Gonadotropin Releasing Hormone, GnRH, is administered to induce a surge of luteinizing hormone, LH, resulting in ovulation of the dominant follicle and a new follicular wave begins. Progesterone, P4, is the active hormone in intravaginal progesterone releasing devices, also known as CIDR, and melengestrol acetate, MGA. P4 is the hormone of pregnancy and is intended to prevent ovulation of the growing follicles via negative feedback on GnRH when used for estrous synchronization. Lastly, prostaglandin F2 alpha, PG, is intended to induce luteolysis, or degradation, of the corpus luteum (site of ovulation) and removes the negative feedback of P4.

         Cows with approximately 3/8 or more Bos Indicus influence often have physiological differences resulting in differing reproductive outcomes when using common reproduction methods. One distinction is their slightly longer gestation, or pregnancy, length (~292 days) compared to Bos Taurus cattle (~282 days) on average. However, there are other important factors to consider when building your reproductive management plan.

         For instance, Bos indicus females tend to have a lower responsiveness to exogenous GnRH, especially when administered at a random stage of the cycle, resulting in a lower intensity LH surge upon administration. Additionally, circulating P4, produced by the corpus luteum, plays a role in the female’s responsiveness to GnRH. Bos indicus cattle are more sensitive to circulating P4 and the P4 introduced to the system by inserting a CIDR due to their lower liver metabolism. These type of cattle are often more prone to the effects of external stressors, which can further diminish their responsiveness to GnRH.

        When using a breeding program based on estrus expression, it is important to note that Bos indicus influence cows typically experience a shorter period of standing estrus and often exhibit signs of estrus late in the evening. This can affect the accuracy of identifying the timing of the onset of estrus and therefore, the timing of breeding.

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         In other regions, estrogen or estradiol products are commonly used to enhance estrous synchronization; however, the U.S. Food and Drug Administration does not approve the use of these hormones. Therefore, alternative methods are promoted in the United States.

        Alternative methods include administering a PG injection at the start of the synchronization protocol, rather than GnRH, to induce luteolysis and decrease the concentration of circulating P4. Two PG injections can also be given prior to the start of the synchronization protocol 12 to 14 days apart to improve synchronization among cows when beginning the protocol.

        A less common approach involves inserting a previously used CIDR (up to 14 days) within a synchronization protocol. This aids in reducing the circulating P4, improving responsiveness to GnRH, and promotes the growth of the dominant follicle. If using this method, it is essential to properly clean the CIDRs between uses. CIDRs can be sanitized using Nolvasan or Chlorohexidine.

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