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Genetic and phenotypic relationships between gestation length, litter size and litter birth weight in Yankasa sheep
Animal Reproduction Science, 34 ( 1993 ) 111-118
111
0378-4320/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Genetic and phenotypic relationships between gestation length, litter size and litter birth weight in Yankasa sheep O.A. Osinowo*, B.Y. Abubakar, A.R. Trimnell National Animal Production Research Institute, Ahmadu Bello University, Zaria, Nigeria (Accepted 29 January 1993)
Abstract
The genetic and phenotypic relationships between gestation length, litter size and litter birth weight m Yankasa sheep were investigated using records of 834 lambings collected over an 8 year period (1983-1990). Litter size, year of birth and litter birth weight had significant influences on gestation length, while the effects of parity, season of birth and sex of lamb were not significant (P> 0.05 ). Single lambs had a gestation length longer by 1 day than twin lambs (151.9 vs. 150.9 days). The relationship between litter birth weight and gestation length was curvilinear with partial regression coefficients of 0.67 and - 0 . 2 4 for the linear and quadratic components respectively'. Mean gestation length for the breed was 151.4 days. Heritability estimates by paternal half-sib analyses, taking geslalion as a trait of the foetus, were 0.20, 0.11 and 0.05 for gestation length, litter size and litter birth weight, respectively. Genetic and phenotypic correlations between gestation length and litter size, gestation length and litter birth weight, and litter size and litter birth weight were - 0 . 2 9 and - 0.15, 0.93 and -0.04, and 0.25 and 0.74 respectively. The negative genetic correlation between gestation length and litter size is an indication that selection for large litter size would be accompanied b~ a reduction in gestation length.
Introduction
Gestation length is the interval from fertile service to parturition and ranges from 140 to 159 days in sheep (Hafez and Jainudeen, 1975 ). It is principally determined by genetic factors as evidenced by characteristic gestation lengths for each mammalian species and, to a lesser extent, breed. For example, gestation lengths of 147 and 146 days obtained for Canadian Arcott and Suffolk ewes respectively, both meat-type sire breeds, were longer than the 145 days for Outaouais and Rideau Arcott ewes and 144 days for Finsheep ewes, all fecund-type dam breeds (Shrestha and Heaney, 1990). The limited but definite variability in gestation length is accounted for by maternal, foetal and environmental factors (Holm, 1967; Terrill, 1975; Trimnell et al., 1988). *Corresponding author.
112
O,A. Osinowo et al. / Animal Reproduction Science 34 (1993) 11 l - 118
Maternal factors include parity or age and endocrine activities of the dam associated with gestation and parturition. Gestation length tends to increase with age or parity. In the case of foetal factors, gestation length is shorter for twin lambs than for single lambs while it is sometimes longer for ram than for ewe lambs (Smith, 1967; Swamy, 1978). Lambs of heavier birth weight tend to have longer gestation (Mullaney and Lear, 1969 ). The endocrine activities of the foetus are also important as foetal corticosteroids cause a fall in progesterone, a rise in oestrogen and release of prostaglandin F2~, leading to uterine contractility and foetal expulsion. The foetus thus exerts significant influence on gestation length as does the dam, and gestation length can be considered either as a trait of the foetus or of the dam. Environmental factors reported to exert significant effects on gestation length include season, year and plane of nutrition (Kishore et a l . , 1980; Nivsarkar et al., 1981 ). Underfeeding tends to reduce gestation length, especially in late pregnancy. There appears to be no report on gestation length as a trait of the foetus while no estimates of genetic correlations between gestation length, litter size and litter birth weight have been reported for any tropical African sheep breed. The present study was conducted to determine the effects of some genetic, environmental, foetal and maternal factors on gestation length in Yankasa sheep. Materials and methods
General Data for this study were obtained from the Yankasa Sheep Breeding Project, National Animal Production Research Institute, Shika, Nigeria. The station is located on latitude 11 ° 12' N, longitude 7 ° 33' E and an altitude of 610 m, in the subhumid zone of the country. The mean annual temperature is 24.4 ° C while the mean annual rainfall of 1107 m m is seasonally distributed as follows: 0.1% in the late dry season (January-March), 25.8% in the early wet season (April-June), 69.6% in the late wet season (July-September), 4.5% in the early dry season (October-December). Pasture quality is good in the late wet and early dry seasons but poor in the other seasons. The management of the flock was semi-intensive as previously described (Osinowo and Ekpe, 1985 ). The sheep were housed overnight and grazed for 6-8 h daily on improved pastures. Concentrate supplement ( 15% CP) was fed daily at the rate of 300-500 g per head per day. Measurements Advantage was taken of the absence of a breeding season for sheep in this latitude to routinely breed the ewes to lamb approximately every 6.5-7
0./1. Osinowo et al. /Animal Reproduction Science 34 (1993) 111-118
1 13
months. During the breeding periods, raddled rams were either joined to ewes in the paddock with daily checks of the ewes for crayon markings to record mating dates, or the ewes were checked for oestrus using vasectomized or aproned rams and then assigned for artificial insemination or natural mating. Gestation length is defined as the interval from mating to parturition. Litter size is the total number of lambs born (live or dead) at each parturition. Litter birth weight is the total litter weight taken within 4-14 h after birth. The 834 lambings on which this study was based were produced by 362 dams and 108 sires.
Statistical analysis Least-squares procedures were used to analyse environmental effects on gestation length. Fixed effects of year of birth (1983-1990), parity (1-6), litter size (single or twin), sex of lamb (male, female or male and female in a twin set), lambing season (late dry, early wet, late wet or early dry) and residual error were included in the general statistical model. Litter birth weight was also included in the model as a covariate. The effects of parity, season and sex were not significant ( P > 0.05 ) and the three factors were therefore excluded from the final model. The effect of litter birth weight was partitioned into linear and quadratic components. Gestation length was investigated as a trait of the foetus after adjustment for year of birth effect. Sires and dams with less than two records each were excluded from the genetic analyses. This reduced the number of records to 667 with 78 sires and 215 dams. Genetic parameters for gestation length, litter size and litter birth weight were estimated by both paternal and maternal half-sib analyses using the mixed model least-squares and maximum likelihood computer program (Harvey, 1990). Heritability estimates ( _+SE), genetic, phenotypic and environmental correlations for the three traits were obtained from the variance and covariance components. Results
The gestation length frequency distribution for Yankasa sheep in this study is shown in Fig. 1. The modal gestation length was 151 days. Least-squares analysis showed that the effects of parity, season of birth and sex of lamb were not significant ( P > 0.05), whereas litter size, year of birth and litter birth weight had significant influences on gestation length (Table 1 ). Single lambs had a longer gestation than twin lambs by 1 day ( 151.9 vs. 150.9 days). Yearly variations in gestation length did not follow a consistent pattern. The relationship between litter birth weight and gestation length was curvilinear with partial regression coefficients of 0.67 and - 0 . 2 4 for the linear and quadratic
114
O.A. Osinowo et al. / Animal Reproduction Science 34 (1993) 111-118 NUMBER OF CASES
200
14:3 144 145 146 147 148 149 150 151 152 153 ' 5 4 GESTATION LENGTH (DAYS)
155 156 157 158
Fig. 1. Frequency distribution o f gestation length in Yankasa sheep.
Table 1 Effects of litter size, litter birth weight and year of birth on gestation length in Yankasa sheep Variable
Subclass
N
Least-squares (mean_+ SE)
Overall
834
151.4 + 0.2
Litter size Single Twin
662 172
151.9 _+0.2 a 150.9+0.5 b
Year o f birth 1983 1984 1985 1986 1987 1988 1989 1990
63 115 151 84 84 230 72 35
152.7+0.5 a 152.0+0.4 a 151.0+0.3 b 150.0 + 0.4 c 151.8+0.4 a 150.7 + 0.3 b 151.3_+0.5 b 151.6 + 0.7 a'b
Regression Litter birth weight Linear Quadratic
0.67 + 0.20** - 0.24 _+O. 1O*
"'b'CMeans within each variable not followed by the same superscripts differ significantly. *P< 0.05; **P< 0.001.
components respectively. Least-squares mean gestation length for all 834 lambings in this study was 151.4 _+0.2 days. The genetic analysis was carried out after elimination of sires and dams with less than two records which reduced the number oflambings to 667 with 78 sires and 215 dams. Least-squares means and coefficients of variation for
0.,4. Osinowo et al. / Animal Reproduction Science 34 (I993) 111-118
1I5
Table 2 Estimates of heritability (h 2), genetic (r 8), phenotypic (rp) and environmental (re) correlations between gestation length, litter size and litter birth weight in Yankasa sheep Traits
Variance or covariance components Among
h 2 _ SE
rg _+SE
rp
r~
Within
0.11_+0.10 0.0046 a 0.1667 (0.0279) b (0.1435) (0.65_+0.17) Litter birth weight 0.0130 0.9860 0.05 _+0.09 (0.1914) (0.8082) (0.77_+0.17) 0.20_+0.11 Gestation length 0.6152 11.7884 (0.7990) (11.5972) (0.26_+0.16) Litter size and litter 0.0019 0.3039 birth weight (0.0591 ) (0.2470) Litter size and -0.0154 -0.2095 gestation length ( - 0.0660) ( - 0.1588 ) Litter birth weight and 0.0831 - 0.2366 gestationlength (0.0540) ( - 0 . 2 0 8 7 )
Litter size
0.25+0.80 (0.81 +0.07) -0.29_+0.53 (-0.44_+0.36) 0.93_+0.98 (0.14+0.28)
0.74 -0.15 -0.04
0.78 (0.60) -0.113 (0.05) -0.16 (-0.25)
aEstimates from paternal half-sibs. bEstimates from maternal half-sibs in parentheses.
gestation length, litter size and litter birth weight for this data set were 151.7+_0.1 days and 22%, 1.22+_0.02 and 31% and 3.08+_0.05 kg and 29% respectively. Estimates of heritability, genetic, phenotypic and environmental correlations by paternal and maternal half-sib analyses for gestation length, litter size and litter birth weight are presented in Table 2. Heritability estimates by paternal half-sib analyses were 0.11, 0.05 and 0.20 for litter size, litter birth weight and gestation length respectively. Genetic and phenotypic correlations between gestation length and litter size, gestation length and litter birth weight, and litter size and litter birth weight were - 0 . 2 9 and - 0 . 1 5 , 0.93 and - 0 . 0 4 , and 0.25 and 0.74 respectively. Discussion The physiology of parturition in sheep clearly reveals the significant role of the foetus in its initiation. The general view that parturition in the species is initiated by a rise in foetal cortisol triggering a series of physiological events culminating in increased myometrial activity and foetal expulsion (Liggins et al., 1973; Flint and Ricketts, 1979 ) confirms that gestation length can be considered as a trait of the foetus as well as of the dam. This would mean that though the trait is measured on the dam, it is to a significant extent determined by the foetal genotype. The mean gestation length of 151.4 days obtained in this study agrees closely
116
O.A. Osinowo et aL / Animal Reproduction Science 34 (1993) 11 I - 118
with an earlier estimate derived from a smaller data set from the same flock (Trimnell et al., 1988 ) and falls within the range of 140-159 days expected for sheep (Hafez and Jainudeen, 1975 ). The insignificant influences of parity, season of birth and sex on gestation length in the present study contrasts with the significant effects of litter size, litter birth weight and year of birth. Although some reports seem to indicate an increase in gestation length with age or parity (Terrill, 1975 ) and a longer gestation for ram lambs than for ewe lambs (Smith, 1967; Swamy, 1978 ), these could not be confirmed in this study. Shrestha and Heaney (1990) reported that sex of lamb, age and body weight of the ewe had no significant influence on gestation length. Preliminary analysis in the present study showed a significant seasonal effect on gestation length which however disappeared with the inclusion of litter birth weight in the statistical model as a covariable. This is an indication that seasonal effect on gestation length was mediated through its influence on birth weight under the semi-intensive production system, where seasonality in forage availability and quality is pronounced. Most reports have indicated significant year of birth effect on gestation length, in agreement with the present finding. The significant influences of litter size and litter birth weight on gestation length agrees with previous reports (Smith, 1967; Mullaney and Lear, 1969; Terrill, 1975 ). Shrestha and Heaney (1990) reported a progressive reduction of gestation length within breed as prolificacy increased from ewes bearing single to quintuplet lambs. The latter authors also reported that gestation length increased significantly with the litter birth weight of lambs in agreement with the present report. However, the relationship between gestation length and litter birth weight was shown in the present study to have both significant linear and quadratic components, indicating a curvilinear relationship. Paternal half-sib estimates of heritability for gestation length and litter size of 0.20 and 0.11 respectively compare favourably with estimates of 0.18 reported for both gestation length and prolificacy by Shrestha and Heaney (1990). The latter authors obtained a lower heritability estimate of 0.04 for multiple birth. The distinction between prolificacy and multiple birth was possible in their study where litter size ranged from one to five. In the present study, litter size in Yankasa sheep was either one or two, hence the distinction between prolificacy and multiple birth was not possible. In the present study also, gestation length was considered as a trait of the foetus, with the genetic analyses involving the sires and dams of the lambs rather than those of their dams. The heritability estimate of 0.05 obtained for litter birth weight was the lowest among the three traits. The heritability estimates from the maternal half-sib analyses were 0.11, 0.77 and 0.26 for litter size, litter birth weight and gestation length respectively, indicating the magnitude of the non-genetic effects in the dam vari-
O.A. Osinowo et al. / A nimal Reproduction Science 34 (1993) 11 I - 118
117
ance components compared with the paternal half-sib estimates. The genetic correlation between gestation length and litter size was - 0 . 2 9 , indicating as expected that selection for increased litter size would be associated with a decrease in gestation length. The phenotypic and environmental correlations between gestation length and litter size were also negative but smaller ( - 0. ! 5 and - 0 . 1 3 respectively). The genetic, phenotypic and environmental correlations between litter size and litter birth weight were 0.25, 0.74 and 0.78 respectively, while the corresponding correlations between gestation length and litter birth weight were 0.93, - 0.04 and - 0.16. It may be concluded from this study that litter size, year of birth and litter birth weight exert significant influences on gestation length in Yankasa sheep. Consideration of gestation length as a trait of the foetus yielded reasonable estimates of genetic parameters for the trait, similar to other reports in which it was considered as a trait of the dam. There was a negative genetic correlation between gestation and litter size, indicating that selection for increased prolificacy would result in a reduction in gestation length. Acknowledgements The authors gratefully acknowledge the technical contributions of the field staff of the Yankasa Sheep Breeding Project and I.A. Adeyinka. The secretarial assistance of P.N. Ogala and Isa B. Aliyu is also acknowledged with thanks.
References Flint, A.P.F. and Ricketts, A.P., 1979. Control of placental endocrine function: a role of enzyme activation in the onset of labour. J. Steroid Biochem., 11: 493-500. Hafez, E.S.E. and Jainudeen, M.R., 1975. Gestation, prenatal physiology and parturition. In: E.S.E. Hafez (Editor), Reproduction in Farm Animals, 3rd. edn. Lea & Febiger, Philadelphia, pp. 166-202. Harvey, W.R., 1990. Mixed model least-squares and maximum likelihood computer program PC-2. Ohio State University. Holm, L.W., 1967. Prolonged pregnancy. Adv. Vet. Sci., 11:159-205. Kishore, K., Gour, D., Rawat, P.S. and Arora, C.L., 1980. Note on gestation length in crossbred sheep. Indian J. Anim. Sci., 50: 565-567. Liggins, G.C., Fairclough, R.J., Grieves, S.A., Kendall, J.Z. and Knox, B.S., 1973. The mechanism of initiation of parturition in the ewe. Rec. Prog. Horm. Res., 29:1 l 1-159. Mullaney, P.D. and Lear, D., 1969. Duration of pregnancy in Merino ewes in relation to survival of lambs. Aust. Vet. J., 45: 366-367. Nivsarkar, A.E., Singh, R.N., Bohra, S.D.J., Kuma, M. and Bapna, D.L., 1981. Note on variation in the gestation length of Malpura and Sonadi sheep and their crossbreds. Indian J. Anim. Sci., 51: 1176-1177. Osinowo, O.A. and Ekpe, G.A., 1985. Post-partum intervals to oestrus and conception in Yankasa sheep. J. Agric. Sci. Camb., 104: 253-255. Shrestha, J.N.B. and Heaney, D.P., 1990. Genetic basis of variation in reproductive perform-
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ance. 2. Genetic correlation between gestation length and prolificacy in sheep. Anim. Reprod. Sci., 23: 305-317. Smith, I.D., 1967. Breed differences in the duration of gestation in sheep. Aust. Vet. J., 43: 6364. Swamy, M.N., 1978. Factors affecting gestation length in Bannur sheep. Gujvet, 9: 8-11 (Anim. Breeding Abstr., 48: 74. Abstr. No. 640, 1980). Terrill, C.E., 1975. Sheep. In: E.S.E. Hafez (Editor), Reproduction in Farm Animals, 3rd. edn. Lea & Febiger, Philadelphia, pp. 166-202. Trimnell, A.R., Osinowo, O.A., Olorunju, S.A.S. and Buvanendran, V., 1988. Environmental effects on gestation length in Yankasa sheep. J. Anim. Prod. Res., 8: 33-38.
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0378-4320/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Genetic and phenotypic relationships between gestation length, litter size and litter birth weight in Yankasa sheep O.A. Osinowo*, B.Y. Abubakar, A.R. Trimnell National Animal Production Research Institute, Ahmadu Bello University, Zaria, Nigeria (Accepted 29 January 1993)
Abstract
The genetic and phenotypic relationships between gestation length, litter size and litter birth weight m Yankasa sheep were investigated using records of 834 lambings collected over an 8 year period (1983-1990). Litter size, year of birth and litter birth weight had significant influences on gestation length, while the effects of parity, season of birth and sex of lamb were not significant (P> 0.05 ). Single lambs had a gestation length longer by 1 day than twin lambs (151.9 vs. 150.9 days). The relationship between litter birth weight and gestation length was curvilinear with partial regression coefficients of 0.67 and - 0 . 2 4 for the linear and quadratic components respectively'. Mean gestation length for the breed was 151.4 days. Heritability estimates by paternal half-sib analyses, taking geslalion as a trait of the foetus, were 0.20, 0.11 and 0.05 for gestation length, litter size and litter birth weight, respectively. Genetic and phenotypic correlations between gestation length and litter size, gestation length and litter birth weight, and litter size and litter birth weight were - 0 . 2 9 and - 0.15, 0.93 and -0.04, and 0.25 and 0.74 respectively. The negative genetic correlation between gestation length and litter size is an indication that selection for large litter size would be accompanied b~ a reduction in gestation length.
Introduction
Gestation length is the interval from fertile service to parturition and ranges from 140 to 159 days in sheep (Hafez and Jainudeen, 1975 ). It is principally determined by genetic factors as evidenced by characteristic gestation lengths for each mammalian species and, to a lesser extent, breed. For example, gestation lengths of 147 and 146 days obtained for Canadian Arcott and Suffolk ewes respectively, both meat-type sire breeds, were longer than the 145 days for Outaouais and Rideau Arcott ewes and 144 days for Finsheep ewes, all fecund-type dam breeds (Shrestha and Heaney, 1990). The limited but definite variability in gestation length is accounted for by maternal, foetal and environmental factors (Holm, 1967; Terrill, 1975; Trimnell et al., 1988). *Corresponding author.
112
O,A. Osinowo et al. / Animal Reproduction Science 34 (1993) 11 l - 118
Maternal factors include parity or age and endocrine activities of the dam associated with gestation and parturition. Gestation length tends to increase with age or parity. In the case of foetal factors, gestation length is shorter for twin lambs than for single lambs while it is sometimes longer for ram than for ewe lambs (Smith, 1967; Swamy, 1978). Lambs of heavier birth weight tend to have longer gestation (Mullaney and Lear, 1969 ). The endocrine activities of the foetus are also important as foetal corticosteroids cause a fall in progesterone, a rise in oestrogen and release of prostaglandin F2~, leading to uterine contractility and foetal expulsion. The foetus thus exerts significant influence on gestation length as does the dam, and gestation length can be considered either as a trait of the foetus or of the dam. Environmental factors reported to exert significant effects on gestation length include season, year and plane of nutrition (Kishore et a l . , 1980; Nivsarkar et al., 1981 ). Underfeeding tends to reduce gestation length, especially in late pregnancy. There appears to be no report on gestation length as a trait of the foetus while no estimates of genetic correlations between gestation length, litter size and litter birth weight have been reported for any tropical African sheep breed. The present study was conducted to determine the effects of some genetic, environmental, foetal and maternal factors on gestation length in Yankasa sheep. Materials and methods
General Data for this study were obtained from the Yankasa Sheep Breeding Project, National Animal Production Research Institute, Shika, Nigeria. The station is located on latitude 11 ° 12' N, longitude 7 ° 33' E and an altitude of 610 m, in the subhumid zone of the country. The mean annual temperature is 24.4 ° C while the mean annual rainfall of 1107 m m is seasonally distributed as follows: 0.1% in the late dry season (January-March), 25.8% in the early wet season (April-June), 69.6% in the late wet season (July-September), 4.5% in the early dry season (October-December). Pasture quality is good in the late wet and early dry seasons but poor in the other seasons. The management of the flock was semi-intensive as previously described (Osinowo and Ekpe, 1985 ). The sheep were housed overnight and grazed for 6-8 h daily on improved pastures. Concentrate supplement ( 15% CP) was fed daily at the rate of 300-500 g per head per day. Measurements Advantage was taken of the absence of a breeding season for sheep in this latitude to routinely breed the ewes to lamb approximately every 6.5-7
0./1. Osinowo et al. /Animal Reproduction Science 34 (1993) 111-118
1 13
months. During the breeding periods, raddled rams were either joined to ewes in the paddock with daily checks of the ewes for crayon markings to record mating dates, or the ewes were checked for oestrus using vasectomized or aproned rams and then assigned for artificial insemination or natural mating. Gestation length is defined as the interval from mating to parturition. Litter size is the total number of lambs born (live or dead) at each parturition. Litter birth weight is the total litter weight taken within 4-14 h after birth. The 834 lambings on which this study was based were produced by 362 dams and 108 sires.
Statistical analysis Least-squares procedures were used to analyse environmental effects on gestation length. Fixed effects of year of birth (1983-1990), parity (1-6), litter size (single or twin), sex of lamb (male, female or male and female in a twin set), lambing season (late dry, early wet, late wet or early dry) and residual error were included in the general statistical model. Litter birth weight was also included in the model as a covariate. The effects of parity, season and sex were not significant ( P > 0.05 ) and the three factors were therefore excluded from the final model. The effect of litter birth weight was partitioned into linear and quadratic components. Gestation length was investigated as a trait of the foetus after adjustment for year of birth effect. Sires and dams with less than two records each were excluded from the genetic analyses. This reduced the number of records to 667 with 78 sires and 215 dams. Genetic parameters for gestation length, litter size and litter birth weight were estimated by both paternal and maternal half-sib analyses using the mixed model least-squares and maximum likelihood computer program (Harvey, 1990). Heritability estimates ( _+SE), genetic, phenotypic and environmental correlations for the three traits were obtained from the variance and covariance components. Results
The gestation length frequency distribution for Yankasa sheep in this study is shown in Fig. 1. The modal gestation length was 151 days. Least-squares analysis showed that the effects of parity, season of birth and sex of lamb were not significant ( P > 0.05), whereas litter size, year of birth and litter birth weight had significant influences on gestation length (Table 1 ). Single lambs had a longer gestation than twin lambs by 1 day ( 151.9 vs. 150.9 days). Yearly variations in gestation length did not follow a consistent pattern. The relationship between litter birth weight and gestation length was curvilinear with partial regression coefficients of 0.67 and - 0 . 2 4 for the linear and quadratic
114
O.A. Osinowo et al. / Animal Reproduction Science 34 (1993) 111-118 NUMBER OF CASES
200
14:3 144 145 146 147 148 149 150 151 152 153 ' 5 4 GESTATION LENGTH (DAYS)
155 156 157 158
Fig. 1. Frequency distribution o f gestation length in Yankasa sheep.
Table 1 Effects of litter size, litter birth weight and year of birth on gestation length in Yankasa sheep Variable
Subclass
N
Least-squares (mean_+ SE)
Overall
834
151.4 + 0.2
Litter size Single Twin
662 172
151.9 _+0.2 a 150.9+0.5 b
Year o f birth 1983 1984 1985 1986 1987 1988 1989 1990
63 115 151 84 84 230 72 35
152.7+0.5 a 152.0+0.4 a 151.0+0.3 b 150.0 + 0.4 c 151.8+0.4 a 150.7 + 0.3 b 151.3_+0.5 b 151.6 + 0.7 a'b
Regression Litter birth weight Linear Quadratic
0.67 + 0.20** - 0.24 _+O. 1O*
"'b'CMeans within each variable not followed by the same superscripts differ significantly. *P< 0.05; **P< 0.001.
components respectively. Least-squares mean gestation length for all 834 lambings in this study was 151.4 _+0.2 days. The genetic analysis was carried out after elimination of sires and dams with less than two records which reduced the number oflambings to 667 with 78 sires and 215 dams. Least-squares means and coefficients of variation for
0.,4. Osinowo et al. / Animal Reproduction Science 34 (I993) 111-118
1I5
Table 2 Estimates of heritability (h 2), genetic (r 8), phenotypic (rp) and environmental (re) correlations between gestation length, litter size and litter birth weight in Yankasa sheep Traits
Variance or covariance components Among
h 2 _ SE
rg _+SE
rp
r~
Within
0.11_+0.10 0.0046 a 0.1667 (0.0279) b (0.1435) (0.65_+0.17) Litter birth weight 0.0130 0.9860 0.05 _+0.09 (0.1914) (0.8082) (0.77_+0.17) 0.20_+0.11 Gestation length 0.6152 11.7884 (0.7990) (11.5972) (0.26_+0.16) Litter size and litter 0.0019 0.3039 birth weight (0.0591 ) (0.2470) Litter size and -0.0154 -0.2095 gestation length ( - 0.0660) ( - 0.1588 ) Litter birth weight and 0.0831 - 0.2366 gestationlength (0.0540) ( - 0 . 2 0 8 7 )
Litter size
0.25+0.80 (0.81 +0.07) -0.29_+0.53 (-0.44_+0.36) 0.93_+0.98 (0.14+0.28)
0.74 -0.15 -0.04
0.78 (0.60) -0.113 (0.05) -0.16 (-0.25)
aEstimates from paternal half-sibs. bEstimates from maternal half-sibs in parentheses.
gestation length, litter size and litter birth weight for this data set were 151.7+_0.1 days and 22%, 1.22+_0.02 and 31% and 3.08+_0.05 kg and 29% respectively. Estimates of heritability, genetic, phenotypic and environmental correlations by paternal and maternal half-sib analyses for gestation length, litter size and litter birth weight are presented in Table 2. Heritability estimates by paternal half-sib analyses were 0.11, 0.05 and 0.20 for litter size, litter birth weight and gestation length respectively. Genetic and phenotypic correlations between gestation length and litter size, gestation length and litter birth weight, and litter size and litter birth weight were - 0 . 2 9 and - 0 . 1 5 , 0.93 and - 0 . 0 4 , and 0.25 and 0.74 respectively. Discussion The physiology of parturition in sheep clearly reveals the significant role of the foetus in its initiation. The general view that parturition in the species is initiated by a rise in foetal cortisol triggering a series of physiological events culminating in increased myometrial activity and foetal expulsion (Liggins et al., 1973; Flint and Ricketts, 1979 ) confirms that gestation length can be considered as a trait of the foetus as well as of the dam. This would mean that though the trait is measured on the dam, it is to a significant extent determined by the foetal genotype. The mean gestation length of 151.4 days obtained in this study agrees closely
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O.A. Osinowo et aL / Animal Reproduction Science 34 (1993) 11 I - 118
with an earlier estimate derived from a smaller data set from the same flock (Trimnell et al., 1988 ) and falls within the range of 140-159 days expected for sheep (Hafez and Jainudeen, 1975 ). The insignificant influences of parity, season of birth and sex on gestation length in the present study contrasts with the significant effects of litter size, litter birth weight and year of birth. Although some reports seem to indicate an increase in gestation length with age or parity (Terrill, 1975 ) and a longer gestation for ram lambs than for ewe lambs (Smith, 1967; Swamy, 1978 ), these could not be confirmed in this study. Shrestha and Heaney (1990) reported that sex of lamb, age and body weight of the ewe had no significant influence on gestation length. Preliminary analysis in the present study showed a significant seasonal effect on gestation length which however disappeared with the inclusion of litter birth weight in the statistical model as a covariable. This is an indication that seasonal effect on gestation length was mediated through its influence on birth weight under the semi-intensive production system, where seasonality in forage availability and quality is pronounced. Most reports have indicated significant year of birth effect on gestation length, in agreement with the present finding. The significant influences of litter size and litter birth weight on gestation length agrees with previous reports (Smith, 1967; Mullaney and Lear, 1969; Terrill, 1975 ). Shrestha and Heaney (1990) reported a progressive reduction of gestation length within breed as prolificacy increased from ewes bearing single to quintuplet lambs. The latter authors also reported that gestation length increased significantly with the litter birth weight of lambs in agreement with the present report. However, the relationship between gestation length and litter birth weight was shown in the present study to have both significant linear and quadratic components, indicating a curvilinear relationship. Paternal half-sib estimates of heritability for gestation length and litter size of 0.20 and 0.11 respectively compare favourably with estimates of 0.18 reported for both gestation length and prolificacy by Shrestha and Heaney (1990). The latter authors obtained a lower heritability estimate of 0.04 for multiple birth. The distinction between prolificacy and multiple birth was possible in their study where litter size ranged from one to five. In the present study, litter size in Yankasa sheep was either one or two, hence the distinction between prolificacy and multiple birth was not possible. In the present study also, gestation length was considered as a trait of the foetus, with the genetic analyses involving the sires and dams of the lambs rather than those of their dams. The heritability estimate of 0.05 obtained for litter birth weight was the lowest among the three traits. The heritability estimates from the maternal half-sib analyses were 0.11, 0.77 and 0.26 for litter size, litter birth weight and gestation length respectively, indicating the magnitude of the non-genetic effects in the dam vari-
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ance components compared with the paternal half-sib estimates. The genetic correlation between gestation length and litter size was - 0 . 2 9 , indicating as expected that selection for increased litter size would be associated with a decrease in gestation length. The phenotypic and environmental correlations between gestation length and litter size were also negative but smaller ( - 0. ! 5 and - 0 . 1 3 respectively). The genetic, phenotypic and environmental correlations between litter size and litter birth weight were 0.25, 0.74 and 0.78 respectively, while the corresponding correlations between gestation length and litter birth weight were 0.93, - 0.04 and - 0.16. It may be concluded from this study that litter size, year of birth and litter birth weight exert significant influences on gestation length in Yankasa sheep. Consideration of gestation length as a trait of the foetus yielded reasonable estimates of genetic parameters for the trait, similar to other reports in which it was considered as a trait of the dam. There was a negative genetic correlation between gestation and litter size, indicating that selection for increased prolificacy would result in a reduction in gestation length. Acknowledgements The authors gratefully acknowledge the technical contributions of the field staff of the Yankasa Sheep Breeding Project and I.A. Adeyinka. The secretarial assistance of P.N. Ogala and Isa B. Aliyu is also acknowledged with thanks.
References Flint, A.P.F. and Ricketts, A.P., 1979. Control of placental endocrine function: a role of enzyme activation in the onset of labour. J. Steroid Biochem., 11: 493-500. Hafez, E.S.E. and Jainudeen, M.R., 1975. Gestation, prenatal physiology and parturition. In: E.S.E. Hafez (Editor), Reproduction in Farm Animals, 3rd. edn. Lea & Febiger, Philadelphia, pp. 166-202. Harvey, W.R., 1990. Mixed model least-squares and maximum likelihood computer program PC-2. Ohio State University. Holm, L.W., 1967. Prolonged pregnancy. Adv. Vet. Sci., 11:159-205. Kishore, K., Gour, D., Rawat, P.S. and Arora, C.L., 1980. Note on gestation length in crossbred sheep. Indian J. Anim. Sci., 50: 565-567. Liggins, G.C., Fairclough, R.J., Grieves, S.A., Kendall, J.Z. and Knox, B.S., 1973. The mechanism of initiation of parturition in the ewe. Rec. Prog. Horm. Res., 29:1 l 1-159. Mullaney, P.D. and Lear, D., 1969. Duration of pregnancy in Merino ewes in relation to survival of lambs. Aust. Vet. J., 45: 366-367. Nivsarkar, A.E., Singh, R.N., Bohra, S.D.J., Kuma, M. and Bapna, D.L., 1981. Note on variation in the gestation length of Malpura and Sonadi sheep and their crossbreds. Indian J. Anim. Sci., 51: 1176-1177. Osinowo, O.A. and Ekpe, G.A., 1985. Post-partum intervals to oestrus and conception in Yankasa sheep. J. Agric. Sci. Camb., 104: 253-255. Shrestha, J.N.B. and Heaney, D.P., 1990. Genetic basis of variation in reproductive perform-
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ance. 2. Genetic correlation between gestation length and prolificacy in sheep. Anim. Reprod. Sci., 23: 305-317. Smith, I.D., 1967. Breed differences in the duration of gestation in sheep. Aust. Vet. J., 43: 6364. Swamy, M.N., 1978. Factors affecting gestation length in Bannur sheep. Gujvet, 9: 8-11 (Anim. Breeding Abstr., 48: 74. Abstr. No. 640, 1980). Terrill, C.E., 1975. Sheep. In: E.S.E. Hafez (Editor), Reproduction in Farm Animals, 3rd. edn. Lea & Febiger, Philadelphia, pp. 166-202. Trimnell, A.R., Osinowo, O.A., Olorunju, S.A.S. and Buvanendran, V., 1988. Environmental effects on gestation length in Yankasa sheep. J. Anim. Prod. Res., 8: 33-38.