DEVELOPMENT OF METHODS TO SYNCHRONIZE ESTRUS

The development of methods to control the estrous cycle of the cow has occurred in five distinct phases. The physiologic basis for estrus synchronization followed the discovery that progesterone inhibited preovulatory follicular maturation and ovulation. Regulation of estrous cycles was believed to be associated with control of the corpus luteum, whose lifespan and secretory activity are regulated by trophic and lytic mechanisms. Phase I included efforts to prolong the luteal phase of the estrous cycle or to establish an artificial luteal phase by administering exogenous progesterone. Later, progestational agents were combined with estrogens or gonadotropins in phase II, whereas phase III involved PGF_2α and its analogues as luteolytic agents. Treatments that combined progestational agents with PGF_2α characterized phase IV.

Precise monitoring of ovarian follicles and corpora lutea over time by transrectal ultrasonography expanded our understanding of the bovine estrous cycle and particularly the change that occurs during a follicular wave. Growth of follicles in cattle occurs in distinct wavelike patterns, with new follicular waves occurring approximately every 10 days (6- to 15-day range). We now know (phase V) that precise control of estrus and ovulation requires the manipulation of both follicular waves and luteal lifespan.

A single injection of GnRH to cows at random stages of their estrous cycles causes release of luteinizing hormone leading to synchronized ovulation or luteinization of most large dominant follicles. Consequently, a new follicular wave is initiated in all cows within 2 to 3 days after GnRH administration. Luteal tissue that forms after GnRH administration is capable of undergoing PGF_2α-induced luteolysis 6 or 7 days later.² This method is referred to as the GnRH-PGF_2α protocol throughout this chapter. The GnRH-PGF_2α protocol increased estrus synchronization rate in both beef^3,4 and dairy⁵ cattle. A drawback of this method is that approximately 5% to 15% of the cows are detected in estrus on or before the day of PGF_2α injection, so that a smaller proportion of females are detected in estrus and inseminated during the synchronized period.⁶

THE MELENGESTROL ACETATE PROGRAM

This section reviews recently developed methods using MGA to control estrous cycles of cows or heifers in breeding programs involving natural service or artificial insemination. Four methods are outlined for using MGA (MGA Premix, Pfizer Animal Health, New York, NY) to facilitate estrus synchronization in heifers or cows. The choice of which system to use depends largely on a producer’s goals. MGA is the common denominator in each of the systems described. MGA is an orally active progestogen that will suppress estrus and prevent ovulation when consumed by cows or heifers on a daily basis.

MGA may be fed with a grain or a protein carrier and either top-dressed onto other feed or batch-mixed with larger quantities of feed. MGA is fed at a rate of 0.5 mg/animal/day in one feeding. The duration of feeding may vary between protocols, but the level of feeding is consistent and critical to success. Animals that fail to consume the required amount of MGA on a daily basis may prematurely return to estrus during the feeding period. This can be expected to reduce the synchronization response. Therefore, adequate bunk space must be available so that all animals consume feed simultaneously.

Animals should be observed for behavioral signs of estrus each day of the feeding period. This may be done as animals approach the feeding area and before feed distribution. This practice will ensure that all females receive adequate intake. Cows and heifers will exhibit estrus beginning 48 hours after MGA withdrawal, and this behavior will continue for 6 to 7 days. It generally is recommended that females exhibiting estrus during this period not be inseminated or exposed to natural service because of the reduced fertility among such animals at the first heat after MGA withdrawal.

Method 1: MGA with Natural Service

The simplest MGA-based method of estrus synchronization involves using bulls to breed synchronized groups of females. This practice is especially useful in helping producers make a transition from natural service to AI. In this process, cows or heifers receive the normal 14-day feeding period of MGA and are then exposed to fertile bulls about 10 days after MGA withdrawal (Fig. 37-1).

Fig. 37-1 Melengestrol acetate (MGA) and natural service.

(Adapted from Patterson et al.⁹)

This system works effectively; however, careful attention to bull-to-female ratios is indicated. A ratio of 15 to 20 synchronized females per bull is recommended. Age and breeding condition of the bull and results of breeding soundness examinations should be considered carefully.

Method 2: MGA plus Prostaglandin

A more precise means of estrous cycle control involves the combination of MGA with PGF_2α. PGF_2α is a luteolytic compound normally secreted by the uterus of the cow. PGF_2α can induce luteal regression but cannot inhibit ovulation. When PGF_2α is administered in the presence of a functional corpus luteum (CL) during days 6 to 16 of the estrous cycle, premature regression of the CL begins and the cow returns to estrus.

In this program, PGF_2α should be administered 19 days after the last day of MGA feeding. This treatment places all animals in the late luteal stage of the estrous cycle at the time of injection, which shortens the synchronized period and maximizes conception rate (Fig. 37-2). Although a 19-day interval is optimal, 17- to 19-day intervals produce acceptable results and provide flexibility for extenuating circumstances.^7,8 Any of the four available PGF_2α products can be used for synchronization of estrus in cattle after the MGA treatment. Label-approved dosages differ with each of these products; the practitioner should carefully read and follow directions for proper administration before their use.

Fig. 37-2 The melengestrol acetate–prostaglandin F_2α (MGA-PGF_2α) protocol.

(Adapted from Brown et al⁷ and Lamb et al.¹⁴)

Figure 37-3 illustrates the distribution of estrus comparing the MGA-PGF_2α system and an MGA-only system.⁹ The combined MGA-PGF_2α system is best suited for use with AI programs because of the high degree of synchrony that can be achieved, which decreases the amount of time required for detection of estrus. Under natural mating conditions, distributing estrus over several additional days may be advantageous, to prevent overworking of bulls used in these programs.

Fig. 37-3 Distribution of estrus comparing the melengestrol acetate–prostaglandin F_2α (MGA-PGF_2α) system with an MGA-only system.

(Adapted from Patterson et al.⁹)

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