Systematic Approach To Reproductive Management Of Heifers
Posted: September 6, 2004
Artificial insemination (AI) of dairy heifers provides numerous advantages. Foremost is the opportunity to improve the genetic merit of the entire herd. Several years ago a survey was conducted of Pennsylvania dairy producers who do not use AI for heifers. They were asked to give the reasons why they do not use this management tool. The major reasons were the perception of lowered conception rates with AI, difficulty or time involved with estrous detection, heifers were housed at an inconvenient location, and lack of adequate restraint facilities.
Many programmed breeding programs have been introduced to synchronize estrus and ovulation and minimize time spent for heat detection. There are three basic programs for heifers with some modifications of each: prostaglandin (PG), the controlled internal drug-releasing device (CIDR) program using a vaginal insert containing progesterone combined with an injection of PG and finally melengestrol acetate (MGA), an orally active progestin fed for prescribed period of time followed by an injection of PG at a set time following the withdrawal of the MGA from the feed.
The prostaglandin programs depend upon PG regressing the corpus luteum (CL) followed by formation of a dominant follicle, estrus and ovulation. Prostaglandin will not regress a CL that is present on the ovary during the first five days of the estrous cycle. Consequently, the two-injection PG program was developed with a 14-day interval between injections so that a high percentage of heifers possess a functional CL at the time of the second PG injection. Even though a larger percentage of the females will be in heat with the two-injection system, the onset of heat among a group of heifers occurs over a 5 or 6-day period. This system is not effective on prepubertal heifers that are eligible for breeding based on size but have not exhibited heat.
The CIDR + PG program which uses both progesterone and PG to regulate the estrous cycle is generally more effective. The release of progesterone over a seven-day period ensures regression of the CL in response to PG because most heifers should have a CL that has developed over those seven days. The synchronization rate of heifers on the CIDR+ PG system was compared to a group of heifers that were not synchronized or heifers that received a single PG injection. The CIDR was inserted for seven days and the cattle were injected with PG on day 6. Overall the CIDR+PG treatment increased the percentage of heifers in estrus over a three-day period with acceptable conception rates compared to the other two groups. It is a simple system, which provides good synchrony of estrus and is easy to manage. Heifers exhibit heat over a shorter period of time compared to the PG systems so time spent for heat detection is productive and timing of insemination is more accurate. Another advantage of this system is that it will induce a percentage of prepubertal heifers to begin cycling. The basic CIDR program can be modified so that the PG injection is more conveniently given on day 7 when the insert is removed. The only disadvantage is the onset of heat among a group of heifers will be spread over more days. However, if the heat detection is routine and accurate, good conception rates can be obtained. Other modifications using GnRH with the CIDR + PG program are being used in some herds for appointment breeding. The results from research trials with these systems have been variable but some herds in Pennsylvania are obtaining good results.
The MGA + PG program is also an effective program. The major disadvantages are acquiring a source of MGA and then ensuring that the heifers consume the prescribed amount on a daily basis over the 14-day feeding period.
There are modifications of all these programs. I suggest dairy producers who are planning to adopt a programmed breeding system for the first time keep the system simple until you gain experience. To achieve success with any synchronization and AI program it is important to follow the system and pay attention to details. It should be noted that heifers that fail to conceive to the first programmed breeding will generally return to heat as a group. Consider using heat detection aids such as heat mount detectors or tail head markings so that a high percentage of synchronized and return heats are observed. Discuss these programs with your AI representative and veterinarian to determine which system best suits your situation.
Dairy and Animal Science Extension