Gordon King, Animal Science, University of Guelph
One of the difficulties associated with operating an animal breeding enterprise is that almost everything relating to an actual mating must be done when the female is ready, rather than at the manager's discretion. Thus, there has been considerable interest in development of procedures to manipulate ovarian activity so that ovulation is regulated to allow mating at predetermined times. At present, no technique precisely regulates ovulation but reasonably effective pharmacological and physical methods are available to synchronise estrus in mature, cyclic females or to stimulate follicular phases in near pubertal or acyclic animals. Effective procedures to influence ovarian activity so that a number of females show estrus around the same time have a number of practical uses in livestock production. One of the major applications is to facilitate artificial insemination of beef cows or females of any other species maintained under extensive conditions. Animals can be concentrated at a convenient location and additional labour provided during the treatment period. Since the induced follicle phase should occur around the same time in most treated females, arrangements can be made for insemination of all females without estrus detection or for only those showing behavioural signs. The insemination period is usually limited to immediately after treatment or for one additional cycle and males are subsequently placed with the females to service any that did not conceive to inseminations. Such programs are usually applied in beef or sheep herds but dairy operators may also resort to controlled mating of heifers when these are kept away from the milking herd and regular observation is difficult. Also, dairy farmers directly involved in crop production may have difficulty finding the time for routine estrus detection during periods of heavy field work. In such instances, controlled breeding could be initiated for short periods as a convenience. Another important use of estrous cycle control is to programme embryo transfer recipients so that sufficient numbers are available when needed.
Sows possess a reasonably precise estrous regulatory mechanism since most remain acyclic while nursing but initiate follicular development, usually progressing to estrus and ovulation, immediately after weaning. In contrast, gilt cycles are quite random, creating problems in programming replacements into batch farrowing groups. Thus, estrous control of gilts can be a useful management aid in large swine units attempting to operate a regular batch farrowing system. The induction of sexual maturity in prepubertal gilts through injection of hormonal combinations also has cycle regulating potential but fertility is usually low or pregnancy terminates early if they are mated at an estrus induced by currently available pharmacological procedures.
To be acceptable for routine use in commercial livestock production units, any estrous controlling procedure must fulfil a number of essential criteria.
Any compound or procedure under consideration should be evaluated for its ability to satisfy these criteria.
Male exposure. Perhaps the simplest procedure for regulating estrus is the introduction of a mature ram into a flock of acyclic ewes just before onset of their normal breeding season or exposure of near pubertal gilts to a boar. Contact with the ram will usually stimulate ovarian activity in many of the ewes and a substantial proportion show estrus during the first week. The breed, nutritional status and timing in relation to when seasonal activity would normally start are all factors that influence how effective the response will be. Successful induction of estrus in some animals has also been reported soon after placing bulls with groups of postpartum cows or apparently acyclic heifers, but the effectiveness of this treatment is still questionable. The estrous response is usually quite dramatic when nearly mature gilts are exposed to mature boars. A number of commercial herds routinely mix potential replacement gilts with boars each day as the females reach 5-6 months of age and approach market weight. Expectations are that at least half of the treated gilts will show estrus within 7 to 10 days of initial male exposure. Gilts can be mated at the first estrus but, since most responders will subsequently cycle regularly, service can be delayed for three or six weeks. If substantially more animals than the required number of replacements are exposed, the responding gilts can be retained for breeding and the remainder marketed before they are overweight.
Environmental Manipulation. Controlled lighting is used routinely to regulate onset of sexual maturity in pullets and regimes have been devised to stimulate earlier initiation of ovarian activity in seasonal breeding mammals. Short day breeders like ewes and does may be programmed to cycle if they are maintained in a light-tight building with photoperiod gradually reduced over an 8 to 12 week period. Such facilities might be applicable for propagation of elite sheep or dairy goat breeding stock but are impractical for commercial production. Does, however, can be housed in conventional stables and exposed to a 20 h photoperiod through a combination of natural and artificial light during the winter months, with abrupt termination of artificial prolongation and return to shorter natural photoperiod in early March. A high proportion of the treated does should commence cycling and be successfully mated 7 to 10 weeks after the return to natural light only. The bucks must also be exposed to this schedule to obtain high fertility.
The breeding season of mares may be advanced by gradually increasing the photoperiod by artificial lighting each week during the late fall and early winter, to about 16 h per day and maintain this photoperiod until cycling commences. Mares are long day breeders so must be exposed to a lengthening photoperiod for commencement of ovarian activity. The natural breeding season for most mares in the northern hemisphere is May through August. This can be advanced by exposing the females to increased photoperiod commencing six to eight weeks before the desired time for mating. The most commonly employed system is to supplement natural daylight with incandescent (200 watt) or fluorescent (40 watt) light so the total photoperiod reaches 14 to 16 h. The light intensity should be about 10 foot-candles at the level of the animal's eye. If an owner wishes to begin breeding around the middle of February, mares should be brought into individual stalls and exposed to artificial light commencing about the beginning of December. Stallions should also be subjected to the increased photoperiod to insure fertility at the advanced mating time.
The separation of beef calves from their dams for 48 h sometime in the second month after parturition can induce estrus and ovulation in cows. This short-term calf removal has been quite successful in some trials but no response was obtained in others. Presumably, other factors such as body condition, nutrition and possibly environmental conditions are also important in determining the response. Hopefully, further research will provide answers and a reliable procedure. Short-term removal requires no inputs besides additional labour and restraint facilities so could have considerable application in developing countries.
. Pharmacological control of ovarian activity involves administration of exogenous hormones that modify the luteal phase to regulate the occurrence of estrus. Since luteolysis is the event that governs when the next estrus and ovulation occurs in normally cycling females, it is the manipulated activity. One method to accomplish this is through artificially extending the luteal phase by continued administration of progesterone or some other progestagen compound, and then discontinuing treatment just before estrous is desired. Alternately, the luteal phase can be terminated prematurely by injection of a luteolytic agent such as prostaglandin F2a into most species. Treatment may be given to individual animals but is more commonly applied to groups of females.
Regulation of ovarian function by induction of luteolysis can be
accomplished by administration of any effective luteolytic agent. The compound
causes regression of corpora lutea and a decrease in circulating progesterone,
allowing resumption of gonadotrophic stimulation and follicular maturation so
that responding females should show estrus signs around 72 to 96 h after
treatment. The most potent luteolytic compounds currently available are
prostaglandin F2a and its analogues. These are usually given by intramuscular
or vulvar subcutaneous injections, but can also be infused into the uterus.
Bovine corpora lutea are highly susceptible to very small doses of
prostaglandins any time between days 6 and 16 of the estrous cycle. Since
approximately half of the cows in any randomly cycling group will be between
days 6 and 16 postestrus and a quarter between days 16 and 20 at any particular
time, 66 to 75% of the animals should show estrous signs within a few days
after a single injection. A number of treatment and mating options are possible
and these can be modified to suit most situations. Prostaglandins, however, are
relatively expensive and can only function in animals with active corpora
lutea. Thus, unless the herdsperson knows that almost all group members are
cycling regularly, it may be economical to have the females examined rectally
by a qualified individual to select suitable candidates. Porcine corpora lutea
are not susceptible to prostaglandin induced lysis until very late in the
luteal phase so the procedure has very limited use for cycle control in this
species. Caution must always be exercised when using these drugs since they are
likely to produce abortions if given to pregnant females. Some of the possible
methods for control of the bovine estrous cycle using prostaglandins are:
i). Inject all cows, or only those cows with palpable corpora lutea, observe for estrus over the next 7 to 10 days and inseminate animals when detected during that period. Then place bulls with the herd to mate any cows not detected or returning from insemination
ii). Inject all cows and inseminate any that show estrous signs during the next 10 days. Any that have not been inseminated are re-injected on day 11 and subsequently observed.
iii). A simpler modification of the above method would be to observe and inseminate over a 7 to 10 day period and any apparently normal females that are not detected by that time could then be injected.
iv). Inject an entire group with prostaglandin and then repeat the injections for all animals 10 to 12 days later with mating following the second treatment. Any females that were too early in their luteal phase to be susceptible at first injection should be responsive at the second one or almost ready to return to estrus without treatment. Similarly all those that responded and ovulated should be far enough into their next cycle to be again susceptible.
Theoretically, this method should be 100% effective but perfect results are seldom achieved in actual practice. Results from a trial comparing several of these methods with conventional detection and mating are presented in the following Table. One important distinction between these results is that all cows treated with the prostaglandin analogue were detected and mated within a six week period but some of the control animals were not.
| Cycle control
AI when detectedb
| Cycle control
Detected + fixed time AIc
|No. of cows||67||29||28|
|Calving to 1st AI, %||57||52||39|
|Calving to 3rd AI, %||71 or 60d||72||69|
aRoutine observation for estrous signs and
inseminated when detected.
bOne injection of prostaglandin F2a and inseminated when detected,
cA second injection of prostaglandin F2a given on day 11 to all cows not observed in estrus following the first injection and fixed time AI at 72 and 96h later.
dPregnancy rate in the conventionally mated group was 71% for mated cows but only 60% for all cows that should have been mated.
The occurrence of estrus can be regulated by giving progesterone or some other progestagen compounds with similar biological activity for extended periods to cycling females. Effective treatments should suppress gonadotropin secretion as long as they are administered and release should resume immediately after withdrawal, so all the treated animals will begin a follicular phase together. In any group of sexually active females, estrous periods will be randomly distributed and members will be at various stages on any single day. The progestagen treatment must therefore be continued for a period approaching the normal cycle duration to regulate all members effectively. Agents that can be given in the feed are convenient for confined and regularly fed animals, but their prolonged administration to ruminants usually results in depression of fertility while shorter treatment periods provide less control. Thus, products such as medroxyprogesterone acetate or melengesterol acetate are no longer used alone as estrus regulating agents but might be administered for shorter intervals in combination with prostaglandins or estrogens (reviewed by Patterson et al., 1989). In contrast, two other progestens, methalabure and altrenogest can be fed to sows for 15 or 20 day periods to achieve very good synchronization and fertility dose not seem to be affected by the treatment. However, methalabure is teratogenic when fed to pregnant females so it is no longer marketed in many countries. Altrenogest is currently available in a number of countries but is only approved in North America for use in mares. (Data from King et al., 1983. Can Vet J, 24:105)
Progestagens can be injected but treatments must be repeated daily or oftener for optimum results with most of the agents so this is not convenient or desirable. The various implant methods, illustrated in Figure 19.3, are the most practical means of administration currently available since animals need only be handled for insertion and again for removal, which precisely controls drug withdrawal. The first implants were progestagen impregnated sponges that were compressed and placed in vaginas of ewes, by means of a speculum, so the drug could be absorbed directly through the mucosa. After an 8 to 12 day period the sponges were removed and most treated females came into estrus within a few days. These were effective since most ewes tolerated the foreign object within their vaginas so the rejection rate was low. In contrast, when sponges were inserted into cows or sows, most females commenced vigorous straining and the devices were expelled quickly.
The Progesterone Releasing Intravaginal Device (PRID) available for cows or Controlled Intravaginal Releasing Device (CIRD) available for cows and ewes are sophisticated systems for intravaginal administration of progestagens. They consist of a stainless steel coil (PRID) or looped structure (CIRD) covered by a layer of progesterone impregnated silastic (Fig. 19.2). The progesterone is released slowly and absorbed into the system. These, like sponges, are inserted through a speculum and usually left in place for about 12 days. A small gelatin capsule containing estradiol benzoate is attached to the inner surface of the PRID coil. This estrogen is quickly absorbed through the vaginal wall and should destroy any functioning corpora lutea. Thus, the luteal phase condition will be maintained entirely by progesterone absorbed from the PRID and all treated cows should commence a follicular phase immediately after removal. Good synchronization of estrus and acceptable fertility has been obtained with treatments using a combination of progesterone releasing devices and prostaglandins (Folman et al., 1981. Anim Reprod Sci. 4:117).
Another progestagen method for cycle control is a norgestomet containing silastic implant, marketed commercially as Syncro-Mate B. These are inserted under the loose skin at the base of the ear using a special implanting device. A small injection of estradiol valerate is usually given at the time of insertion to cause luteolysis in succeptable females. The implant must be removed after 9 days by making a small incision in the skin above the silastic and forcing it out. As with the previous devices, animals should show estrus a few days after removal of the implant.
Intravaginal progestin sponges combined with an injection of equine chorionic gonadotrophin given at sponge removal will usually stimulate estrus in sexually mature ewes during the nonbreeding season but fertility is low and cycles do not usually continue. Ovine gonadal activity is regulated by melatonin secretion from the pineal gland so exogenous treatment can influence function. Both timed and continuous melatonin administration to acyclic ewes effectively induce breeding activity around mid-summer probably following exposure to a critical interval of long photoperiod. In less seasonal Merino crossbreds melatonin treatment can be effective before the summer solstice. The length of melatonin exposure may also be important in stimulating ovarian activity. Some ewes are able to respond to very short exposure periods but an interval of at least 36 days is necessary to obtain normal cyclicity in most animals.
The acceptance of any new technology involves practical considerations on costs associated with procuring new inputs, facility modifications that might be required for administration and the results that could be anticipated. When current reproductive efficiency is average or above, there is little incentive for adopting a totally controlled breeding system. Cycle control with or without fixed time insemination might be used occasionally for convenience since it would negate the necessity for routine estrous detection during periods when animal attendants might be occupied with other activities such as seeding or harvesting that drastically reduce the time available for animal observation. Regulation of ovarian activity is also useful for heifer mating in units where replacements are raised away from the main herd and not visited on a regular basis. Estrous control might be beneficial some herds with below average fertility when malnutrition or genital diseases can be eliminated as primary causes. If it appears that most cows are cycling normally and the main problem seems to be poor detection, cycle control should provide the operator with a good indication of when estrus is expected so that attention can be focused on individual animals during this critical period. This could facilitate detection and assist in mating at appropriate times, improving pregnancy rates and shortening calving intervals. Management of reproduction may have even more appeal for many herds if recombinant bovine somatotrophin is approved for lactation enhancement since it could provide a method to achieve mating before animals were placed on stimulatory treatment. Any such manipulations should be combined with early pregnancy indication by milk progesterone determinations and subsequent confirmation by rectal examination.
Veterinarians must be thorough in assessing causes for reproductive problems and initiating potential solutions. The introduction of controlled breeding techniques might assist in educating some attendants to improve detection and insemination timing but will have little benefit in herds were feeding deficiencies are resulting in nutritional acyclicity. Veterinarians, in consultation with individual herd managers, must decide if ovarian cycle control could be used advantageously in a particular management programme. The increased cost of the compound, plus any expenses for professional supervision of treatment, must be assessed against the anticipated improvement in reproductive performance.