Considerable information is available
on the reproductive function of goats, but research on reproductive management of goats in
the U.S. has focused mostly on milk and fiber production systems and has not been directed
at meat as the primary product. In a meat production system, however, reproductive
performance is of paramount importance since productivity is largely a function of the
number of offspring born and weaned and the frequency with which they are produced. The
main reproductive concerns in meat production from goats therefore must be an optimum
litter size (2-3 kids) with a high survival to weaning and, secondly, the flexibility to
strategically breed does to produce kids that will fit a specific market niche to command
a maximum price.
Goat Reproductive Biology
The Doe
Goats are seasonally polyestrous
under the temperate climatic conditions of the U.S. During the period of seasonal
breeding, reproduction in the doe is controlled by the estrous cycle. Some of the
characteristics of the estrous cycle of concern to producers are listed in Table 1.
Although the estrous cycle length of goats (21 days) is 3-4 days longer than in sheep,
gestation length, duration of estrus and timing of ovulation are similar between the two
species. Goats are often considered more prolific than sheep, but there appears to be more
variation between breeds within species (i.e. Nubian vs. Angora; Finnsheep vs.
Rambouillet) than there is between species.
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.
The Buck
In the buck, the primary sex organs
are the testis, which weigh 100-150 grams in the mature animals and fluctuate in size with
changes in breeding season. Similar to the ovaries in the female, the testis produces the
male gametes (spermatozoa) and sex hormones (i.e. testosterone). The spermatogenic process
takes place inside the seminiferous tubules, whereas the Leydig cells in the interstitial
tissue are responsible for hormone production. The testes are located in the scrotum,
which aids in the thermoregulation of the testes. Spermatozoa produced by the testis enter
the epididymis, also located in the scrotum, which serves as the site of sperm maturation
(acquisition of motility and fertilizing capacity) and storage. The vas deferens connects
the epididymis to the ampulla and accessory sex glands. The latter provide the spermatozoa
with the fluids that make up the ejaculate of the buck and are located in the pelvic
region. The penis is the final component of the male reproductive tract and is used to
deposit the semen into the female. In the buck, erection is achieved through the extension
of the sigmoid flexure that allows an extension of up to 30 cm and the filling of the
cavernous tissues with blood. In the non-erect state the glans of the penis is contained
in the sheath.
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 in the goat,
as in other mammals, is a process of gradual maturation of the interaction between the
hypothalamus, pituitary and gonads, initiated during embryonic development. Postnatal
sexual development is dominated by negative feedback of estradiol in association with
changes in the secretory pattern of LH. Puberty is generally defined as the point of
sexual development at which the animal becomes capable of reproduction (first ovulation in
the female and first spermatozoa in the ejaculate of the male), but often animals are not
fully sexually competent at this stage. In both the male and female goat, puberty may also
often be reached without having achieved adequate physical growth to support reproduction
and in the doe first ovulation may not necessarily coincide with first estrus.
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
The environmental cue most
dominantly affecting seasonal breeding in small ruminants is the annual change in
daylength. Photoperiodic control of reproductive patterns is mediated through rhythmic
secretions of melatonin by the pineal gland during darkness, which influence the
gonadotropin-releasing hormone pulse generation and the hypothalamic-pituitary-gonadal
feedback loop. However, following extended exposure to decreasing daylength, animals
become photorefractory to the short day stimulus and will cease cyclic activity, unless a
period of long day photostimulation is supplied.
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.
Pregnancy Diagnosis
While not of immediate concern in
extensive goat operations that utilize extended natural mating, the early determination of
pregnancy can be a useful management tool under more intensive production conditions, or
when A.I. and embryo transfer is employed. Pregnancy diagnosis will identify the females
requiring repeat breeding or insemination and/or will allow the separation of pregnant and
open females for differential management. When fetal numbers can be determined as part of
the pregnancy diagnosis, different feeding regimes can be applied to single and multiple
litter bearing females.
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
A buck should posses
characteristics that will advance the production potential of the herd in which he is
used, while being able to successfully mate to transmit these characteristics. As was
indicated earlier, spermatogenesis is susceptible to outside influences such as elevated
temperature, season of year and nutrition and breeding males need to be evaluated for
reproductive soundness 3-4 weeks prior to mating season.
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.
Estrus synchronization
Approaches towards synchronizing
estrus in livestock have to focus on either the manipulation of the luteal or the
follicular phase of the estrous cycle. In the doe the window of opportunity is generally
greater during the luteal phase, which is of longer duration and more responsive to
manipulation. Different approaches have been concerned with either extending the luteal
phase by supplying exogenous progesterone or with shortening this phase through removal of
the corpus luteum. Successful techniques must not only establish synchrony, but also
provide a reasonable level of fertility in the synchronized cycle (Table 3).
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.
Out-of-Season Breeding
Some of the pharmaceutical
techniques used for out-of-season breeding in small ruminants are essentially the same
progestogen-gonadotropin treatments described for estrus synchronization above. Estrus
response and subsequent fertility for the out-of-season application of intravaginal
sponges are similar to that reported for does during the breeding season. An alternative
pharmacological means of modifying the seasonal breeding patterns is through the
manipulation of the melatonin signal. Exogenous melatonin can administered to supplement
the endogenous release and thus mimic the 'short days' associated with the onset of
breeding season in fall. Melatonin can be supplied either as an orally active compound, by
injection or as an implant (subcutaneous or intravaginal), all of which have been
similarly effective. For the successful application of this treatment the melatonin
stimulus has to be continuous and in case of the orally active form requires daily feeding
between 1500 and 1600 hours. A prerequisite for the advancement of the breeding season
through melatonin treatment is for animals to have experienced a sufficient period (30-60
days) of long days. The response to melatonin treatment is related to the timing of the
treatment in relation to onset of breeding season for a given breed at a specific
location. A commercially available, subcutaneous slow release melatonin implant
(Regulin®, see Table 4) has been marketed overseas, no commercial products are currently
available in the U.S.
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.
Superovulation
As multiple litter bearing
animals, ovulation rate and litter size have a major impact on the reproductive efficiency
of goats. Ovulation rate is influenced by the stage of breeding season, nutrition,
genotype and parity. However, it can also be manipulated by pharmacological means.
Superovulation through gonadotropins (primarily FSH and PMSG), used in higher
(pharmacological) doses to elicit a superovulatory response, is commonly used to prepare
does for ova collection in embryo transfer. PMSG is more easily administered than FSH,
usually as a single injection of up to 1500 to 2000 i.u., but the superovulatory response
to PMSG can quite variable and is usually lower than in a FSH-induced superovulation.
Problems associated with PMSG-induced superovulation are a high number of non-ovulated
follicles and short, irregular estrous cycles. FSH is usually administered in decreasing
doses of 1 to 5 mg, injected in 12 hour intervals over a period of 3 to 5 days around the
time of termination of the progestogen treatment. Acceptable ovulation rates in does
following FSH range from 10 to 25, but the number of viable embryos may be significantly
lower. Improvements in the consistency and predictability of the superovulatory response
have been achieved through co-treatment with prostaglandin and LH. The latter acts in
synergism with FSH to achieve follicular stimulation and the ratio of FSH to LH has been
considered of some importance in achieving a satisfactory superovulation response.
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.
Artificial Insemination
The process of AI ca be broken
down into semen collection, semen processing and storage and the actual insemination. The
first two parts (collection and processing) are usually not of great concern to the
producer, unless bucks are collected on the farm. The actual insemination process,
however, is often carried out by producers with frozen semen shipped to the farm. Does can
be inseminated with fresh and extended, non-frozen semen stored chilled for up to 48
hours, but for most practical purposes semen originates frozen from outside the farm.
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.
Embryo Transfer
The ET process can be broken down
into the basic steps of 1) estrus synchronization of the donor and recipient, 2)
superovulation of the donor, 3) fertilization of the donor, 4) recovery of the embryos and
5) the actual transfer of embryos to recipients. Success in all of the above steps is
vital to achieve implantation and carriage to term of the transferred embryo. The ability
to culture embryos following collection has allowed us to transfer the fertilization from
inside the donor to the culture dish and also to further manipulate the embryo (embryo
splitting and gene transfer).
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.
Conclusions
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.
| Table 1: Reproductive Characteristics of Does and Bucks |
| Criteria |
Mean |
Range |
Doe |
Cycle length (d)
follicular phase (d)
luteal phase (d) |
20
4
17 |
17-24
-
- |
| Duration of estrus (hrs) |
30 |
16-50 |
| Ovulation after estrus (hrs) |
33 |
30-36 |
| Gestation length (d) |
150 |
144-155 |
| Litter size |
- |
1-4 |
Buck |
| Daily testicular sperm production (billion) |
6.0 |
4.8-7.2 |
| Ejaculate volume (ml) |
1.0 |
0.5-1.5 |
| Ejaculate concentration (billion/ml) |
3.0 |
1.5-5.0 |
| Table 2. Comparisons of techniques available for pregnancy diagnosis in the doe |
| Technique |
Sensitivity Range (Days) |
Fetal
Numbersa |
Accuracy (%) |
Practical
Application |
| Sterile harnessed male |
> 20 |
no |
65 - 90 |
high |
| Abdominal palpation |
60 - 115 |
no |
60 - 90 |
moderate |
| Progesterone assay |
18 - 22 |
no |
90 - 95 |
moderate |
| Estrone assay |
> 60 |
no |
90 - 95 |
low |
| Real-time ultrasound |
40 - 100 |
yes |
90 - 95 |
high |
| A/B-mode ultrasound |
60 - 120 |
no |
85 - 95 |
high |
| Doppler ultrasound |
60 - 90 |
no |
85 - 90 |
moderate |
| Radiography |
>50 |
yes |
90 - 95 |
low |
a techniques allowing the determination of litter size with high
degree of accuracy (>95%)
| Table 3. Advantages Associated With The Synchronization of Estrus in Does |
|
| - Facilitates the use of artificial insemination |
| - Prepares for the use of embryo transplantation |
| - Assists in the induction for out-of-season breeding |
| - Concentrates time of breeding and parturition for closer
management |
| - Allows for optimal nutritional management of dam and
offspring |
|
|