Some beef producers find it profitable to control estrus and ovulation so that they can follow it with natural breeding or artificial insem-ination.
This practice increases input costs but can also reduce labour requirements and produce a more concentrated breeding season.
Many options are available to control reproduction, and which one is used depends on how simple or complicated the producer wants to get.
To understand how these systems work, it’s necessary to review the estrus cycle and how it is controlled by hormones.
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The estrus cycle has three phases: follicular, estrus and luteal.
* Follicular – Follicles develop on the ovaries.
- Estrus – A surge in luteinizing hormone (LH) causes follicles to mature and release their eggs, which is ovulation. It is the time for breeding.
- Luteal – The ovulated follicle develops into a corpus luteum (CL), which produces hormones such as progesterone to maintain pregnancy.
Early synchronizing systems used a prostaglandin, PGF2a, for its luteolytic properties of causing regression of the CL.
Once the CL shrinks, estrus occurs three to five days later. For this prostaglandin to have any effect, the cow or heifer must have cycled to produce a CL. This means many animals are missed.
PGF2a can be used more efficiently by giving two injections 10 to 14 days apart. The first injection brings cows possessing a CL into estrus. Cows in the follicular or estrus phase are unaffected.
The second injection theoretically catches all the cows. They should have come into estrus from the injection or through natural cycling and thus have a CL present. The result is an entire herd cycling together.
This PGF2a system requires limited economic input and produces acceptable results in cycling postpartum cows and pubertal heifers. However, it won’t bring noncycling animals into estrus.
A heat detection program is critical if PGF2a is used for timed AI because the time of estrus is variable. It happens over a period of three days, or not at all if cattle are not cycling.
A better way to control estrus is to add gonadotropin releasing hormone (GnRH), which controls the follicular phase.
This hormone triggers the release of LH, a gonadotropin, from the pituitary gland. LH, in turn, causes ovulation of the follicle. If no follicles are present, GnRH has no effect.
Several systems marry the effect of GnRH with PGF2a.
For example, the Select Synch method starts with an injection of GnRH to initiate the LH surge and ovulate the follicles, which puts the cows into the luteal phase.
An injection of PGF2a is administered seven days later. This causes regression of the CL, which is present in three-quarters of cattle. At this point, the cattle are checked for heat and bred.
Another system, the CO-Synch (evaluated in Colorado, hence CO), is the same except that 48 hours after the PGF2a injection, another GnRH injection is administered at the same time cattle are bred. The advantage of this system is that no heat detection is required.
A variation is to breed one day after the GnRH injection, but studies show pregnancy rates do not change. Using the CO-Synch program and breeding at the same time as the injection allows cows to be worked one less time.
Estrus response to GnRH systems is 60 to 90 percent. Conception rates average 50 percent.
