The benefits of artificial insemination (AI) are well understood in many beef cattle operations, however the synchronization protocols can seem to be in a different language when one sits down to plan out their breeding season. In order to understand these protocols, understanding the hormones that are used to regulate estrous cycle can prove to be very helpful.
A list of protocols is approved by the Beef Reproduction Task Force each year for the use of synchronization of beef cows and heifers. These protocols outline the timing of administration of hormones to allow for the synchronization of the estrous cycle. In order to control the estrous cycle to allow for a successful breeding season, three different hormones are approved for use.
Prostaglandin F2α (PGF)
Prostaglandin F2α is a hormone that is produced by the uterus to regress the Corpus Luteum (CL) and allow the female to come into estrus. The administration of a single dose of exogenous (from outside source) PGF will regress a CL on days 6 to 18 of the estrous cycle. Prior to day 6 the CL has not reached full maturity and will not respond to PGF, and after day 18 the CL will naturally be regressing.
Gonadotropin Releasing Hormone (GnRH)
Gonadotropin Releasing Hormone is used to induce ovulation and initiation of a new follicular wave in synchronization protocols. Administration of exogenous GnRH will begin a cascade of hormonal responses that will cause follicles that are greater than 10mm in size to ovulate. Following ovulation a new follicular wave will be initiated. This is why GnRH is giving at the start of several synchronization protocols and also at time of insemination during a Fixed-Time AI, when we want to force the female to ovulate at time of artificial insemination.
Following ovulation the female forms a CL which produces progesterone (P4), this is the hormone that holds the female out of estrus. It is also the hormone that is responsible for maintaining pregnancy. There are two products used in synchronization protocols to mimic the CL: Synthetic progestins, the orally dosed MGA product, and progesterone, through the use of a CIDR. Both of these products can mimic the effect of CL on the estrous cycle. Administration of exogenous P4 will keep cows and heifers out of estrous, stopping ovulation, even if a CL is not present. This allows for the regulation of ovulation.
Having a basic understanding of what hormones are used to synchronize a herd and the purpose of their administration will help to ensure that synchronization protocols are followed properly. It is hard to explain these complex systems and hormonal cascades without extreme detail, thus this is not a complete overview of these processes, but will bring some light to the use and purpose of their administration. Remember it is key to give the right shots on the right days, strictly following the protocols set forth by the Beef Reproductive Task Force to have a successful AI season.
Source: Kalyn Waters