Reproduction traits in the Boer goat doe
J.P.C. Greyling
*Department of Animal Science, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa
Abstract
The aim of this review is to give insight into the reproduction potential of the Boer goat doe. Assessment of the reproduction traits in the Boer goat doe demonstrates a mean body weight at puberty of between 30.6 and 27.5 kg, depending on the dietary energy level. Kids weaned during the natural breeding season (April/May) exhibit oestrus or puberty earlier than those weaned outside the natural breeding season. The mean age at the onset of puberty in the Boer goat doe is 191.1 and 157.2 days for kids born in August (late winter) and January (mid-summer). Although periods of complete anoestrus was not observed, the peak of sexual activity occurred during autumn and the period of lowest sexual activity from late spring to mid-summer. The duration of the oestrous cycle was recorded as being 20.70.7 days, with the mean duration of the oestrous period being 37.48.6 h and the position of the LH peak (indicative of ovulation) being 8.01.5 h following the onset of
oestrus. The time of ovulation was recorded as occurring 36.8 h after the onset of oestrus, with a mean ovulation rate of 1.720.9 ovulations per doe. The mean gestation period is quoted as being 148.23.7 days, with multiple births having no
signi®cant effect on gestation length. Involution of the Boer goat uterus is macroscopically complete by day 28 post-partum with the duration of the post-partum anoestrous period in the Boer goat being 55.524.9 days. The mean interval from partus to conception recorded, was 62.020.2 days. To optimise the reproductive ef®ciency in the Boer goat doe, it is essential that its reproductive potential be known and exploited.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Boer goat doe; Puberty; Cyclic activity; Gestation; Post-partum period; Reproduction
1. Introduction
The level of reproductive performance is dependant on the interaction of genetic and environmental fac-tors, but this performance is particularly susceptible to the latter, for example, the seasonal availability of nutrients can affect reproduction considerably (Riera, 1982). Although Indigenous goat breeds have an excellent ability to accommodate and adapt to ¯uctua-tion in environment, this often involves some degree of reproductive failure (Devendra and Burns, 1983).
Reproduction ef®ciency in female goats is deter-mined by many different processes. These processes include, for example, the length of the breeding season, cyclic activity, ovulation rate, fertilisation rate, the post-partum anoestrous period and the growth and viability of the offspring. Reproductive ef®ciency as such can be measured and expressed as the kidding rate, weaning rate, kidding interval, liveweight of kids born or weaned and the length of the reproductive cycle (Greyling, 1988). The goat is the most proli®c of all domestic ruminants under tropical and sub-tropical conditions and certain breeds are able to breed throughout the year (Hofmeyr et al., 1965; Devendra and Burns, 1983), while other breeds like, for exam-*Tel.:27-51-401-2210; fax:27-51-448-0692.
E-mail address: hester@landbou.uovs.ac.za (J.P.C. Greyling)
ple, the Angora have a restricted breeding season (Shelton, 1978; Van der Westhuysen, 1980). One of the most favourable attributes of the Boer goat as a meat producing animal, is its high rate of reproduction and the fact that it has an extended breeding, espe-cially as reproduction is a major contributing factor to the ef®ciency of meat production (Naude and Hof-meyr, 1981). The Boer goat is also an above-average milk producer, which not only holds promise for the raising of their offspring, but also possibly in provid-ing milk for human consumption (Raats et al., 1983; Raats, 1988). However, in order to determine the reproductive and productive potential of the Boer goat, a thorough knowledge of the reproduction physiology is essential. This review thus concentrates on repro-ductive aspects relevant to the Boer goat.
2. Puberty
Puberty can be de®ned in several ways. For the purposes of the present text, it is considered as the age of the female at which oestrus is ®rst detected and is followed by characteristic cyclic ovarian activity in the non-pregnant animal. In the male, puberty is expressed as the stage when spermatogenesis is initiated (Louw and Joubert, 1964). The direct cause of sexual maturation at puberty is given as a rise in the output of the pituitary hormones, leading to an increase in size and activity of the gonads. Underlying this process is some form of maturation of the hypothalamo-pituitary axis, permitting changes in neurosecretion and thereby secretion of the gonado-trophic hormones (Hunter, 1980; Cupps, 1991).
In sheep the ability of the ewe lamb to respond to the oestradiol positive feedback becomes established within a few weeks following birth (Foster and Karsch, 1975). At the onset of puberty, there is a rise in plasma LH due to an increase in the rate of pulsatile LH discharges, which result in one or more of the follicles developing towards the pre-ovulatory stage and in a steady increase in oestradiol production Ð which eventually activates the LH surge mechanism. This is seen as a change in frequency, rather than amplitude of pulsatile LH releases (Foster and Ryan, 1979). In the young, bodyweight is of great signi®-cance in the attainment of puberty, as the occurrence of puberty is dependant on the animal attaining a certain critical bodyweight (Gordon, 1975). Generally
breeding in goats should be delayed until the animal has attained 60±75% of its mature bodyweight (Smith, 1980).
There are several factors that could play a role in the attainment of puberty in the female goat. These factors include season of weaning, nutritional status and the male-effect. The mean body weight at puberty of Boer goat does has been set at 30.67.2 kg for animals on a high energy diet (9.613 MJ/kg DM) and 27.54.3 kg on a lower energy diet (7.688 MJ/kg DM) (Greyling, 1988). These body weights compare well to those for Saanen goats and are heavier than that reported for Creole goats (24 kg) in Venezuela and 25±30 kg in northern Mexico (Delgadillo and Malpaux, 1996). Boer goat kids weaned during the normal breeding season (April/May) exhibit oestrus signi®cantly (P<0.05) earlier than those weaned in December (out-side the natural season). This phenomenon could not be ascribed to a higher mean bodyweight within respective kidding seasons, the level of nutrition and/or the ram effect. It would seem as if season of birth is the main cue for the onset of puberty (Greyling and Van Niekerk, 1990). In northern Mexico, Creole does born in January demonstrate their ®rst oestrus at an age of 250 days and on average at an age of 172 days for females born in August or December. Accord-ing to Delgadillo and Malpaux (1996), age at puberty is very dependent upon the season of birth. The mean age at the onset of puberty in Boer goat does recorded, is 191.1 and 157.2 days for does born during August (late winter) and January (mid-summer), respectively. Kids weaned during the normal breeding season (April/May) thus exhibit oestrus signi®cantly (P<0.05) earlier (157.2 days) (Greyling, 1988). This age at puberty is somewhat earlier than in the Saanen (217.9 days), the Angora (240 days), Black Bengal (196.5 days) and Barbari nannies (213 days) (Riera, 1982; Amoah and Bryant, 1984; Bhattacharyya et al., 1984). It is evident that the permanent presence of the male had a marked (P<0.05) effect on does exhibiting oestrus (or the attainment of puberty) (Greyling and Van Niekerk, 1990). It is dif®cult to isolate a single factor involved in the attainment of puberty, as a sequence of events is involved in the process of sexual maturation and an interaction possibly exists between male effect, seasonality and nutrition.
releases. From serum LH levels, it is evident that the pituitary in the Boer goat is active from 13 weeks (weaning) of age, irrespective of season, ram effect or nutrition. The fact that animals weaned during the natural breeding season maintained a signi®cant (P<0.05) higher mean serum LH level, compared to those weaned outside the normal breeding season, indicates greater pituitary activity during the breeding season (Greyling, 1988). From the serum progesterone values obtained for Boer goat kids weaned in Decem-ber (outside the normal breeding season), it would seem that cyclic activity started in high and low energy diet groups at 7.63.45 and 9.14.3 weeks, respec-tively, following weaning (3 months of age). For animals weaned in April (breeding season) activity was observed at 7.63.5 and 12.76.8 weeks for the high and low diet groups following weaning, respec-tively (Greyling and Van Niekerk, 1990). The serum progesterone values obtained for Boer goat female kids are in agreement with other researchers who report basal concentrations of serum progesterone of 0.3 ng/ml in prepubertal sheep (Bernardinelli et al., 1980) and values of between 0.2 and 0.53 ng/ml in prepubertal goats (Amoah and Bryant, 1984; Bhatta-charyya et al., 1984).
3. Seasonal cyclic activity
Seasonal ¯uctuations in daylight length and tem-perature are important factors affecting the length of the breeding season (Chemineau, 1983). In goats, breeding season usually commences as the days become shorter. There is evidence to indicate that the pineal gland, through its secretion of melatonin is involved in mediating the effects of photoperiod on gonadal function (Hafez, 1974). One characteristic often recorded and which is a good index of seasonal reproductive activity, is the interval between two consecutive kiddings. A highly signi®cant and posi-tive correlation has been found between kidding rate and the latitude of the area where the goats are raised (Delgadillo and Malpaux, 1996). Reproductive ef®-ciency in the female is thus greatly determined by this seasonality (length of the breeding season).
In Boer goat does, the sexual activity for all the months of the year were signi®cantly (P<0.01) lower than that observed in April and May (autumn), with
these two months not being signi®cantly different from one another. Although periods of complete anoestrus within the Boer goat herd was never observed (Fig. 1), it would appear that the Boer goat doe is seasonally poli-oestrus, with an extended breed-ing season. The peak of sexual activity occurred during April and May (autumn) and the period of lowest sexual activity occurred from October through to January (late spring to mid-summer). The mean number of oestrous periods per doe per annum recorded for the Boer goat doe (11.74.4) (Greyling and Van Niekerk, 1987) is similar to that obtained by Hofmeyr et al. (1965) of 11.32.5, but, as could be expected, much greater than in the seasonal Angora doe (4.72.5) (Marincowitz, 1962). Bodyweight (mean 67.2 kg) of the Boer goat doe could not be correlated to the annual sexual activity observed, although a signi®cant (P<0.05) negative correlation (rÿ0.654) was recorded between sexual activity and daylight length. One of the factors contributing to the months of low sexual activity was the high frequency (approximately 30%) of long oestrous cycles (40±60 days) observed in the Boer goat doe (Greyling and Van Niekerk, 1987). Natural breeding programmes with Boer goat does must be executed taking into account the peak natural sexual activity (autumn) of the ani-mals for optimal reproductive performance.
4. The oestrous cycle and oestrous period
The duration of the oestrous cycle in the mature Boer goat doe is 20.70.7 days with a high incidence of short (<13 days) and long (>25 days) cycles. The frequency of short and long cycles in the Boer goat doe has been recorded as being 16.6 and 10.2%, respec-tively (Greyling, 1988). Prasad and Bhattacharyya (1979) categorised the oestrous cycle of the Barbari goat breed into short, medium and long cycles, with the frequency of each category being 19.7, 68.8 and 11.5%, respectively. This variation in oestrous cycle length could be related to the season of the year and stage post-partum.
respectively) (Greyling, 1988). Oestrous cycles are found to be signi®cantly shorter during periods of the year with moderate climatic conditions, compared to extreme cold-dry and hot-wet periods (Prasad and Bhattacharyya, 1979). Marais (1968) found the oes-trous period of the Angora doe to be shorter at the onset and end of the breeding season, compared to the months of peak sexual activity (autumn).
The duration of the oestrous period of the Boer goat appears to be variable in length, but in line with the common duration of oestrus reported in goats of 36 h, with a variation of between 22 and 60 h (Riera, 1982). Serum progesterone is very low on the day of oestrus (mean 0.35 ng/ml) and increases to maximum levels (mean 5 ng/ml) on approximately Day 13 of the cycle. The position of the LH peak (indicative of ovulation) varied in the Boer goat doe, being 8.01.5 h following the onset of oestrus.
The time of ovulation in the goat is reported as occurring towards the end of the oestrous period (Van der Westhuysen et al., 1985). In the Boer goat doe (mean bodyweight of 44.514.5 kg), the time of ovulation is recorded as occurring 36.8 h (86.7% of does ovulating by 38 h after the onset of oestrus) after the onset of oestrus, with the mean time interval between the LH peak and ovulation being 24.7 h. The position of the LH peak relative to the onset of oestrus was recorded as being between 4 and 20 h after the onset of oestrus. Serum LH concentration during the oestrous period ranged between 3.8 and 53.2 ng/ml for individual serum peak values.
ovulation rates quoted for local Malawian goats (1.68) by Zerfas et al. (1993) or the 1.39 for BoerSmall East African does (Reynolds, 1985) and that quoted by Hobson et al. (1986) and Lawrenz (1986) for Angora does (1.15±1.58). The ovulation rate recorded in the Boer goat is however substantially lower than the 4.0 quoted by Rao and Bhattacharrya (1980) in Black Bengal does. The percentage singletons, twins, triplets and quadruplets born in the Boer goat are quoted as being 24.5, 59.2, 15.3 and 1%, respectively (Campbell, 1994). Nevertheless, Boer goat does can be considered as being one of the more proli®c goat breeds in the world. No signi®cant correlation (r0.49) was recorded between the bodyweight of the females and the ovulation rate (Greyling and Van Niekerk, 1990).
5. The gestation period and uterine involution
The mean gestation period for the Boer goat is recorded as being 148.23.7 days, with the duration being 149.13.6, 147.83.6 and 146.84.7 days for does bearing singletons, twins and triplets, respec-tively. There was no signi®cant difference in the gestation length between does bearing singletons or triplets and the season of mating had no signi®cant effect on the gestation length (Greyling, 1988). The in¯uence of nutrition on foetal development during certain months of pregnancy does tend to shorten or lengthen the gestation period, but the variation due to this factor was only 1.5 days (Riera, 1982). During gestation, there is little evidence of any dramatic increase in serum progesterone concentration above levels attained during the luteal phase of the oestrous cycle, suggesting the CL as being the main source of progesterone. A positive relationship between multi-ple pregnancies and maternal serum progesterone levels was not observed. Similarly the serum LH levels in the Boer goat during pregnancy are not static, but ¯uctuate apparent at random, at relatively low levels, ranging between 0.850.13 and 5.721.15 ng/ml (Greyling, 1988).
Morphological changes or their delay in the post-partum uterus and ovaries of farm animals exert limitations on the reproductive performance of females following parturition. After distention and distortion of uterine tissues during pregnancy and the heightened glandular development for support of the conceptus, the uterus must undergo contraction
and loss of weight, together with extensive regenera-tion of its epithelial layers during the process of uterine involution (Hunter, 1980). The interval from parturition to a subsequent pregnancy is a factor of major economic importance and hence the involution of the post-partum uterus must be seen as one of the important limitations in achieving the goal of optimal reproduction ef®ciency.
In the Boer goat, macroscopic changes of the post-partum uterus in weight and volume rapidly decline from parturition to approximately Day 12 post-par-tum. This is demonstrated by the fact that by Day 12 post-partum, the uterus weight is 15% of its weight at parturition. By Day 20, it is 8% of that at parturition and only 27% more than the uterus weight of maiden Boer goats. Decrease in uterine horn length and diameter was less pronounced and the external dia-meters were back to normal by Day 28 post-partum. Regarding the decrease in caruncle diameter, the day of no change, according to the model used, in Boer goats was 26.3 days. According to these observations, it would seem that the involution process of the Boer goat uterus is macroscopically complete by approxi-mately 28 days post-partum (Greyling and Van Nie-kerk, 1991a). These observations are veri®ed by histological changes recorded in the post-partum uterus of the Boer goat. The ®rst new CL's were observed 32 days following parturition in the Boer goat and were micro-morphologically approximately 15 days old Ð implicating thus that these animals had ovulated about 17 days post-partum. These CL's were much smaller and had a smaller mass than the CL viteum (Greyling and Van Niekerk, 1991b).
6. The post-partum anoestrous period
to the ®rst oestrous period post-partum tended to de-crease when the bodyweight of the doe was high (above 47.25 kg) and when the goats were older than 3 years (Torres-Acosta et al., 1996). The post-partum anoestrous periods of does kidding in May (late autumn) were signi®cantly (P<0.01) shorter than those Boer goats which kidded in October (early summer) (37.312.5 versus 59.918.0 days, respectively) (Greyling, 1988). This is in agreement with Restall and Starr (1977) who suggested that the season of kidding could have an effect on the post-partum anoestrous period.
According to Riera (1982), this interval varies among goat breeds, lactational status and nutrition. A factor that also plays an important role in the determination of this post-partum period to ®rst oes-trus is the suckling stimulus (Rigor et al., 1984). An interesting observation made, was the fact that in a group of Boer goat does which kidded in October (early summer), 40% actually reconceived within 99 days post-partum. The mean interval from partus to conception recorded, was 62.020.2 days. There was no signi®cant difference in the post-partum anoestrous interval for does giving birth to different numbers of offspring. Regarding the reproductive endocrine status of the does during the post-partum period, the serum progesterone levels of two kidding seasons (early summer versus late autumn) did not differ signi®-cantly for the entire sampling periods. There was a tendency for the mean serum progesterone concentra-tion in the early summer kidding group to be higher from 76 days (just prior to the natural breeding season) post-partum Ð suggesting higher ovarian activity during this period (Greyling, 1988). Resumption of cyclic activity in the goat is very susceptible to external factors, such as season, suckling and a factor that also has a role to play, is the presence of the male.
7. Lamb mortalities
It is common experience that multiple births in goats are associated with a high mortality rate (Deven-dra and Burns, 1983). There seems to be however, no biological reason why mortality should be high, pro-vided nutrition and management is adequate. It should just be pointed out that the full meat production potential of the Boer goat could only be utilised by exploiting their proli®cacy. To this end, intensive
management and high nutritional levels might be economically worthwhile. Under intensive conditions, Boer goat does successfully raise twins and triplets. It is, however, necessary to pay special attention to triplets during the ®rst few days after birth (Naude and Hofmeyr, 1981).
8. Conclusions
The productivity of any breeding female is deter-mined by the number of progeny delivered in a given period of time. The interval from parturition to a sub-sequent pregnancy is a factor of major economic importance and any factor that could place limitations on this period could hamper the goal of optimal repro-duction ef®ciency (Greyling and Van Niekerk, 1991b). Considered in the light of the health-consciousness and rural nutritional needs that prevail, the Boer goat yields lean meat of a high quality, particularly at a young age, while also having an abundence of milk. Boer goats should be marketed between the ages of 6 and 15 months and carcasses should weigh no more than 23 kg to produce an acceptable product. With characteristics of being hardy, adaptable, resistent to diseases, highly fertile, proli®c, excellent growth rates and having a longevity of approximately 10 years, this makes the breed the sought-after goat in the world (Ueckermann, 1969; Anonymous, 1996).
In order to optimise the reproductive potential of the Boer goat, it is essential that a reproductive manage-ment programme be implemanage-mented that takes into account all the reproductive physiology aspects. With the bene®cial reproductive characteristics of the Boer goat, it can be seen as the source of animal protein to alleviate the need in the developing countries and as a means of helping in the social upliftment of the rural poor communities. The Boer goat however with its ex-tended breeding season, also has great potential in the commercial industry and can be utilised even in more intensive production systems for meat production. It is this wide-spread application potential and the ability of the Boer goat that makes it popular world-wide.
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