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REPRODUCTIVE MANAGEMENT OF THE MEAT GOAT
By: "Stephan Wildeus"
Goat Reproductive Biology
The reproductive tract of the mature doe consists of the ovaries, which weigh 0.5 to 3 grams dependent on the stage of the reproductive cycle. The ovaries are the primary sex organ, containing the eggs and secreting the female reproductive hormones (i.e. progesterone, estrogen). The oviducts (10-12 cm long) transport the ova to the uterus and act as the site of fertilization. The uterus (15-20 cm long in the non-pregnant state) is the site of fetal implantation and consists of two uterine horns with a common uterine body. The uterus provides the environment that supports the conceptus throughout gestation. Closure of the uterus is provided by the cervix (4-7 cm long), a muscular canal with several cervical folds or rings that must be at last partially penetrated during artificial insemination. The exterior component of the doe reproductive tract is the vagina which is the site of semen deposition during natural mating; it also supplies a fluid environment to support this process during the appropriate stage of the estrous cycle.
The events of the estrous cycle are largely controlled by the hormonal interactions of the ovaries with the secretory glands (pituitary, hypophysis) located at the base of the brain. In addition to internal stimuli, this system is also responsive to external stimulation such as changes in day length and the presence and absence of males. In short, primary follicles in the ovaries, containing primary oocytes (eggs), develop in successive waves to develop Graafian follicles that will rupture and release a secondary oocyte during ovulation. The released oocyte transverses the oviduct to join with spermatozoa, whereas the Graafian follicle transforms into corpus luteum. The development of the follicle is under the control of gonadotropins (follicle stimulating hormone - FSH and luteinizing hormone - LH) released by the pituitary gland. The gonadotropins, via a hormonal feedback loop, also control the release of estrogens by the ovary, which control the estrous behavior displayed by the doe (flagging, mounting etc.). Following ovulation the luteinized follicle (corpus luteum) secrets progesterone which prepares the uterus for a possible pregnancy and suppresses the secretion of gonadotropins to suspend further follicular development. Failure to establish pregnancy will result in the release of prostaglandin from the non-pregnant uterus, which regresses the corpus luteum and allows a new cycle to proceed. Knowledge of these processes facilitates an understanding of the techniques that can be used to control reproduction (superovulation, estrus synchronization etc.) in the doe.
In contrast to the female, where all primary oocytes (eggs) that will be developing are present at birth, primary spermatocytes are produced through mitotic divisions continuously throughout the reproductive life of the male. A further difference between oogenesis in the female and spermatogenesis in the male is that the meiotic divisions of the primary oocytes yield only one functional ova, while primary spermatocytes produce four spermatozoa. The final step of sperm cell production is a process of metamorphosis in which the spermatids, the product of the second meiotic division, develop the characteristics of the functional spermatozoa (head, acrosome, midpiece and tail). Spermatozoa , approximately 60 microns long, are ejaculated in a dense suspension with seminal plasma (Table 1). The seminal plasma activates motility (5-15 mm/min progressive forward motility) and supplies substrates to buffer and nourish the sperm cells. Similarly to the ovaries, the events in the testis are controlled through the gonadotropins LH and FSH.
Onset of Puberty
Sexual development is influenced by both genetic and environmental factors. In does and bucks the age at puberty ranges from 150 to 230 days, dependent on nutrition, location and season of birth. Nutrition is among the most significant factors influencing reproductive development and the onset of puberty. A low plane of nutrition delays first estrus and reduces uterine and ovarian weights, while having no effect on the partitioning of fat and protein and the weight of other organs. Increasing the overall plane of nutrition generally advances the onset of puberty, but overfeeding will decrease subsequent fertility and impair mammary gland development. Season of birth also has a significant impact on the timing of puberty in both the doeling and buckling, with sensitivity to photoperiodic cues already being in effect in the prenatal stages of development. Puberty in spring-born kids has to be achieved in the same year's fall breeding season or will be delayed until the following year's breeding season. There are some indications that the introduction of bucks may induce estrus and ovulation in the pubertal doe. The physiological basis for this response is attributed to be partly pheromonal and partly neurological.
Seasonality of Breeding
Differences exist in the onset and duration of seasonal breeding between various breeds of goats and even between individual animals within a breed. Geographical location, particularly degree of latitude, has a significant impact on timing and length of the breeding season. At locations close to the equator and in tropical breeds of goats animals often are aseasonal and breed throughout the year. In the seasonally breeding does, the breeding season is framed by transitional periods during which cyclic activity can be induced through appropriate management techniques (i.e. introduction of males).
Goat Reproductive Management Techniques
Reproduction should be a vital component of the overall herd management scheme and closely integrated with nutritional and health aspects, as well as form part of a comprehensive recording system. Diets and feed supplies have to be adjusted to account for the physiological stage of production of the goat, particularly in the female (lactation, gestation). Prior to breeding (2-3 weeks) does should be placed on a gaining plane of nutrition to stimulate higher ovulation rates. Once does are bred and pregnancy has been determined, does should be preferentially fed based on pregnancy status (and litter size if fetal numbers were determined; see pregnancy diagnosis below). Does nursing their kids are nutritionally challenged and may require supplemental feed if pastured to ensure adequate milk supply for multiple litters.
There are currently no major reproductive diseases affecting goats in the U.S., however, goats need to be maintained in good health (dewormed and vaccinated) to ensure proper reproductive function. Meat-type does should be capable of giving birth and raising their offspring unassisted, but help may have to be provided with complications during parturition and the acceptance of the newborn. Records should be collected on kidding and weaning performance (litter size and weight) to be used for selection of breeding stock. Replacement does should be managed closely to achieve a level of sexual maturity that allows an early mating (at 60-70% of adult body weight) within one year of age, thus increasing life time production of the doe. Similarly, young bucks should be mated early in life to decrease the generation interval and achieve maximum genetic progress.
To be most useful to the producer, pregnant animals need to be identified as early as possible in gestation and provide an estimate of fetal numbers. A variety of approaches have been explored for the early detection of pregnancy and possibly fetal numbers (Table 2). Techniques have either focused on the detection of physical changes resulting from pregnancy (fluid accumulation and presence of a detectable fetus) through palpation and ultrasound, or been concerned with the identification of maternal and fetal physiological signals (progesterone, uterine proteins) associated with pregnancy.
The most promising technique currently available for pregnancy diagnosis in the goat is the use of real-time ultrasound scanning. The arrival of lower cost, portable veterinary scanners, combined with the advantages of their use (fetal number determination, minimal animal restraint, high throughput), has made the application of this technology economically feasible on the farm level. Transcutaneous real-time ultrasonography allows reliable pregnancy diagnosis as early as 35 days of gestation, whereas transrectal examination will reduce this period further to 25 days. Ultrasound examination can be expanded through the application of fetometry, allowing the aging of the fetus. Guidelines for fetal aging have been developed for the goat, using biparietal diameter as the main measurement. Linear array and sector scanners are available for use in transcutaneous ultrasonography and 5 and 7.5 MHz linear transducers can be used for transrectal examinations. The latter can also be successfully employed for the examinations of ovarian structures.
In view of the versatility and benefits provided by the real-time ultrasonography, many of the techniques listed in Table 2 will find only limited application for routine diagnostic purposes. The use of A- and B-mode and Doppler sound ultrasonic devices has now been succeeded by the real-time linear array and sector scanners. Techniques using hormonal or metabolic (i.e. blood glucose) signals have not found widespread use in small ruminants. With the introduction of animal-side testing for blood and/or milk progesterone by enzymeimmunoassay and the validation of these techniques for goats, the turn-around time for laboratory analysis has been reduced. However, progesterone and estrogen determinations for pregnancy diagnosis should not be expected to find wide-spread application. Success to predict litter size from progesterone and estrogen concentrations has only been moderate (around 60%) and is confounded by breed differences.
Breeding Soundness Examination
Part of such a 'breeding soundness examination' is an evaluation of the overall condition of the buck and includes his health history, physical soundness, particularly of feet and legs, and examination (palpation) of the external genitalia (scrotum and scrotal content, sheath and penis) for signs of infections and other abnormalities. There are currently no age and breed standards for scrotal circumference in meat-type breeds and there is a need for guidelines to be developed. The second part of the examination involves the collection and evaluation of an ejaculate. In trained bucks this is achieved using an artificial vagina, but in most instances an electroejaculator has to be used. The method of collection has some effect on the ejaculate characteristics, the volume generally being larger in an electroejaculate. The ejaculate is immediately scored for motility under low (mass motility) and high magnification (percentage motile sperm) of a light microscope on a pre-warmed slide. Morphological abnormalities and viability are determined from stained semen smears. In the final part of the examination bucks are allowed access to estrous does to evaluate libido and mating behavior.
Bucks are classified as either sound, questionable or unsatisfactory, based on all components of the examination. No firm guidelines have been developed to assign bucks into these categories and interpretation rests largely with the experience of the examiner. Animals deficient in any part of the examination should be considered questionable and retested after several weeks. A second failed test would indicate reproductive deficiencies and such a buck should not be used in natural mating.
Manipulation of Reproduction in Goats
The utilization of reproductive management techniques has only limited application in an extensively managed herd, but can be an useful tool to improve performance of a more closely managed herd. Additional inputs will be needed in labor and handling facilities and in the area of nutritional management. Unfortunately most of the commercial pharmaceutical products developed for reproductive manipulation in goat and sheep are not available and/or approved for use in the U.S. and have only been applied in the U.S. on an experimental basis. However, a description of these techniques is relevant to familiarize the producer with the options that may become available or can be applied under extra-label use in cooperation with a licensed veterinarian. There are some reproductive manipulations that can be performed without the aid of pharmaceutical compounds, such as the use of the male effect and controlled lighting and they will also be discussed briefly.
The treatment of choice for estrus synchronization, and also out-of-season breeding, in goats has been the intravaginal sponge, impregnated with 45-60 mg of a synthetic progesterone (Table 4). Sponges are widely used either in conjunction with pregnant mare serum gonadotropin (PMSG), FSH or prostaglandin to more tightly synchronize and/or induce a superovulatory response. Under research conditions sponges impregnated with natural progesterone in higher doses (400-500 mg) have been used and similar synchrony and fertility to that of commercial sponges were achieved. An alternative means of supplying continuous, exogenous progesterone has been the intravaginal pessary (CIDR-G®) developed for goats in New Zealand. The CIDR device is constructed from a natural progesterone impregnated medical silicone elastomer molded over a nylon core. In large scale trials with cashmere goats in Australia CIDR devices were equally effective to intravaginal sponges in controlling ovulation and fertility following A.I.
A number of synchronization systems for goats have been evaluated under research conditions that use compounds approved for other species and/or applications (Table 4). One of these systems is based on the extra-label use of the norgestomet ear implant supplied with the estrus synchronization system Synchromate-B®, developed for cattle. Does are implanted with the norgestomet implants for a period of approximately 14 days and a gonadotropin, either FSH or PMSG, is administered around the time of implant removal. There will usually not be an adequate response and synchrony of estrus without the gonadotropin treatment. The estradiol valerate injection provided in the product combination for cattle should not be used for goats due to their increased sensitivity to estrogens. Studies have indicated that the implant dose provided for cattle (6 mg norgestomet) can be reduced to 2-3 mg by cutting the implant. Following synchronization does and ewes come into estrus within 72 hours. Melengestrol acetate (MGA) is an orally-active, synthetic progestogen, approved for use in feedlot cattle, that can be used for the induction and synchronization of estrus in does in conjunction with zeranol and PMSG. Prostaglandin F2a, or rather its analogues, are widely used for estrus synchronization in cattle, but results have not been as satisfactory in goats. A functional corpus luteum is required for prostaglandin to regress, thus making this technique only suitable for synchronization during the breeding season. Synchronization with prostaglandin analogue generally produces a more synchronized estrus than that obtained with a progestogen-gonadotropin treatment, but subsequent fertility is somewhat reduced.
The application of estrus synchronization schemes requires an increased level of management either through the utilization of A.I. or the proper management of bucks. With a larger number of females showing estrus at the same time, the female : male ratio should not exceed 7:1, or alternatively, timing of the induced estrus should be staggered (i.e. spreading the removal of intravaginal sponges over several days). Hand mating of males, as a modification of A.I., can also be used. Fertility of the synchronized estrus is generally high, but responses to PMSG and prostaglandin co-treatment have at times been variable. The repeated use of PMSG in conjunction with progestogen treatment has resulted in reduced fertility in subsequent years and was attributed to an active immunization against PMSG.
Artificial lighting, either by itself or in conjunction with melatonin and/or the male effect, can provide effective manipulation of the breeding season in goats. Since melatonin can be conveniently used to mimic short days, artificial lighting under practical conditions is mostly employed for 'long day' simulation. Long days under artificial lighting are usually administered as 16 hours of daylight to 8 hours of darkness. To simulate long days it is, however, not necessary to provide the entire 16-hour light period, but treatment can be divided into the natural daylight period followed by an appropriately timed 1 hour light stimulus at the time of desired dusk. Goats will distinguish between a gradual decrease in daylength as opposed to a sudden shift from short to long days. Models for light controlled year-round breeding of goats have been proposed and experimentally validated and would subject animals to a 2-month short day-long day cycle. Results indicated that the period of cyclic activity was extended, but that periods of acyclicity remained and a lack of continuity in cycles developed. Most practical systems have focused on the extension of the natural breeding season, combining a period of long days followed by melatonin treatment for short day simulation.
Exposing anestrous females to intact males or androgen-treated castrates, following isolation, has been demonstrated to induce estrus and ovulation in the doe. The physiological basis for this response is partly pheromonal and partly neurological, with neither aspect completely accounting for the response. However, it is documented that the stimulus will elicit a pulsatile LH release sufficient in length and magnitude to initiate the ovulatory process. The male-induced estrus is usually synchronized, with ovulation occurring within 2-3 days of stimulation. The response to male stimulation can be quite variable and is influenced by breed, completeness of prior isolation, "depth" of anestrus, nutrition and stage postpartum. Unless male-induced cyclic activity is initiated preceding the natural breeding season of a given breed at a given location the response is transient in nature. Hence the practical application of the 'male effect' lies primarily in inducing an early breeding season, or in combination with some pharmacological out-of-season breeding manipulation.
Goats generally respond more favorably to out-of-season breeding using melatonin, artificial lighting and the male effect than sheep. Differences have been attributed to the higher and more variable endogenous night-time melatonin levels in sheep compared to goats, as well as to the need for progesterone priming before estradiol will generate behavioral estrus.
Increases in ovulation rate have also been achieved through the immunization of does to steroids. Steroid immunization has become commercially available overseas as Fecundin®, which immunizes females to androstenedione (Table 4). Immunization is achieved through two subcutaneous injections (2 ml) administered initially 2-3 weeks apart and in a single annual boosters thereafter. A period of 3 weeks is suggested between the booster immunization and the time of optimum ovulation. Due to the long term effects and the relative ease of application of the product, steroid immunization can be used for the improvement of ovulation rate and subsequently litter size in more extensively managed flocks. The animal response in ovulation rate and litter size varies with breed and location, but improvements of ovulation rate (+1.0) and litter size (+0.5) have been achieved in does.
A number of other pharmacological treatments to manipulate reproductive function in goats are subject to investigation and development under research conditions. However, it is not clear to what extend these approaches may prove to be biologically and/or economically feasible. Among the concepts under investigation are (i) the immunization against inhibin, which selectively suppresses FSH, but not LH, (ii) the use of GnRH in conjunction with progestogen-based superovulation treatments, and (iii) the administration of betamethasone for the induction of kidding.
Assisted Reproduction in Goats
The techniques of artificial insemination (AI) and more recently embryo transfer (ET) in livestock production present producers with unique opportunities to maximize the number of progeny from animals with superior genetic make-up and move their germplasm around with relative ease. Drawback of these technologies are the need for experienced personnel with the appropriate equipment to achieve the desired success. The costs involved are most likely prohibitive for producers of goats that are marketed for meat, but has great potential for producers of breeding stock, propagating animals with outstanding production characteristics. In the fledgling meat goat industry the recent introduction of the Boer goat is an excellent example for the need to apply assisted reproductive technologies for the dissemination of stock. As other superior meat-producing germplasm is identified, the application of AI and ET is likely to rise in the area of meat goat production.
For AI, semen is usually collected from bucks trained to serve an artificial vagina, adjusted to the appropriate temperature and pressure. Once a collection schedule is initiate, bucks can be collected 2-3 times daily on alternate days. Semen is immediately evaluated for quality and the concentration is determined. The semen is then diluted in a medium containing egg yolk, sugars and buffer to provide an insemination dose of 20 million (frozen, laparoscopic intra-uterine) to 300 million (fresh, vaginal) spermatozoa, dependent on the intended insemination technique. When semen is intended for frozen use, glycerol is included in the diluent as a cryoprotectant. Semen can be either frozen as a pellet, using an engraved block of dry ice, or aspirated into straws and frozen using liquid nitrogen vapor. Once frozen in liquid nitrogen, semen can be stored for extended periods of time.
The success of the actual insemination depends to a large degree on the appropriate timing in relation to estrus and ovulation. Does must be observed closely for the onset of estrus (flagging, restlessness, frequent urination, vaginal swelling and mucus discharge), or can be synchronized (see above), and should be inseminated 12-18 hours after the onset of estrus. In case of transcervical insemination the thawed semen will be deposited in the restrained doe either in the cervix or in the uterine body, adjacent to the cervical opening, using an insemination pipette and speculum. In case of intra-uterine insemination, the semen is deposited into the uterus through the abdominal cavity via an insemination pipette manipulated through a laparoscope. When using this technique the doe is restraint in a cradle in a ventral position. Though laparoscopic insemination is more involved, fertilization rates are high, even when using small doses of frozen semen.
Techniques used for estrus synchronization of donor and recipient and for the superovulation of the donor with gonadotropins (FSH, PMSG) are similar to those described above. Insemination of donor does should occur either naturally or through vaginal AI, rather than intra-uterine AI, to additional manipulation of the uterus and oviducts. For the actual collection, the uterus of the donor is flushed 3-5 days following mating. Traditionally this is done in goats under anesthesia using a midventral or flank laparotomy, involving the exteriorization of the uterus. Particularly in case of repeated collections this may cause adhesions interfering with subsequent collections. More recently collection techniques using laparoscopy have been developed and reported good success in goats (76% pregnancy). Following collection, the flushing medium is examined to identify fertilized (cleaved) ova, determine the recovery of ova (based on the number of corpa lutea) and evaluate ova quality. Only high grade embryos should be used for frozen storage, whereas embryos of less quality may be used for fresh transfer. Embryos should be transferred into the uterine horn of the same side containing an ovary with a corpus luteum. Multiple transfer into recipients without a corresponding number of corpa lutea is not recommended. Following a sufficient period of rest donor does can be repeatedly collected.
Appropriate reproductive management is vital to a successful meat goat enterprise. Much of the profit to be realized will depend the frequency with which litters are produced, the size of litters and the survival to weaning of multiple litters. A number of the reproductive techniques described here may not have an immediate application for the producer of goat meat, but any success in meat goat production will require sound knowledge of the reproductive biology of these animals. Since goat meat production in the U.S. as a primary enterprise is still in its beginnings, much of the germplasm evaluation and multiplication, completed for many other livestock breeds, will still have to take place. The application of reproductive technology (AI, ET) will form an important part of this process.
a techniques allowing the determination of litter size with high degree of accuracy (>95%)
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