Manipulation of the Heat Cycle

Manipulation of the heat cycle of cattle can be a handy tool available to the Highland breeder who chooses to use artificial insemination as the sole means to breed a number of cattle. 

These same techniques can be utilized in a number of other circumstances including inducing estrus (heat) in non-cycling females, treating certain diseases, (cystic ovaries or pyometra)and synchronization of a number of females to come in heat at the same time, allowing for mass insemination and restriction of a calving season to a limited time frame.

The Heat Cycle

The heat cycle of the cow is controlled by hormones produced in the pituitary gland of the brain and those produced in the ovary. During the 21 day heat cycle (average) waves of follicles grow and then regress until a single follicle emerges as the dominant follicle from which the egg will be ovulated.  

Each hormone has a specific function, and target location.  The follicle produces estrogen, responsible for visible signs of heat; and after ovulation, the corpus luteum develops on the ovary in the follicle’s place.  The corpus luteum (CL) produces progesterone, and is responsible for maintaining pregnancy if it occurs. High levels of progesterone block the follicles from developing.

If we call Day 0 of the cows heat cycle standing heat, then during day 1-2 of the estrus cycle, the corpus luteum is starting to develop.  By day 7, the CL has reached its mature size, and is producing the maximum amount of progesterone.  After day 17, the CL begins to regress, progesterone levels fall, and the body’s feedback mechanism to the hypothalamus in the brain causes the release of Gonadotropin-releasing hormone (GnRH) which in turn stimulates the release of Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland.  These are responsible for the maturation of the follicle with the subsequent increase in levels of estrogen with its outward expression of heat and subsequent ovulation.

Manipulation of the heat cycle involves using drugs replace those ordinarily produced in the bovine estrus cycle in a effort to control precisely when ovulation will occur.


Prostaglandins such as Lutalyse or Estrumate work by causing luteolysis or  regression of the CL when it is present on the ovary. Prostaglandins only work when the CL is mature, therefor they will only function during days 7-17 of the heat cycle. Basically, an injection of one of these drugs will put the appropriate cow’s cycle right to day 18, with destruction of the CL leading to decreases progesterone levels,  an increase in follicular growth, increase in estrogen production, and heat and subsequent ovulation will occur 2-5 days after the prostaglandin injection.

Progesterone-like compounds, such as  MGA (melengestrol acetate- a feed additive approved for suppressing heat in feedlot heifers) and Eazi-Breed CIDR (controlled intravaginal drug releaser) will not allow the follicle to ovulate or the cow to exhibit signs of heat, just as a functional CL during days 6-17 would.

GnRH (gonadatropin-releasing hormone) causes the release of luteinizing hormone and follicle-stimulating hormone from the anterior pituitary gland   These hormones will stimulate the ovary to grow follicles to produce an egg and stimulate its release from the follicle.

Sychronizing Estrous for Artificial Insemination

Control of the estrous cycle in beef cattle can be an advantage for those herds utilizing artificial insemination.  Synchronization is very successful at grouping cows into a close time frame for heat, but it will not put all cows into heat at exactly the same time.  Nor is the use of estrous controlling drugs a guarantee of fertility.  The cows must be in good physical condition, the AI technique must be good and the ability to detect cows in heat must be excellent.  Likewise, handling facilities must be adequate, as all the drug used will do no good if the holding pen breaks open and the cows in heat aren’t present to be inseminated.

Inducing Estrus

There are two basic methods used to synchronize estrus.  One is to induce destruction of the corpus luteum using a prostaglandin. The other method is to create an artificial luteal period with a progestational compound, then abruptly remove the compound and allow the cow to come into heat.  

Double Injection of Prostaglandin

A relatively simple technique to synchronize a number of cows to come into estrus at about the same time is a double injection of a prostaglandin.  Say we have ten cycling cows that we want to inseminate at about the same time.  If we inject all of those cows with Lutalyse, those with a mature CL on their ovary will respond by the regression of the CL and will return to heat in 1-5 days. 

However, some of those cows will have just been in heat,(days 0-6 of the heat cycle) or are ready to come into heat on their own. (Day 17-20 of the heat cycle.)  If we reinject the same drug 11 or 14 (some authorities say 11 days, but information suggests that the 2nd injection 14 days after the first will result in more cattle in heat) days after the first injection, we will have synchronized all the cows to come into heat at about the same time. 

Those cows that responded the first time will have come into heat in 1-5 days, and with a second injection in 11 days, will be between 12-16 days in their cycle and will have another mature CL that will respond to a shot this time as well.

Those cows who had just been in heat (days 0-6 ) for the first shot, and could not respond to it, will now be between day 11 and 17 days in their cycle and will now respond with the regression of a CL.   Those cows that were just coming into heat on their own(days 17-20) will now be in day 7-10 (see note below) of their cycle and should also now respond. 

All cattle should now come back into heat 1-5 days after the second injection.  You will need to observe these animals for signs of heat, and AI them 12 hours after they are detected in standing heat.

Cattle in day 7 of their cycle may or may not respond  to a prostaglandin injection. If the second injection is given after 11 days, these cows will be either in day 8 or 18 of their ycle, and will either respond to the prostaglandin or be in heat on their own. If the 2nd prostaglandin injection is give 14 days after the first, these cattle may actually be in heat the day of the injection and should be bred instead.

AM-PM Rule for Artificial Insemination

Those cattle seen in heat in the evening are inseminated the following morning.  Those cattle seen in heat in the morning are inseminated that evening.)

Single Injection and Observation

An alternative to breeding all cattle in the same 1-5 day span is to inject all animals with a prostaglandin and observe for 1-5 days for heat.  Breed those cows that come into heat after one shot of Lutalyse, and then 11-14 days after the initial injection, reinject those cows that were not bred and expect them to come into heat within another 1-5 days. 

Remember that you must not reinject those cattle that you previously bred after the first injection.  To do so will return them to heat, and if pregnant they will abort.

Progestatioinal Compounds

The use of progestational compounds can also be used to help synchronize estrus in cattle.  The progesterone-like compounds put the cow in an artificial luteal phase, so that she is held at about day 17 of her cycle as long as the drugs are continued.  Once these drugs are stopped, the body’s natural feedback mechanism allows the development of a dominant follicle and subsequent heat. 

While signs of estrus are seen frequently when using progestational compounds, pregnancy rates tend to be quite low when using just the progestins.  Increased pregnancy rates are seen if the progestins are used in conjunction with prostaglandin injections.

Two relatively simple protocols using progesterone-like compounds are available:

The first involves the feeding of MGA (melengestrol acetate) mixed in the ration at a dose of .5 mg per head per day.  This drug is approved for use in feedlot heifers to prevent them from coming into heat. In this particular protocol, MGA is fed for 14 days, effectively holding these females at day 17 of their cycle.

When the drug is stopped, the cows will exhibit heat within 2-5 days. This heat is a subfertile heat, and breeding the females at this time is not recommended.  These animals will ovulate an aged (and generally less fertile) egg at this heat.  17-19 days after the MGA withdrawal, a CL will be present on the ovary, and an injection of prostaglandin at this time will result in a fertile heat usually within 48-72 hours. 

Females should be bred 12 hours after standing heat. Mass mating of all females that have not shown signs of heat at 72 hours (heifers) or 80 hours (cows) will result in additional pregnancies. This particular method may induce estrous cycles in some females that are not yet cycling. Generally this method will be more effective in heifers because it takes 31 days to administer, and many cows will not have calved early enough to be cycling normally and respond to this treatment.

A second protocol showing promise is a relatively new device called EAZI-BREED CIDR.  This is an intravaginal device impregnated with progesterone that is inserted into the vagina of the cow, and is retained until removal ( its T-shape is naturally resistant to premature expulsion.) This device is inserted on Day 0, then on Day 6, an injection of Lutalyse is given IM.  On  day 7, the Eazi-Breed CIDR is removed. It has a string that is visible at the vaginal opening, and is simply pulled out. Heat should occur in 1 - 4 days.

Like the MGA prototcol, this method may induce estrous cycles in cattle that had not previously been cycling, even heifers.  Unlike the MGA protocol, the heat induced by the injection of Lutalyse on day 6 should be a fertile heat. However, if the Eazi-breed CIDR is left in place longer than the recommended time frame, the subsequent heat may be subfertile, just as in the MGA protocol. The follicle that finally matures will be older than normal, and less fertile.

Gonadotropin Releasing Hormone (GnRH)

The last of the drugs routinely used to control the heat cycle in cattle is gonadotropin releasing hormone, (Cystorelin, Factrel or Feragyl). GnRH will induce the development of a new folliclular wave by stimulating the pituitary gland to produce Follicle Stimulating Hormone (FSH). GnRH will also induce ovulation because it also stimulates the pituitary gland to produce a  luteinizing- hormone (LH) surge.  GnRH will also stimulate a cystic follicle to luteinize, thus being the treatment of choice for cystic ovaries.

GnRH is not generally used by itself to synchronize heat, but in combination with some of the other protocols previous mentioned. Unless these methods include the use of a progestin compound, they are not considered as effective in heifers as they are in cows.  When used on cows, they should be at least 45 days post-calving.

One use of this product to synchronize ovulation rather than standing heat, per se, was named Ovsynch, obviously derived from Ovulation Synchronization.  This method utilizes an injection of GnRH at any random part of the heat cycle, followed by an injection of prostaglandin (Lutalyse) 7 days later, then a second GnRH injection 36-48 hours after the Lutalyse, with timed Artificial Insemination 8-18  hours following the last GnRH injection.  These cattle will not necessarily show any signs of heat, but will ovulate close to the 12-16 hour time limit.

A variation of this technique is called Select Synch.  The GnRH injection is given, then 7 days later the Lutalyse injection is given. Females are observed for signs of heat beginning 30-36 hours before and up to 6 days following the Lutalyse. Cows are inseminated 12 hours after standing heat is observed.  This method also can be utilized for natural service with a bull, for known breeding dates. This program allows the females to show signs of heat for the bull to respond to, unlike the Ovsynch approach. Co-Synch is very similar to Ovsynch, but the second GnRH injection is given at 48 hours following the Lutalyse injection, and AI is performed at the same time.

A variation of Co-Synch or any of the synchronization protocols, for that matter, that can increase pregnancy rates 5-8% involves the same drug schedule, but the calves are separated from their dams on day seven when the prostaglandin injection is given, and are kept separate for 48 hours until the 2nd GnRH injection and insemination occur.  This technique requires secure pens or pastures so cows and calves remain separated for the entire 48 hours. Obviously, the calves require high quality grass or hay and clean water during the separation. It has been shown that short term calf separation will enhance induction of ovulation, even in some anestrus females.  By adding calf separation, it increases the overall effectiveness of the entire program.


Techniques that are more effective for heifers who have never been bred, and cows that have not yet cycled normally after calving (generally will be less than 45 days post-partum) involve the use of the progestin compounds, prostaglandins and GnRH together.  The use of progesterone products will help induce normal estrous cycles in these potentially problematic groups of cattle. These protocols can also be used on cows that are cycling normally if it is convenient.

CIDR Co-Synch utilizes the CIDR insert and GnRH injection on the first day, followed by an injection of Lutalyse and CIDR removal 6 days later.  All females are given a second GnRH injection and inseminated 48 hours following the Lutalyse injection.

MGA -Short Synch involves an initial injection of GnRH and the addition of MGA in the feed for a total of 5 days.  24 hours following the withdrawal of MGA, an injection of Lutalyse is given, followed in another 48 hours with a 2nd injection of GnRH and AI at the same time.

MSU Synch with CIDR involves an initial injection of GnRH along with the insertion of the CIDR.  Seven days later, an injection of Lutalyse is given, the insert is removed, and then females are observed for signs of heat over the next 3 days.  If observed in heat, they are inseminated as appropriate.  At 72 hours after the Lutalse injection, any females not inseminated are given another injection of GnRH and inseminated at that time.

The use of either MGA in the feed or the CIDR insert combined with a GnRH and Lutalyse protocol seems to be more effective in that these combinations will successfully induce ovulation in both anestrus cows and peripubertal heifers, be more likely to induce a fertile heat, increase the pregnancy rate and increase the likelihood of successful fixed-time artificial insemination.

Achieving Success

The success of manipulating the heat cycle of cattle depends however, on not just the drugs and timing, but healthy animals that are disease free and on a good nutrition program.  Producers need to know how to use the products correctly and must have good facilities to adequately restrain their cattle.  If not using a method for fixed time AI, then excellent heat detection is mandatory.

Injections should be given in the muscles of the neck, (according to Quality Beef Assurance Guidelines) and require 1 ½ inch needles to insure that the drugs are actually deposit in the neck muscles of the cow.

Good restraint is required so that the measured amount of the injections are delivered without loss either from poorly placed or loose needles. While many of these protocols can be adapted to use for natural (bull ) service, the competence of the inseminating technician also comes into play.

With healthy cattle, good facilities and attention to detail, any of the above listed protocols can be used to advantage in both the small and large Highland herd.














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