Human chorionic gonadotropin (hCG) can induce ovulation and stimulate development of luteal tissue in beef females. Those actions make it attractive as a possible substitute for gonadotropin-releasing hormone (GnRH) in synchronization programs or as a treatment intended to increase progesterone after breeding or embryo transfer. However, hCG is not simply a more powerful version of GnRH. Its prolonged action can be useful in selected circumstances, but it can also disrupt normal follicular and luteal patterns and, in some cases, reduce pregnancy rates. Producers should view hCG as a targeted reproductive tool and not a routine add-on to all protocols.

What is hCG?

hCG is a glycoprotein hormone produced by the human placenta during pregnancy. Although its name reflects its human origin, cattle ovaries recognize it because its biological activity closely resembles luteinizing hormone (LH). In the cow, LH completes maturation of the dominant follicle, triggers ovulation and supports transformation of follicular cells into the corpus luteum (CL). The CL then produces progesterone, the hormone that prepares the uterus and supports early pregnancy. 

GnRH and hCG reach a similar endpoint by different routes. GnRH acts first on the pituitary gland, causing the cow to release her own short-lived surge of LH and follicle-stimulating hormone. hCG acts directly at LH receptors in the ovary and remains biologically active longer. That lasting, LH-like signal often produces a strong ovulatory response and can stimulate more luteal tissue. When hCG ovulates a first-wave dominant follicle after breeding, the resulting “accessory” CL may raise circulating progesterone. Yet the same extended stimulation may alter ovulation timing, oocyte maturation, estrous cycle length and the cow’s response to prostaglandin.

How can hCG be effectively used?

Because hCG reliably induces ovulation when a responsive dominant follicle is present, researchers have tested it in place of GnRH at the beginning or end of timed artificial insemination (AI) protocols. Direct ovarian action may be helpful when pituitary LH release is limited, and a prolonged signal may increase the percentage of follicles that ovulate. But synchronization requires more than ovulation. Ovulation must occur within the correct breeding window and yield a competent oocyte, a functional CL and a uterus that remains synchronized with embryo development.

We compared hCG with GnRH in a Co-Synch program. Overall AI pregnancy rates were approximately 48% in GnRH-treated cows and 34% in hCG-treated cows. More hCG-treated cows also developed short estrous cycles after timed insemination and age influenced the response, where younger cows did not respond exactly like older cows; but the overall conclusion was that hCG was not a suitable general replacement for GnRH in beef cows. 

The preferred use of hCG may occur after ovulation rather than as a routine ovulation trigger. Progesterone is essential during early embryo development. Treating cattle about five to seven days after ovulation can ovulate the first-wave dominant follicle and create an accessory CL. Studies commonly report more luteal tissue and increased progesterone after successful accessory-CL formation. Administering hCG five to seven days after insemination improved pregnancy rates by approximately 6%. Embryo-transfer researchers have also used the same strategy in recipients, attempting to improve the luteal environment before or near transfer. Treatment of recipients at the same time as embryo transfer (i.e., seven days after ovulation) resulted in a 5-8% improvement in pregnancy rates. This increase largely occurs in females that are in poorer body condition.  

What causes a decline in fertility in females treated with hCG?

Pregnancy begins with a sequence, not a single event. If hCG triggers ovulation before the oocyte has completed maturation, or later than expected relative to insemination, fertilization can suffer even though an ovulation is recorded. The prolonged LH-like exposure may also alter steroid production within the follicle. Recent reviews caution that hCG before insemination can disturb physiological events required for ovulation of a viable oocyte.

After ovulation, an accessory CL and added progesterone can also complicate the next estrous cycle. hCG treatment in the early luteal phase has lengthened estrous cycles and reduced the proportion of cows returning to estrus in some studies. Additional luteal tissue may undergo incomplete or delayed regression following prostaglandin. That can create asynchronous follicular development, delayed estrus or animals that fail to fit the next management schedule. These effects are especially important in programs that depend on tight resynchronization of open females.

Under what conditions should you consider using hCG?

hCG may deserve consideration when a veterinarian or producer has identified a specific limitation: poor luteal function, a low-fertility embryo-recipient group, or a herd that may be in poor condition. It may also have value when ultrasound is used to evaluate ovarian structures and select animals most likely to respond. In contrast, adding hCG to a successful synchronization program “for insurance,” or swapping it dose-for-dose for GnRH, is difficult to justify, and may sometimes be detrimental.

The bottom line

hCG is a potent, long-acting LH-like hormone that can induce ovulation, form an accessory CL and increase progesterone in cattle. Those effects can benefit selected low-fertility animals or embryo-transfer recipient groups, especially when treatment is correctly timed. In estrous synchronization, hCG may shift ovulation away from the ideal insemination window, interfere with oocyte competence, produce short or prolonged cycles and reduce pregnancy rates. In embryo transfer programs, improved luteal tissue and structures do not always translate into more calves. The best use of hCG is therefore precise, evidence-based and measured against a proven control protocol. For most producers, the question is whether that response produces enough additional pregnancies to justify the biological risk, expense and potential additional animal handling. 

Editor’s note: Cliff Lamb is the animal science department head and a professor at Texas A&M University in College Station, Texas.