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Gestation length in Carthusian Spanishbred mares
Livestock Production Science 82 (2003) 181–187 www.elsevier.com / locate / livprodsci
Gestation length in Carthusian Spanishbred mares ´ *, I. Rodrıguez, ´ C.C. Perez J. Mota, J. Dorado, M. Hidalgo, M. Felipe, J. Sanz ´ , Spain Department of Animal Medicine and Surgery, Veterinary Faculty, University of Cordoba, Campus de Rabanales, 14014 Cordoba Received 15 November 2001; received in revised form 19 November 2002; accepted 8 January 2003
Abstract The length of gestation in Carthusian Spanishbred mares reared in southern Spain was calculated on the basis of 364 spontaneous full-term deliveries taking place in the 7-year period 1994–2000. Ultrasonography was used to determine follicular dehiscence (1st day of gestation) and to confirm conception in mares. Mean gestation length was 338.9569.55 (SD) days, and a normal range of 319–359 days was established for this breed. Pregnancy with a female foal was 2.17 days shorter than with a male foal (P , 0.05). Gestation length shortened as the breeding season progressed: for each month later in the breeding season that a mare was mated there was an decrease in gestation length of 2.75 days. The variation in gestation length was greater when mating took place during the reproductive season. 2003 Elsevier B.V. All rights reserved. Keywords: Horse; Gestation length; Andalusian; Sex effect; Breeding month effect; Review
1. Introduction A number of studies have confirmed wide variations in gestation length for various breeds of horse (Salerno and Montemurro, 1966). The duration of pregnancy may be influenced by, among other factors, the ability to prolong gestation (embryonal diapause, reported by Vandeplassche, 1992; Lofstedt, 1992), an ability found in horses, but not in other domestic animals or humans. This is one reason why gestational length, alone, is insufficient for determining foetal readiness for birth. Lofstedt (1992) reports that a delayed embryonic development may be
observed between day 20 and 40, associated with lower plasma progesterone levels. The length of pregnancy is also governed by genetic factors (breed, sire, foal sex, genotype) and environmental factors (age of dam, parity, breeding season, photoperiod, foetal nutrition). It is, therefore, difficult to establish the limits beyond which a gestation period should be considered shortened or prolonged. The aim of this study was to determine average gestation length and normal variability in Carthusian mares, the purest and most homogeneous Spanish-bred lineage. The possible repercussions of foetal sex and breeding season were also examined.
1.1. Normality and deviation in gestation length *Corresponding author. Tel.: 134-957-218-716; fax: 134-957211-093. ´ E-mail address: [email protected] (C.C. Perez).
According to Laing and Leech (1975) and to Rossdale (1993), an equine pregnancy lasting be-
0301-6226 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0301-6226(03)00027-7
´ et al. / Livestock Production Science 82 (2003) 181–187 C.C. Perez
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Table 1 Normal range of gestation lengths in different equine breeds Authors a
Pavlovic (1960) Detkens (1953)a Becze (1958)a Vesovic (1953)a Bettini (1955)a Hrasnika (1944)a Salerno and Montemurro (1966) Bettini (1955)a Pozo-Lora (1954) Pozo-Lora (1954) Matassino (1962) Ilancic (1958)a Kurtz Filho et al. (1997) Hintz et al. (1979) Demirci (1988) Unian and Pereira (1991) Hintz et al. (1992) ´ Perez et al. (1997) Giger et al. (1997) Giger et al. (1997) ´ Sanchez (1998) Marteniuk et al. (1998) Blesa (1999) a
Duration
Breed
328.0 338.8 336.5 333.7 333.8 339.160.9 340.968.7 342.2 343611.4 343.6612.1 337.8613.0 333.5 315–360 305–365 314–361 317–363 319–364 322–359 307–361 315–369 306–381 302–383 322–346
Nonius Throughbred Belgian Arabian Belgian Poney Bosnien Salernitain Percheron Arabian Spanishbred Avelignese Lippiziano Throughbred Throughbred Arabian Arabian Crossbred; Throughbred Crossbred Throughbred Carthusian Spanishbred Freiberger Freiberger3donkey Throughbred Standardbred Spanishbred
Data reviewed by Salerno and Montemurro (1966).
tween 320 and 360 days is to be considered normal, although these values depend on the breed and the equine population studied (Table 1). Gestations of less than 320 days are generally considered short, and resulting foals may present, among other features covered by the term ‘prematurity’, predisposition to illness, low birth weight and inability to stand. Gestations shorter than 300 days are not viable because vital foetal organs are not fully developed (Rossdale, 1976). A gestation period of over 360 days is considered long or prolonged. In such cases, the newborn foal is postmature and might display weakness, large size with poor muscular development, irregular dental eruption, hoof overgrowth and inadequate transference of colostral passive immunity (Putnam et al., 1991; Rossdale, 1993). Braunton (1990) reported a gestational period of 403 days in a three-quarter thoroughbred mare, but the longest reported pregnancy resulting in a live foal is 419 days (West, 1994). Furthermore, neither maternal nor foetal complications are necessarily observed in 300-day gestations or in those exceeding 400 days.
1.2. Factors involved in equine gestation length Genetic factors strongly influence gestation length, in terms of both individual animals and breeds (Sato ´ et al., 1973; Giger et al., 1997; Sanchez et al., 1999). Roberts (1986) reported that pregnancies last longer in light breeds than in draft breeds, according to results reviewed by Langlois (1973). Longer gestational age at foaling has been reported in mares bred with donkeys (Holm, 1967; Arora et al., 1983; Giger et al., 1997). Sire effects are also involved, and Marteniuk et al. (1998) suggest that sires associated with exceptionally long gestation periods should be bred early in the reproductive season, while those associated with short gestations should be used later on. The relationship between coat colour and gestation length has been studied, although the results obtained were not significant (Dring et al., 1981). A number of authors have calculated pregnancy length inheritability, reporting values of 0.25–0.38 in thoroughbred mares (Rollins and Howell, 1951; Hintz et al., 1979; Dring et al., 1981). In Spanishbred horses, Rodero and Pozo-Lora (1960) give a value of
´ et al. / Livestock Production Science 82 (2003) 181–187 C.C. Perez
0.22, and report that duration can be modified by selection. Influence of foal sex on gestation length has been reported by Blesa (1999) in Spanishbred mares, but was not found in early studies by Pozo-Lora (1954) in Arabian and Spanishbred horses. Several studies in other breeds report that male-product gestations are slightly longer (2–3 days) than their female counterparts (Rophia et al., 1969; Hintz et al., 1979; ´ Platt, 1984; Marteniuk et al., 1998; Sanchez, 1998), although the reason for this variation is not clear. Moreover, numerous studies have addressed the influence of environmental factors on gestation length. About age of dam, Demirci (1988) observed longer pregnancies in older mares and Platt (1979) also noted more variable and prolonged gestations in thoroughbred mares over 15 years old. In view of these findings, the influence of dam age can be considered in any statistical model used to calculate gestation length, even though other authors report no ´ significant differences (Hintz et al., 1979; Perez et ´ al., 1997; Kurtz Filho et al., 1997; Sanchez, 1998). Gestation length is also influenced by number of births or parity. Pool-Anderson et al. (1994) report a 10-day longer duration in nulliparous vs. multiparous Quarter Horse mares; Zowlinski (1964) reported similar findings, although other researchers have failed to record significant differences (Arora et al., ´ 1983; Sanchez, 1998). With regard to breeding month, longer gestational periods are reported early in the breeding season (Howell and Rollins, 1951; Rophia et al., 1969; Hintz et al., 1979; Arora et al., 1983; Giger et al., ´ ´ 1997; Sanchez, 1998). Perez et al. (1997) observed that the gestation period was 11 days longer in mares bred in winter (beginning of the breeding season) than at the end of season (June). Ousey et al. (2000), in ponies, noticed that gestations were shorter in May and July breeding, increasing in September. Platt (1984), in a study of thoroughbred mares, reports that the timing of birth has a more marked effect than the breeding month on gestation length. Photoperiod seems to be the principal source of variation in gestation length, through its action on the maternal brain, although the precise mechanisms involved remain unclear. In 1960, Astudillo et al. observed that gestations were extended when daylight periods were shorter, and later, in 1982, Hodge
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et al. demonstrated that pregnant mares subjected to 16 h of light per day from 1 December, advanced the date of birth. Although no direct information exists on pregnant mares, Sharp (1988) suggested that melatonin may be involved in translating photoperiod information to the mare which then physiologically alters the duration of pregnancy. This suggestion echoes the findings of Langlois (1973), who reports that the hormonal environment in pregnant mares may be altered by variations in circadian rhythm. The moon has no proven influence on gestational length in horses, although some studies report effects in cattle (Subramaniam et al., 1991) and humans (Kelly and Martens, 1994; Periti and Biagiotti, 1994; Ghiandoni et al., 1998). Langlois (1973) indicated that well-nourished mares have shorter gestational lengths than mares on maintenance diets. Moreover, nutrition can accelerate or delay foetal growth and pre-birth maturity. Pregnant mares ingesting fescue grass parasitized by Acremonium coenophialum show prolonged gestations, with foetal immaturity and decreased serum triiodothyronine concentrations (Boosinger et al., 1995). The balance between foetal and maternal nutritional requirements at the end of gestation indicates the need for regular dietetic input at this stage, and may partly account for variations in gestation length in mares (Fowden et al., 1984).
2. Materials and methods
2.1. Location and animal management A total of 364 gestation periods recorded for reproductive events from 1994 to 2000, were used. All Carthusian mares were from a single stud farm in ´ Jerez de la Frontera (Cadiz, Spain), situated at latitude 368419 north and longitude 068099 west. During the cold season, mares were grazed during daylight, and at night were kept in 800-m 2 barns; in summer, they were housed until sunset, and grazed at night. Mares received an additional dietary supplement of hay and compound feed, and during gestation received extra vitamin A / D 3 / E and calcium. No artificial light was used in barns to modify the natural photoperiod.
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Mating was by natural service using previouslyselected Carthusian stallions. Mares were mated at puberty (roughly 3 years of age). Matured mares were mated at the first postpartum oestrus (9–12 days postpartum), or at subsequent oestrus. From the first day of oestrus, vaginal exploration and ovarian and uterine ultrasonography were performed every 24 h. The presence of a preovulatory follicle ($3.5 cm.) was taken as the start of breeding. The breeding date was considered to be the day immediately prior to that on which ovulation was observed. Early pregnancy diagnosis was made on day 14 by ultrasonography.
2.2. Statistical methods Gestation-length records were grouped on the basis of foal sex (male or female), breeding season (A5transitional period, from September to January; B5reproductive season, from February to May; C5 end of reproductive season, from June to July), breeding month and breeding year (from 1994 to 2000). One-way repeated-measure analysis of variance was used to examine the effect of foal sex, breeding season, month and year on gestation length, using SPSS 7.5 for Windows (Norusis, 1997). Duncan’s multiple comparison test was used when differences were significant (P , 0.05). All possible interactions were tested, but no significant differences were found. Results were expressed as mean6SD.
3. Results and discussion The primary purpose of this study was to determine gestation length in Carthusian mares. The mean duration was 338.9569.55 days. (mean6SD). The shortest and longest periods observed here were 304 and 372 days, respectively, with normal parturition and viable newborn. The normal range for Carthusian mares was set at 319–359 days. Therefore, the incidence of abnormal pregnancy lengths in the equine population under study was 2.75%, of which 1.65% were considered shortened gestation and 1.10% prolonged gestation.
3.1. Effect of foal sex Pregnancy was significantly longer when mares gave birth to colts rather than fillies (339.9669.66 vs. 337.7969.32 days; P , 0.05), a finding similar to that reported by other researchers (Rophia et al., 1969; Langlois, 1973; Roberts, 1986; Vandeplassche, 1992). This effect is not accounted for in the published literature on horses; the only theories available are from studies of variation in length of human pregnancies, which ascribe differences to androgen action (de Zegher et al., 1999) and to sex-chromosome-linked effects (Pergament et al., 1994; Monteiro et al., 1998).
3.2. Effect of breeding season Breeding season factors clearly affected gestation length (P , 0.001): results showed that pregnancies became shorter as the breeding season advanced. Later in the reproductive season, the photoperiod was maximum (June–July) and pregnancies were significatively shorter than in the transitional period or during the reproductive season (333.9868.84 a , 341.0669.95 b and 340.4869.07 b days, respectively); other authors report similar findings (Marteniuk et al., 1998; Ousey et al., 2000). Variation was greater when mares were bred out of season, as reported by Platt (1984). It has been suggested that variations in light may be one cause of the considerable variation in equine gestation length, due to modification of the foetal maturation rate as labour approaches. The results obtained here suggest that the mare may be able to bring parturition forward when more daylight hours are detected, which could be interpreted as the end of the breeding season. This natural adaptative mechanism allows the newborn foal access to the best nutritive and environmental resources.
3.3. Effect of breeding month The monthly distribution of gestation length revealed significant differences (P , 0.001) (Table 2). During the reproductive season, gestation length increased with each successive breeding month, by about 2.75 days per month. Pregnancies were shorter
´ et al. / Livestock Production Science 82 (2003) 181–187 C.C. Perez
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Table 2 Gestation length (mean values) for different breeding months and seasons in Carthusian mares Month
No. observations
Gestation length (days)
Significance P,0.05
January February March April May June July November December
24 18 39 80 87 67 23 11 15
343.71 346.67 342.77 340.79 337.90 334.55 332.30 343.27 335.20
b, c b, a, a, a, a b, a,
Season
No. observations
Gestation length (days6S.E.M.)
Significance P,0.05
Transitional period Reproductive season Late reproductive season
50 224 90
341.0661.41 340.4860.61 333.9860.50
a a b
c c b, c b, c b c b
* Different letters between months or season denote statistical differences.
when conception occurred in December or in June and July, coinciding with maximum photoperiod oscillations (Howell and Rollins, 1951; Astudillo et al., 1960; Hodge et al., 1982; Giger et al., 1997; Ousey et al., 2000).
3.4. Effect of year Data were collected over several years. Significantly shorter gestations were observed in 1994 (332.11610.44 days; P , 0.001), contrasting with the results reported by Marteniuk et al. (1998) in Thoroughbreds. The pluviometric year 1994–1995 was the last of a 5-year dry period, Spain’s secondlongest such period in the last 60 years. This may have influenced the nutritional quality of rations, which might have become deficient and irregular (Silver and Fowden, 1982), leading to an early end of gestation as an mechanism for adjusting to adverse climatic and nutritional conditions.
within the range 319–359 days. Therefore, the incidence of prolonged and shortened gestations was 1.1 and 1.65% respectively. This work confirms the considerable variation in gestation length in Carthusian mares, with a difference of 68 days between the shortest and longest pregnancies. Gestation length is influenced by a number of factors, including foal sex, breeding season and breeding month, which must be taken into account when analysing length.
Acknowledgements The authors wish to thank Yeguada La CartujaHierro del Bocado (Jerez, Spain) for providing both the mares and the data used in this study.
References 4. Conclusions It was concluded from this study that mean gestation length in Carthusian mares is 338.9569.55 days. It is suggested that normal duration must fall
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Gestation length in Carthusian Spanishbred mares ´ *, I. Rodrıguez, ´ C.C. Perez J. Mota, J. Dorado, M. Hidalgo, M. Felipe, J. Sanz ´ , Spain Department of Animal Medicine and Surgery, Veterinary Faculty, University of Cordoba, Campus de Rabanales, 14014 Cordoba Received 15 November 2001; received in revised form 19 November 2002; accepted 8 January 2003
Abstract The length of gestation in Carthusian Spanishbred mares reared in southern Spain was calculated on the basis of 364 spontaneous full-term deliveries taking place in the 7-year period 1994–2000. Ultrasonography was used to determine follicular dehiscence (1st day of gestation) and to confirm conception in mares. Mean gestation length was 338.9569.55 (SD) days, and a normal range of 319–359 days was established for this breed. Pregnancy with a female foal was 2.17 days shorter than with a male foal (P , 0.05). Gestation length shortened as the breeding season progressed: for each month later in the breeding season that a mare was mated there was an decrease in gestation length of 2.75 days. The variation in gestation length was greater when mating took place during the reproductive season. 2003 Elsevier B.V. All rights reserved. Keywords: Horse; Gestation length; Andalusian; Sex effect; Breeding month effect; Review
1. Introduction A number of studies have confirmed wide variations in gestation length for various breeds of horse (Salerno and Montemurro, 1966). The duration of pregnancy may be influenced by, among other factors, the ability to prolong gestation (embryonal diapause, reported by Vandeplassche, 1992; Lofstedt, 1992), an ability found in horses, but not in other domestic animals or humans. This is one reason why gestational length, alone, is insufficient for determining foetal readiness for birth. Lofstedt (1992) reports that a delayed embryonic development may be
observed between day 20 and 40, associated with lower plasma progesterone levels. The length of pregnancy is also governed by genetic factors (breed, sire, foal sex, genotype) and environmental factors (age of dam, parity, breeding season, photoperiod, foetal nutrition). It is, therefore, difficult to establish the limits beyond which a gestation period should be considered shortened or prolonged. The aim of this study was to determine average gestation length and normal variability in Carthusian mares, the purest and most homogeneous Spanish-bred lineage. The possible repercussions of foetal sex and breeding season were also examined.
1.1. Normality and deviation in gestation length *Corresponding author. Tel.: 134-957-218-716; fax: 134-957211-093. ´ E-mail address: [email protected] (C.C. Perez).
According to Laing and Leech (1975) and to Rossdale (1993), an equine pregnancy lasting be-
0301-6226 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0301-6226(03)00027-7
´ et al. / Livestock Production Science 82 (2003) 181–187 C.C. Perez
182
Table 1 Normal range of gestation lengths in different equine breeds Authors a
Pavlovic (1960) Detkens (1953)a Becze (1958)a Vesovic (1953)a Bettini (1955)a Hrasnika (1944)a Salerno and Montemurro (1966) Bettini (1955)a Pozo-Lora (1954) Pozo-Lora (1954) Matassino (1962) Ilancic (1958)a Kurtz Filho et al. (1997) Hintz et al. (1979) Demirci (1988) Unian and Pereira (1991) Hintz et al. (1992) ´ Perez et al. (1997) Giger et al. (1997) Giger et al. (1997) ´ Sanchez (1998) Marteniuk et al. (1998) Blesa (1999) a
Duration
Breed
328.0 338.8 336.5 333.7 333.8 339.160.9 340.968.7 342.2 343611.4 343.6612.1 337.8613.0 333.5 315–360 305–365 314–361 317–363 319–364 322–359 307–361 315–369 306–381 302–383 322–346
Nonius Throughbred Belgian Arabian Belgian Poney Bosnien Salernitain Percheron Arabian Spanishbred Avelignese Lippiziano Throughbred Throughbred Arabian Arabian Crossbred; Throughbred Crossbred Throughbred Carthusian Spanishbred Freiberger Freiberger3donkey Throughbred Standardbred Spanishbred
Data reviewed by Salerno and Montemurro (1966).
tween 320 and 360 days is to be considered normal, although these values depend on the breed and the equine population studied (Table 1). Gestations of less than 320 days are generally considered short, and resulting foals may present, among other features covered by the term ‘prematurity’, predisposition to illness, low birth weight and inability to stand. Gestations shorter than 300 days are not viable because vital foetal organs are not fully developed (Rossdale, 1976). A gestation period of over 360 days is considered long or prolonged. In such cases, the newborn foal is postmature and might display weakness, large size with poor muscular development, irregular dental eruption, hoof overgrowth and inadequate transference of colostral passive immunity (Putnam et al., 1991; Rossdale, 1993). Braunton (1990) reported a gestational period of 403 days in a three-quarter thoroughbred mare, but the longest reported pregnancy resulting in a live foal is 419 days (West, 1994). Furthermore, neither maternal nor foetal complications are necessarily observed in 300-day gestations or in those exceeding 400 days.
1.2. Factors involved in equine gestation length Genetic factors strongly influence gestation length, in terms of both individual animals and breeds (Sato ´ et al., 1973; Giger et al., 1997; Sanchez et al., 1999). Roberts (1986) reported that pregnancies last longer in light breeds than in draft breeds, according to results reviewed by Langlois (1973). Longer gestational age at foaling has been reported in mares bred with donkeys (Holm, 1967; Arora et al., 1983; Giger et al., 1997). Sire effects are also involved, and Marteniuk et al. (1998) suggest that sires associated with exceptionally long gestation periods should be bred early in the reproductive season, while those associated with short gestations should be used later on. The relationship between coat colour and gestation length has been studied, although the results obtained were not significant (Dring et al., 1981). A number of authors have calculated pregnancy length inheritability, reporting values of 0.25–0.38 in thoroughbred mares (Rollins and Howell, 1951; Hintz et al., 1979; Dring et al., 1981). In Spanishbred horses, Rodero and Pozo-Lora (1960) give a value of
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0.22, and report that duration can be modified by selection. Influence of foal sex on gestation length has been reported by Blesa (1999) in Spanishbred mares, but was not found in early studies by Pozo-Lora (1954) in Arabian and Spanishbred horses. Several studies in other breeds report that male-product gestations are slightly longer (2–3 days) than their female counterparts (Rophia et al., 1969; Hintz et al., 1979; ´ Platt, 1984; Marteniuk et al., 1998; Sanchez, 1998), although the reason for this variation is not clear. Moreover, numerous studies have addressed the influence of environmental factors on gestation length. About age of dam, Demirci (1988) observed longer pregnancies in older mares and Platt (1979) also noted more variable and prolonged gestations in thoroughbred mares over 15 years old. In view of these findings, the influence of dam age can be considered in any statistical model used to calculate gestation length, even though other authors report no ´ significant differences (Hintz et al., 1979; Perez et ´ al., 1997; Kurtz Filho et al., 1997; Sanchez, 1998). Gestation length is also influenced by number of births or parity. Pool-Anderson et al. (1994) report a 10-day longer duration in nulliparous vs. multiparous Quarter Horse mares; Zowlinski (1964) reported similar findings, although other researchers have failed to record significant differences (Arora et al., ´ 1983; Sanchez, 1998). With regard to breeding month, longer gestational periods are reported early in the breeding season (Howell and Rollins, 1951; Rophia et al., 1969; Hintz et al., 1979; Arora et al., 1983; Giger et al., ´ ´ 1997; Sanchez, 1998). Perez et al. (1997) observed that the gestation period was 11 days longer in mares bred in winter (beginning of the breeding season) than at the end of season (June). Ousey et al. (2000), in ponies, noticed that gestations were shorter in May and July breeding, increasing in September. Platt (1984), in a study of thoroughbred mares, reports that the timing of birth has a more marked effect than the breeding month on gestation length. Photoperiod seems to be the principal source of variation in gestation length, through its action on the maternal brain, although the precise mechanisms involved remain unclear. In 1960, Astudillo et al. observed that gestations were extended when daylight periods were shorter, and later, in 1982, Hodge
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et al. demonstrated that pregnant mares subjected to 16 h of light per day from 1 December, advanced the date of birth. Although no direct information exists on pregnant mares, Sharp (1988) suggested that melatonin may be involved in translating photoperiod information to the mare which then physiologically alters the duration of pregnancy. This suggestion echoes the findings of Langlois (1973), who reports that the hormonal environment in pregnant mares may be altered by variations in circadian rhythm. The moon has no proven influence on gestational length in horses, although some studies report effects in cattle (Subramaniam et al., 1991) and humans (Kelly and Martens, 1994; Periti and Biagiotti, 1994; Ghiandoni et al., 1998). Langlois (1973) indicated that well-nourished mares have shorter gestational lengths than mares on maintenance diets. Moreover, nutrition can accelerate or delay foetal growth and pre-birth maturity. Pregnant mares ingesting fescue grass parasitized by Acremonium coenophialum show prolonged gestations, with foetal immaturity and decreased serum triiodothyronine concentrations (Boosinger et al., 1995). The balance between foetal and maternal nutritional requirements at the end of gestation indicates the need for regular dietetic input at this stage, and may partly account for variations in gestation length in mares (Fowden et al., 1984).
2. Materials and methods
2.1. Location and animal management A total of 364 gestation periods recorded for reproductive events from 1994 to 2000, were used. All Carthusian mares were from a single stud farm in ´ Jerez de la Frontera (Cadiz, Spain), situated at latitude 368419 north and longitude 068099 west. During the cold season, mares were grazed during daylight, and at night were kept in 800-m 2 barns; in summer, they were housed until sunset, and grazed at night. Mares received an additional dietary supplement of hay and compound feed, and during gestation received extra vitamin A / D 3 / E and calcium. No artificial light was used in barns to modify the natural photoperiod.
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Mating was by natural service using previouslyselected Carthusian stallions. Mares were mated at puberty (roughly 3 years of age). Matured mares were mated at the first postpartum oestrus (9–12 days postpartum), or at subsequent oestrus. From the first day of oestrus, vaginal exploration and ovarian and uterine ultrasonography were performed every 24 h. The presence of a preovulatory follicle ($3.5 cm.) was taken as the start of breeding. The breeding date was considered to be the day immediately prior to that on which ovulation was observed. Early pregnancy diagnosis was made on day 14 by ultrasonography.
2.2. Statistical methods Gestation-length records were grouped on the basis of foal sex (male or female), breeding season (A5transitional period, from September to January; B5reproductive season, from February to May; C5 end of reproductive season, from June to July), breeding month and breeding year (from 1994 to 2000). One-way repeated-measure analysis of variance was used to examine the effect of foal sex, breeding season, month and year on gestation length, using SPSS 7.5 for Windows (Norusis, 1997). Duncan’s multiple comparison test was used when differences were significant (P , 0.05). All possible interactions were tested, but no significant differences were found. Results were expressed as mean6SD.
3. Results and discussion The primary purpose of this study was to determine gestation length in Carthusian mares. The mean duration was 338.9569.55 days. (mean6SD). The shortest and longest periods observed here were 304 and 372 days, respectively, with normal parturition and viable newborn. The normal range for Carthusian mares was set at 319–359 days. Therefore, the incidence of abnormal pregnancy lengths in the equine population under study was 2.75%, of which 1.65% were considered shortened gestation and 1.10% prolonged gestation.
3.1. Effect of foal sex Pregnancy was significantly longer when mares gave birth to colts rather than fillies (339.9669.66 vs. 337.7969.32 days; P , 0.05), a finding similar to that reported by other researchers (Rophia et al., 1969; Langlois, 1973; Roberts, 1986; Vandeplassche, 1992). This effect is not accounted for in the published literature on horses; the only theories available are from studies of variation in length of human pregnancies, which ascribe differences to androgen action (de Zegher et al., 1999) and to sex-chromosome-linked effects (Pergament et al., 1994; Monteiro et al., 1998).
3.2. Effect of breeding season Breeding season factors clearly affected gestation length (P , 0.001): results showed that pregnancies became shorter as the breeding season advanced. Later in the reproductive season, the photoperiod was maximum (June–July) and pregnancies were significatively shorter than in the transitional period or during the reproductive season (333.9868.84 a , 341.0669.95 b and 340.4869.07 b days, respectively); other authors report similar findings (Marteniuk et al., 1998; Ousey et al., 2000). Variation was greater when mares were bred out of season, as reported by Platt (1984). It has been suggested that variations in light may be one cause of the considerable variation in equine gestation length, due to modification of the foetal maturation rate as labour approaches. The results obtained here suggest that the mare may be able to bring parturition forward when more daylight hours are detected, which could be interpreted as the end of the breeding season. This natural adaptative mechanism allows the newborn foal access to the best nutritive and environmental resources.
3.3. Effect of breeding month The monthly distribution of gestation length revealed significant differences (P , 0.001) (Table 2). During the reproductive season, gestation length increased with each successive breeding month, by about 2.75 days per month. Pregnancies were shorter
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Table 2 Gestation length (mean values) for different breeding months and seasons in Carthusian mares Month
No. observations
Gestation length (days)
Significance P,0.05
January February March April May June July November December
24 18 39 80 87 67 23 11 15
343.71 346.67 342.77 340.79 337.90 334.55 332.30 343.27 335.20
b, c b, a, a, a, a b, a,
Season
No. observations
Gestation length (days6S.E.M.)
Significance P,0.05
Transitional period Reproductive season Late reproductive season
50 224 90
341.0661.41 340.4860.61 333.9860.50
a a b
c c b, c b, c b c b
* Different letters between months or season denote statistical differences.
when conception occurred in December or in June and July, coinciding with maximum photoperiod oscillations (Howell and Rollins, 1951; Astudillo et al., 1960; Hodge et al., 1982; Giger et al., 1997; Ousey et al., 2000).
3.4. Effect of year Data were collected over several years. Significantly shorter gestations were observed in 1994 (332.11610.44 days; P , 0.001), contrasting with the results reported by Marteniuk et al. (1998) in Thoroughbreds. The pluviometric year 1994–1995 was the last of a 5-year dry period, Spain’s secondlongest such period in the last 60 years. This may have influenced the nutritional quality of rations, which might have become deficient and irregular (Silver and Fowden, 1982), leading to an early end of gestation as an mechanism for adjusting to adverse climatic and nutritional conditions.
within the range 319–359 days. Therefore, the incidence of prolonged and shortened gestations was 1.1 and 1.65% respectively. This work confirms the considerable variation in gestation length in Carthusian mares, with a difference of 68 days between the shortest and longest pregnancies. Gestation length is influenced by a number of factors, including foal sex, breeding season and breeding month, which must be taken into account when analysing length.
Acknowledgements The authors wish to thank Yeguada La CartujaHierro del Bocado (Jerez, Spain) for providing both the mares and the data used in this study.
References 4. Conclusions It was concluded from this study that mean gestation length in Carthusian mares is 338.9569.55 days. It is suggested that normal duration must fall
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