mammals instead follow an oestrous cycle. Both cycles involve preparing the uterus for pregnancy, but each do this in slightly different ways. One of the main differences is that in an oestrous cycle, animals go through a specific heat period where they are fertile and will accept mating. In a menstrual cycle this is not the case.

 Chemically, progesterone usually functions in a similar way regardless of the type of cycle. Ovaries and ovarian structures produce progesterone, and, during pregnancy, so does the placenta. In most animals, placental progesterone takes over as the most significant source and this maintains the pregnancy.

In cows, the main source of progesterone is the corpus luteum, or yellow body. This is a structure that forms on the surface of the cow’s ovary after the release of an egg. Unlike in many other mammal species, the placenta of a cow doesn’t produce a large amount of progesterone.

What is the role of progesterone in heat in dairy cows?

From the time a heifer reaches puberty, progesterone plays a key part in a cow’s reproductive system. As a calf matures into a heifer, her hormone levels will fluctuate. Just before her first heat cycle, progesterone increases briefly. Once she has had her first heat cycle, she will start to cycle regularly.

Cows follow an oestrous cycle. This is a 21-day hormonal cycle which prepares the cow’s body for pregnancy and fertilisation. For a few days of this cycle, a cow will be in heat and will be fertile. Being able to detect this period is crucial for dairy farmers who use AI, as this is the only time insemination will be successful.

For most of her cycle, a cow’s progesterone will be high. This hormone is produced consistently by the corpus luteum, on the ovary. As other hormones begin to rise, it regresses to allow a new follicle to develop. This inhibits the production of progesterone. As a result, it drops rapidly. Once her it is low, usually less than 1.5ng/ml in milk, a cow is in oestrus. This is the best time to inseminate, and usually the time when any behavioural signals, such as bulling, will be shown.

How does progesterone help during and after pregnancy?

A pregnancy will only be maintained if progesterone is high.
A pregnancy will only be maintained if progesterone is high.

Once a cow is pregnant, her progesterone will remain high. If the it drops too low, it can cause a cow to lose the pregnancy, or induce early labour. The corpus luteum produces the majority of this. As the pregnancy gets more advanced, both the placenta and the cow’s adrenal glands will also produce some. A combination of progesterone and prolactin, another hormone, stimulate the mammary glands to produce milk.

In the few days before the cow gives birth, her progesterone will drop. After she has given birth, the cow’s progesterone levels will stay low until her reproductive system has recovered and she returns to normal cycling.

How can hormone tests help with heat detection?

Heat detection is a well-known essential component of the modern dairy industry. With the significant uptake in AI over the past few decades, determining exactly when a cow is in heat has become paramount.

While there are many heat detection methods, the most reliable is hormone testing. Cows can have silent heats or their heats can be missed. Progesterone testing, on the other hand, will expose false bulling for what it is, and is extremely reliable.

Rapid cow-side dip stick tests like P4Gold allow farmers to take herd fertility into their own hands and get real time information on each cow’s fertility status.