Making profits on your dairy depends upon successful breeding. Neither milk nor meat can be produced without pregnant cows. Getting cows pregnant is achieved through reproduction management. In theory, the process is simple: watch cows for heats or set them up to ovulate and breed at the appropriate time. However, managing reproduction is complex and can be frustrating if not done right. We are continuously looking for that magic solution to solve all of our cows’ reproductive problems. Many will agree the magic solution is dependent on how well reproduction is managed. Understanding how the cow’s estrous cycle works is a good starting point to successfully manage reproduction particularly in optimizing the timing of artificial insemination (A.I.). The estrous cycle, also referred to as the heat cycle, is a series of events and hormonal changes that repeat approximately every 21 days (range 18 to 24 days) in non-pregnant cows. The cycle is shown in Figure 1 and a detailed description follows.
|Figure 1. Estrous Cycle|
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enlarged image of
the estrous cycle.
The Heat Period
The estrous cycle starts on Day 0 when a large fluid-filled blister-like structure on the ovary (called the follicle) creates the hormonal push necessary to initiate estrus, or heat. The follicle contains the egg and produces estrogen, which causes cows to exhibit “estrus behavior” such as: bellowing, secreting cervical mucus, standing to be mounted and the “toning” of the uterus that aids in transportation of semen at the time of insemination. Typically, estrus lasts for 12 hours, but it is common to observe cows in heat for only six to eight hours. Estrogen is also responsible for triggering the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus in the cow's brain. GnRH causes the release of another important hormone, luteinizing hormone (LH) from the pituitary gland.
Ovulation, Fertilization & Formation of the Corpus Luteum
On Day 1, about 32 hours after estrus began, LH will cause the cow’s follicle to ovulate, releasing an egg into the oviduct where it will wait to be fertilized. If sperm are present in the oviduct and fertilization occurs, the fertilized egg (embryo) will remain in the oviduct for three to four days before entering the uterus. Following ovulation, estrogen levels drop, decreasing the hormonal drive for displaying standing estrus behavior. Also after ovulation, an important structure called the corpus luteum (CL) begins to form on the ovary in the place vacated by the follicle. As the CL grows it begins to secrete the hormone progesterone four to five days after heat and reaches mature size and progesterone production nine to 10 days after heat. Progesterone is essential for pregnancy as it prevents the cow from returning to heat and signals the uterus to prepare for a fertilized egg.
Regression of the Corpus Luteum
By Day 12, the CL has grown to its maximum diameter. If fertilization occurs and a viable embryo is present in the uterus by Day 18, the CL is maintained and continues producing large amounts of progesterone, which prevents the return of estrus. However, if fertilization does not occur and the uterus fails to detect a growing embryo, the uterine lining will release the hormone prostaglandin (PG). PG will cause regression of the CL, resulting in the rapid decline of progesterone and triggering the development of another follicle. The cow would then enter standing heat again, completing the 21-day cycle. The decrease in progesterone triggers an increase in GnRH. In addition to stimulating the release of LH, GnRH also stimulates the release of follicle stimulating hormone (FSH). FSH acts on the ovary to stimulate rapid growth of another ovulatory follicle. The follicle will secrete estrogen to cause the next heat and ovulation.
Now that we understand how the estrous cycle works, we can begin to understand the hormones used on estrus synchronization protocols for dairy cows. These hormones are used to manipulate the heat cycle replacing those ordinarily produced in the bovine estrous cycle in an effort to control precisely when ovulation will occur.
Prostaglandins such as Lutalyse, Prostamate or Estrumate work by causing regression of the CL when it is present on the ovary. Prostaglandins only work when the CL is mature. Therefore they will only function during Days 7 to 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 of progesterone levels, an increase in follicular growth and an increase in estrogen production. Heat and subsequent ovulation will occur two to five days after the prostaglandin injection.
Progesterone-like compounds, such as MGA (melengestrol acetate) and EAZI-BREED™ CIDR® (controlled intra-vaginal 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 to 17 would.
GnRH such as Cystorelin, Factrel or Fertagyl 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.
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 any timed A.I method, then excellent heat detection is mandatory.