The Estrus Cycle in Goats
Days are getting shorter, and if you’re lucky, the weather is getting cooler. For those of us who dwell in the Northern Hemisphere, that means the breeding season is upon us. Goats experience seasonal polyestrous, meaning they have multiple heat cycles induced by daylight changes. As the days become shorter, goats begin having their heat cycles. Understanding what controls the estrus cycles in goats helps increase fertility and manipulate the estrus cycle in more management-intensive breeding programs.
How the Hormones Work
There are several key hormones associated with the estrus cycle in goats. Several of these are produced in the brain — GnRH, LH, and FSH.
• GnRH, or gonadotropinreleasing hormone, is produced by the hypothalamus. The release of this hormone is stimulated when longer nights cause an increase in melatonin from the pineal gland. GnRH, in turn, causes the release of LH and FSH from the anterior pituitary.
• LH, luteinizing hormone, is responsible for forming the corpus luteum on the ovary and stimulating ovulation.
• FSH, follicle-stimulating hormone, is, unsurprisingly, responsible for the production of ovarian follicles.
Not every follicle present ovulates during a heat cycle. The dominant follicle is the ovulatory follicle. Goats can have more than one dominant follicle, leading to multiple kids per pregnancy. There are three hormones produced by the reproductive tract itself: Estrogen, progesterone, and prostaglandin f2alpha.
• Estrogen is produced by ovarian follicles, with the dominant follicle producing the most estrogen. Increases in estrogen result in goats exhibiting the outward signs of heat or receptivity to the male during estrus.
• Progesterone is produced by the corpus luteum on the ovary. The corpus luteum is the structure formed on the ovary after ovulation of a dominant follicle. Progesterone suppresses the formation of GnRH, FSH, and LH in the brain. This suppression is what helps to maintain pregnancy.
• Prostaglandin F2alpha is made by the uterus when there’s no pregnancy. It causes the destruction of the ovarian corpus luteum, which in turn stimulates a return of the heat cycle. The entire heat cycle averages around 21 days. However, younger does and does early in the breeding season tend to exhibit shorter and more frequent heat cycles. Standing heat, or estrus, typically lasts around 36 hours. Goats in tropical regions tend to exhibit less seasonality to estrus, as daylight remians more constant throughout the year. During times of decreased melatonin, such as in the spring, goats don’t have regular heat cycles. This time is known as anestrus.
Sexual maturity
Sexual maturity of does varies by age, breed, and body condition. It can also be influenced by geographic location. Some breeds, such as Pygmy goats, can reach sexual maturity as early as 3 months of age. It’s important to note that sexual maturity doesn’t always mean that breeding is appropriate. Body condition, weight, and skeletal maturity are important factors to consider prior to breeding.
Manipulation of the Estrus Cycle
Manipulation of the estrus cycle can help shorten kidding seasons, improve conception rates, and even result in estrus outside of the normal season. And while hormones are required to manipulate the cycle, they don’t always have to be artificial. The “Ram Effect” is commonly referred to in sheep but is also successful in goats. This involves introducing a ram (or buck) to animals transitioning into estrus. This can be used with mature animals at the beginning of the breeding cycle and for young animals to help bring on sexual maturity. A buck is introduced to does that have been isolated from males for 30 days. The pheromones released by the buck stimulate the doe to begin an estrus cycle.
Flushing
Flushing is another method of affecting the estrus cycle of does. Flushing is the practice of increasing the energy intake in goats just prior to and during the first part of the breeding season. The increase in energy results in increased follicular development and can increase ovulation rates. It should be noted, however, that this practice isn’t successful in every doe. Does that are slightly underweight or ideal weight will show an increase in ovulation rates, while those in very poor body condition or who are overweight won’t benefit from flushing.
Pharmaceutical Hormones
Pharmaceutical hormones can also be used to manipulate the heat cycle. During breeding season, hormones can be used to synchronize estrus in a group of does. Having all does coming into heat at the same time can facilitate a short breeding and kidding season, and is also ideal for artificial insemination. Artificial insemination is a wonderful way to bring new genetics into your herd without hunting down the perfect buck. These hormones can also be used to facilitate breeding out of season.
Depending upon your herd goals, a summer or fall kidding may be desired. Commonly used protocols to induce estrus involve a progesterone implant (CIDR), the use of a luteolytic agent — such as PMSG or prostaglandin — and an ovulation-inducing agent containing GnRH.
Choosing an estrus induction protocol depends upon the breeding goal. Goats undergoing artificial insemination will need to have a more precisely timed ovulation than goats destined for natural breeding.
Utilizing methods of manipulating the goat estrus cycle can increase fertility and provide opportunities for breeding outside the normal season. As we enter this breeding season, consider how manipulating the estrus cycle can help you meet your production goals. Your herd veterinarian is always a fantastic resource to consult when considering changes to your herd’s reproductive plan
DR. KATIE ESTILL DVM is a veterinarian consultant for Goat Journal, Countryside & Small Stock Journal, and Countryside online. She works with goats and other large livestock at Desert Trails Veterinary Services in Winnemucca, Nevada.
Originally published in the Fall 2024 issue of Goat Journal and regularly vetted for accuracy.
