On this page:

  • Normal Estrous Bicycle in Cattle
  • Estrous Control and Synchrony in Cattle
    • Table 1—Bachelor FDA-Canonical Drugs to Control and Synchronize Estrous Cycles in Cattle
  • Benefits of FDA-Canonical Animal Drugs
  • Actress-Label Drug Use Not Immune for Estrous Control and Synchrony in Cattle
  • Resource for Yous

With but a ane alphabetic character deviation, the words "estrous" and "estrus" look and sound like, simply in that location's an important deviation. Estrus is a noun and refers to the short period in which a cow is sexually receptive and will stand to be bred. Estrus is normally called "oestrus." Estrous is an adjective used to describe things related to estrus, such equally the behaviors associated with estrus (estrous behaviors) or the menstruation from one estrus to the next (estrous cycle).

Normal Estrous Cycle in Cattle

A good understanding of the normal estrous cycle in cattle can help producers accost reproductive challenges in both heifers (young female dairy or beefiness animals that have non yet had their first dogie) and cows (female dairy or beef animals that have had at least one calf). This understanding is also critical when using a drug regimen to control and synchronize estrous cycles in cattle.

A heifer has her showtime oestrus, or heat, at puberty. The age at puberty is influenced past genetics, nutrition, and body weight. Heifers fed an appropriate diet will generally accomplish puberty between ix and fifteen months of age. Dairy heifers tend to reach puberty earlier, at 9 to 12 months, whereas beef heifers tend to accomplish information technology a bit later, at 13 to 15 months. The age at puberty for some cattle breeds, such as Brahman, tin exist equally late equally 24 months.

After puberty, a heifer continues to have regular estrous cycles every 21 days (the normal range is every 18 to 24 days). The estrous cycle in cattle is complex and regulated by several hormones and organs (see Effigy i).

Phases of the bovine estrous cycle

Figure 1 —Phases of the bovine estrous cycle. Note: E2 = estrogen and Piv = progesterone. (Effigy used with permission from Current Conceptions, Inc. from Pathways to Pregnancy and Parturition, third ed.1 Image cannot be reprinted in whatever other publication in hard copy or electronic grade without written permission from Current Conceptions, Inc.)

The Follicular Phase: Waves of Ovarian Follicles

The follicular phase of the estrous cycle refers to the narrow period of time right before oestrus (estrus) and ovulation (release of the egg for possible fertilization). During this phase, there is rapid growth of a ascendant ovarian follicle and increased estrogen production.

A follicle is a structure containing an egg, also called an ovum, and other cells that can produce estrogen. A heifer or cow will mostly have two or three groups of ovarian follicles—chosen waves—develop during a single estrous wheel (the range is i to four waves). I follicle in each wave volition get the dominant follicle. Early in the estrous bike, when the progesterone level is loftier, the dominant follicle will not ovulate. Instead, it will backslide and allow some other wave of follicles to emerge. The last wave occurs later in the estrous bike when the progesterone level is low. The follicle that emerges every bit ascendant during this wave will not regress. Instead, information technology will grow larger and produce increasingly more than estrogen.

Estrus (Heat) and Ovulation

The loftier estrogen level produced by the dominant ovarian follicle causes the heifer or cow to evidence signs of oestrus. This ways she's sexually receptive—she'southward said to be "in heat"—and will stand to be bred or mounted by other cows, commonly referred to as "standing heat." The heifer or cow may show other signs of rut, such as having a clear mucous vaginal belch and an increased activity level. She may also vocalize more and try to mount other cows. Heat is considered the beginning, or "Day 0," of the estrous wheel.

As well causing the heifer or cow to prove signs of rut, the high level of estrogen produced past the dominant ovarian follicle likewise triggers the hypothalamus—a section of the brain—to release a surge of gonadotropin releasing hormone (GnRH) right before ovulation. GnRH causes the pituitary gland in the brain to release two other hormones—follicle stimulating hormone (FSH) and luteinizing hormone (LH)—which travel in the blood to the ovary to control what happens to the follicles.

FSH is secreted during and presently after heat and causes a new wave of ovarian follicles to develop. A surge of LH causes the dominant ovarian follicle to rupture approximately 24 to 32 hours later, thus releasing the egg. This is ovulation and marks the transition from the follicular phase to the luteal phase.

The Luteal Stage: The Developing Corpus Luteum

During the luteal stage of the estrous cycle, the remnants of the newly ruptured ovarian follicle develop into the corpus luteum and brainstorm to produce progesterone, a steroid hormone needed to back up and maintain a potential pregnancy if the egg is fertilized. Over the first 10 days of the estrous bicycle, the corpus luteum matures and increases in size. The corpus luteum reaches its maximum size and produces the most progesterone at mid-cycle (effectually Days 9 and 10). Nether the influence of high progesterone, waves of follicles continue to sally and regress without a dominant ovarian follicle rupturing. A dominant follicle won't rupture again until the progesterone level falls during the next follicular phase.

No Pregnancy

If the egg isn't fertilized or the early embryo fails to develop, the uterus releases the hormone prostaglandin F2-blastoff around Days 16 to xx of the estrous bicycle. Prostaglandin F2-alpha causes the corpus luteum to regress. This is called luteolysis and leads to a drib in progesterone.

The low progesterone environment allows the dominant ovarian follicle to sally from the last moving ridge of follicles and mature instead of backslide. The dominant follicle produces enough estrogen to cause heat, thus starting the adjacent estrous cycle.

Pregnancy

If the heifer or cow becomes pregnant, the embryo prevents the uterus from releasing prostaglandin F2-alpha and the corpus luteum continues to release progesterone. The high progesterone level stops the heifer or moo-cow from cycling—she won't go into estrus or ovulate. In a normal, healthy pregnancy, the embryo develops into a fetus. Gestation (the flow between when the animal becomes significant and when she calves) lasts about 283 days. It's desirable for a heifer to have her commencement dogie when she's 2-years-erstwhile. For that to happen, the heifer must reach puberty and become pregnant by 14 to 15 months of age.

Cyclicity

When a heifer or moo-cow has an estrous cycle that'southward normal in length and she displays normal estrous behaviors during heat, she's said to be "cycling." When a heifer or cow isn't pregnant just she'due south not ovulating or showing signs of oestrus, she's not cycling. Instead, she's "anestrus," which is sometimes described as being "acyclic." Right afterwards a heifer or cow calves, information technology's normal for her to exist anestrus for a short flow of time. Sometimes a medical trouble, such as ovarian cysts or an infection of the reproductive tract, can cause a moo-cow to stop cycling. Some other reproductive challenge is silent heat—when a heifer or cow appears to exist anestrus simply, in fact, she is cycling normally and just not showing signs of oestrus.

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Estrous Command and Synchrony in Cattle

Exogenous hormones can mimic the hormones of the natural estrous cycle in cattle. By administering exogenous hormones, producers tin control and synchronize estrus in breeding heifers and cows as well every bit shorten their estrous cycles. Producers can grouping animals according to the phase of their estrous bike, which reduces management burdens. Some estrous synchrony regimens may also induce or accelerate estrus in animals that aren't cycling (they're anestrus). For example, a moo-cow is normally anestrus after giving nativity, and an estrous synchrony regimen may be used to advance her first heat subsequently calving. This allows her to exist bred again sooner. These drug regimens may also be used to advance the timing of the kickoff estrus in heifers at puberty.

Some FDA-canonical animal drugs are available for apply in estrous control and synchrony regimens for cattle (see Tabular array 1). These regimens use three classes of drugs:

  • Drugs in the gonadorelin class act similar to gonadotropin releasing hormone (GnRH) naturally released from the hypothalamus during the estrous cycle. These drugs mimic the surge of GnRH right before ovulation to cause the animal's pituitary gland to secrete FSH and LH. Equally described above, FSH causes a new wave of ovarian follicles to develop, and LH causes the dominant follicle to rupture and release the egg (ovulation).

    Five gonadorelin products are FDA-approved to care for ovarian cysts, which is a therapeutic use. (A therapeutic use means the drug is used to treat, command, or prevent disease.) Four of the v gonadorelin products are also FDA-canonical to control or synchronize estrous cycles in cattle when given sequentially with another drug (run across Tabular array 1). Estrous control and synchrony are non therapeutic uses. All five gonadorelin products are prescription merely.

  • Drugs in the progestin grade act similar to the hormone progesterone naturally secreted by the corpus luteum. These drugs can supplement the progesterone level in the heifer's or moo-cow'south trunk to:
    • Influence the release of FSH and LH from the pituitary gland;
    • Suppress estrus (heat); and
    • Block ovulation.

      Merely one progestin, an over-the-counter intravaginal insert, is FDA-canonical for synchronizing estrous cycles or advancing heat in cattle either when given alone or sequentially with another drug (run into Table i). Melengestrol acetate, some other progestin, is FDA-approved for use in manufacturing medicated feed to suppress estrus in heifers but is

      not approved for synchronizing estrous cycles or advancing estrus in cows or heifers.
  • Drugs in the prostaglandin class act similar to the hormone prostaglandin F2-blastoff naturally released from the uterus when there is no pregnancy. These drugs cause luteolysis—the regression of the corpus luteum if information technology's mature enough to respond to the effects of prostaglandin F2-alpha. Afterwards the corpus luteum regresses, a new dominant ovarian follicle emerges and ruptures, causing ovulation and a new estrous wheel to kickoff. Using prostaglandin drugs in the starting time 5 days of the estrous wheel won't crusade the corpus luteum to regress because the early corpus luteum is young and doesn't reply to prostaglandin F2-alpha.

    5 prostaglandin products are FDA-approved for several therapeutic uses. All five products are also approved for estrous synchrony in a single- or double-injection prostaglandin-only regimen. Four of these products are also FDA-approved for sequential use with another drug to synchronize estrous cycles or advance estrus in cattle (see Table 1). All 5 prostaglandin products are prescription only.

Except as described in the various regimens listed in the table below, it's illegal to use whatever of these drugs with another drug for estrous control and synchrony in cattle. Before using any drug, it'southward of import to read the labeling to brand sure the drug is approved for the intended apply and to follow the directions for utilize. Prescription animal drugs must be used past or on the lodge of a licensed veterinarian.

Table 1—Available FDA-Approved Drugs to Control and Synchronize Estrous Cycles in Cattle (see Fauna Drugs @ FDA for specific data most each drug).

 Regimen  Drug Proper noun
(Established Name, besides called Agile Ingredient)		 Awarding Number and Manufacturer Sequential Employ with Another Drug
Gonadorelin-Prostaglandin Factrel® Injection
(gonadorelin injection)		 Nadaa 139-237
Zoetis Inc.		 Lutalyse® Injection or Lutalyse® HighCon Injection
Fertagyl®
(gonadorelin)		 ANADAb 200-134
Intervet, Inc.		 Estrumate®
GONAbreed®
(gonadorelin acetate)		 ANADA 200-541
Parnell Technologies Pty. Ltd.		 cloprostenol sodium
CYSTORELIN®
(gonadorelin)		 Nothing 098-379
Merial, Inc.		 cloprostenol sodium
Progestin-merely EAZI-BREED™ CIDR®
(progesterone intravaginal insert)		 Nix 141-200
Zoetis Inc.		 None
Progestin-Prostaglandin EAZI-Breed™ CIDR®
(progesterone intravaginal insert)		 Zip 141-200
Zoetis Inc.		 Lutalyse® Injection or Lutalyse® HighCon Injection
Prostaglandin-only Lutalyse® Injection & Lutalyse® HighCon Injection
(dinoprost tromethamine injection)		 NADA 108-901 & Null 141-442
Zoetis Inc.		 None
Estrumate®
(cloprostenol injection)		 NADA 113-645
Intervet, Inc.		 None
ProstaMate™
(dinoprost tromethamine)		 ANADA 200-253
Bimeda Animal Health Limited		 None
estroPLAN®
(cloprostenol sodium)		 ANADA 200-310
Parnell Technologies Pty. Ltd.		 None

a New Fauna Drug Application

b Abbreviated New Animal Drug Awarding

Note: Some of the above drugs may be marketed with a different trade name (as well known equally the proprietary proper name) under what'due south chosen "distributor labeling."

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Benefits of FDA-Canonical Animal Drugs

FDA rigorously evaluates an fauna drug earlier approving it. As part of the approval process, the drug company must prove to FDA that:

  • The drug is safe and effective for a specific use in a specific creature species. For a drug intended for employ in food-producing animals, the visitor must also prove that food fabricated from animals treated with the drug is safe for people to consume;
  • The manufacturing process is adequate to preserve the drug'southward identity, force, quality, and purity. The visitor must bear witness that the drug can exist consistently produced from batch to batch; and
  • The drug'southward labeling is true, complete, and not misleading. The company must make sure that the labeling contains all necessary data to use the drug safely and effectively and includes the risks associated with the drug.

FDA's office does not stop after the agency approves an brute drug. As long equally the drug company markets the animal drug, the agency continues to monitor:

  • The drug's safety and effectiveness. Sometimes, the agency'south mail service-approval monitoring uncovers safety and effectiveness issues that were unknown at the time of blessing;
  • The manufacturing process to ensure quality and consistency are maintained from batch to batch;
  • The drug's labeling to make sure the information remains true, complete, and not misleading; and
  • The visitor's marketing communications related to the drug to make sure the information is true and non misleading.

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Under specified conditions, veterinarians are legally allowed to prescribe approved human and animal drugs for actress-label uses in animals. When an canonical human or creature drug is used in a mode other than what is stated on the drug's labeling, information technology's an extra-label employ. This is ordinarily called "off-label" use because the drug is used in a mode that's "off the characterization."

Veterinarians must follow FDA's requirements for actress-label drug use in animals, equally stated in the Federal Food, Drug, and Cosmetic Act and FDA regulations. Using a drug in an actress-label way for estrous command and synchrony doesn't comply with these requirements, which state that for food-producing animals, a veterinary must carefully diagnose a medical condition and any drug he or she prescribes in an actress-label manner must be for a therapeutic employ (to treat, control, or prevent a affliction). Estrous control and synchrony are not therapeutic uses.

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Resource for You

  • Letters to Bovine Veterinarians
    • GONAbreed® (gonadorelin acetate)
    • Factral® (gonadorelin injection) Injection
    • Fertagyl® (gonadorelin)
    • CYSTORELIN® (gonadorelin)
  • The Estrous Cycle of Cattle, Mississippi Country University Extension
  • Back to the Basics: Explaining the Estrous Bicycle, Dairy Cattle Reproduction Council
  • Estrous Bike Learning Module, Institute of Agriculture and Natural Resources, Academy of Nebraska-Lincoln, George Perry, Extension Beef Reproduction Management Specialist

ENDNOTE

1 Senger PL. Pathways to pregnancy and parturition. third ed. Pullman, WA: Current Conceptions, Inc., 2012;144.

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