Estrus synchronization optimizes labor and time, and improves the ease of using artificial insemination (AI) (Lamb et al., 2009). Use of AI allows access to superior genetics, accelerates genetic change within a herd, and is frequently less expensive than natural service (Johnson and Jones, 2004). Synchronized females 1) exhibit estrus at a controlled time, 2) have increased calf uniformity, 3) calve earlier in the season, and 4) wean calves that are older and heavier (Perry, 2004).

There are several protocols for beef heifers or cows to be considered when developing an estrus synchronization program (see 2018 Estrus Synchronization Protocols for Heifers and Cows (Chart), Protocols for Synchronization of Estrus and Ovulation from the Beef Reproduction Task Force). Protocols were originally developed using heat detect and AI (HD), but recent research has shown fixed time AI (FTAI) to have similar pregnancy rates to HD (Johnson and Jones, 2004; Tibbitts et al., 2017). Many protocols can be adapted for natural service, HD, or timed AI (TAI).

Protocols for Heifers

In beef heifers, longer-term progestin protocols (MGA-PG or 14-day CIDR + PG) have provided more consistent estrus response and pregnancy success.

MGA-PG

Melengesterol acetate (MGA) is an orally active progestin. When fed at a rate of 0.5 mg/d per heifer, estrus is suppressed and ovulation is prevented. Level of feeding and consumption of MGA is critical to success. Prostaglandin F2 (PG) is a luteolytic compound normally secreted by the uterus. When administered to females who are on d 6-16 of the estrous cycle, premature regression of the corpus luteum (CL) will occur and the heifer will return to estrus (Patterson et al., 2004). In this program, heifers are offered MGA in their diet for 14 d, followed by a PG injection 19 d later. This will place heifers in the late luteal stage of the estrous cycle at PG injection, and will maximize conception rate. Heifers are AI 72 ± 2 h following PG. An injection of gonadotropin releasing hormone (GnRH) should be given at AI. Administration of GnRH will result in a release of luteinizing hormone (LH) from the brain and cause ovulation of a dominant follicle. See Figure 1.

14-day CIDR-PG

Alternately, if there is a concern about consistent delivery of MGA, producers can utilize a controlled internal drug release (CIDR) insert in place of MGA with similar FTAI and final pregnancy rates to MGA (Vraspir et al., 2014). CIDR inserts are impregnated with progesterone and placed in the vagina to increase circulating progesterone concentrations in the heifer and maintain the CL. Upon removal of the CIDR, an injection of PG is administered to induce luteolysis of the CL. More than 95% of females exhibit estrus within 5 d of CIDR removal (Lamb and Larson, 2004). A CIDR insert is placed in the vagina for 14 d, followed by PG injection 16 d later. Heifers are AI and administered GnRH 66 ± 2 hr following PG injection. See Figure 2.

7-day CO-Synch + CIDR

On d 0, heifers are injected with GnRH and a CIDR insert is placed in the vagina for 7 d. At CIDR removal, an injection of PG is administered. Heifers should be AI 54 ± 2 hr after PG injection. Similar to the previous protocols, an injection of GnRH is administered at AI. See Figure 3.

Protocols for Cows

7-day CO-Synch + CIDR

This protocol is similar to the 7-day CO-Synch + CIDR protocol used in heifers; however, time from CIDR removal and PG injection to AI is increased. On d 0, cows are given an injection of GnRH and a CIDR insert is placed. The CIDR is removed on d 7 and an injection of PG is administered to lyse the CL. Perform TAI 60 - 66 hr after injection of PG and administer GnRH at AI. See Figure 4.

Alternately, estrus synchronization protocols can be used with natural service if AI is not feasible. Synchronization will result in a greater percentage of females coming into estrus earlier in the breeding season, and a greater percentage calving earlier in the season. Early calving cows wean heavier calves, and have longer to resume estrus before start of the following breeding season (See Use of Natural Service Sires with Synchronized Estrus from the Beef Reproduction Task Force). A single injection of PG 5 d after bull turnout is effective in synchronizing beef females in extensive systems.

Many factors (BCS, age, diet quality, etc.) influence success of estrus synchronization protocols. Strict adherence to protocols is imperative to maximizing success. Table 1 provides expected ranges for the aforementioned protocols, as well as typical pregnancy rates for well-managed herds under optimal conditions. See Table 1.

Literature Cited

Johnson, S. K., and R. Jones. 2004. Costs and Comparisons of Estrous Synchronization Systems. In: Applied Reproductive Strategies in Beef Cattle, North Platte, Neb. p 103-115.

Lamb, G. C., C. R. Dahlen, J. E. Larson, G. Marquezine, and J. S. Stevenson. 2009. Control of the estrous cycle to improve fertility for fixed-time artificial insemination in beef cattle: A Review. J Anim Sci 88: 181-192.

Lamb, G. C., and J. E. Larson. 2004. Review of Estrus Synchronization Systems: CIDR. In: Applied Reproductive Strategies in Beef Cattle, North Platte, Neb. p 75-84.

Patterson, D. J., F. N. Kojima, J. E. Stegner, J. F. Bader, G. A. Perry, D. J. Schafer, and M. F. Smith. 2004. Review of Estrus Synchronization Systems: MGA. In: Applied Reproductive Strategies in Beef Cattle, North Platte, Neb. p 53-73.

Perry, G. 2004. Fertility of Natural vs. Synchronized Estrus. In: Applied Reproductive Strategies in Beef Cattle, North Platte, Neb. p 86-98.

Tibbitts, B. T., T. L. Meyer, D. J. Kelly, and R. N. Funston. 2017. Timed Insemination vs. Modified Estrus Detection in Beef Heifers. In: Nebraska Beef Cattle Report. Rep. No. MP 104. Univ. of Nebraska, Lincoln. p. 11-12.

Vraspir, R. A., A. F. Summers, D. O'Hare, L. D. Rowden, and R. N. Funston. 2014. Comparison of long-term progestin-based synchronization protocols on Fixed-time AI pregnancy rate in beef heifers. In: Nebraska Beef Cattle Report. Rep. No. MP 101. Univ. of Nebraska, Lincoln. p. 8-10.