The impact of birthweight on mare health and reproductive efficiency, and foal health and subsequent racing performance



Reasons for performing study: Although low birthweight is a risk factor for neonatal illness, the impact of high birthweight on the health of foals and mares, and on the foals' long-term athletic capability, is unknown.

Objectives: To investigate whether: 1) foals that are excessively heavy are associated with an increased prevalence of maternal illness in the first month post partum and reproductive dysfunction in the following season; 2) excessively light or heavy foals are at an increased risk of illness in the first month of life; and 3) birthweight is associated with racing success.

Methods: Veterinary records from 230 Thoroughbred mares and 409 foalings were reviewed. Data relating to foaling, foal and mare health during the first month post partum and subsequent breeding efficiency were extracted. Foals' racing records were obtained and the association between birthweight and these outcomes examined.

Results: Mares with heavy foals had a higher prevalence of minor nonreproductive problems than those with light and average foals. Compared to average foals, placental weight was lower in the light group and higher in the heavy group. Light foals took longer to stand, nurse and pass meconium, whereas gestation period and duration of Stage 2 parturition were longer in heavy foals. Heavy foals were at increased risk of nonseptic musculoskeletal conditions and conformational defects. Birthweight was not associated with racing outcomes.

Conclusions: High birthweight was associated with increased prevalence of neonatal musculoskeletal conditions but birthweight did not have a major impact on mares' health and reproductive efficiency, or foals' racing success.

Potential relevance: These data add to the understanding of the impact of birthweight on short- and long-term health outcomes in mares and foals.


Low birthweight has long been associated with equine neonatal illness (Rossdale 1976; Platt 1978, 1984; Rossdale et al. 1984) and human infant mortality (Malloy 2007; Mathews and MacDorman 2007). Low birthweight in babies is also associated with a range of diseases in adulthood (Barker 2004). It has been stated that severely undersized foals are not likely to perform successfully, but robust data are lacking (Platt 1978, 1984).

High birthweight reduces the likelihood of human infant mortality (Malloy 2007) but its effect has not been studied in horses, except that it is a potential cause of dystocia (Freeman et al. 1999; Byron et al. 2003; Norton et al. 2007). There are no data on the direct effect of birthweight on subsequent fertility or maternal health, but dystocia can lead to numerous maternal problems including death (Vandeplassche et al. 1974; Freeman et al. 1999). Early reports indicated that fertility was reduced (Vandeplassche et al. 1974; Juzwiak et al. 1990), whereas there are more recent reports of retrospective good foaling rates following dystocia (Byron et al. 2003) and fetotomy (Nimmo et al. 2007). The impact of high birthweight on future racing performance is unknown.

The objectives of this study were to investigate whether: 1) foals that are excessively heavy are associated with an increased prevalence of maternal illness in the perinatal period and first month post partum and reproductive dysfunction in the following season; 2) excessively light or heavy foals are at an increased risk of illness in the first month of life; and 3) birthweight is associated with racing success.

Materials and methods

The veterinary records of 409 foalings in 2003, 2005 and 2007 in the UK were each reviewed by one of 3 researchers (S.W., S.A. or S.S.). Records were reviewed to obtain both objective and subjective observations made by farm personnel; diagnostic procedures performed by attending veterinarians; medications administered; and reports written by veterinarians.

Mare and foaling data

Farm personnel observed all foalings. Events surrounding foaling, such as assistance from farm personnel, veterinarians or caesarean section and gross placental abnormalities, were recorded. Mares' veterinary problems at foaling, within the subsequent 48 h, Days 3–7 from foaling and Days 8–30 from foaling were classified into reproductive and nonreproductive, and major or minor by subjective assessments made by one experienced clinician (C.M.M.). Minor reproductive problems included endometrial cysts, genital tract trauma, infection, uterine enlargements and urine pooling, all noted during routine gynaecological examinations. Major reproductive problems included uterine artery haemorrhage, retained placenta, uterine prolapse and uterine injury with peritonitis. Minor nonreproductive problems included coughing, abdominal pain and various skin and musculoskeletal injuries. Major nonreproductive problems included laminitis and a case of chronic foot abscess present prior to foaling. The time between foaling and first and final cover, number of covers and proportion of mares foaling the following year were recorded.

Foal data

Gestation period, duration of Stages 2 and 3 of labour and the time from birth to the foal first standing, nursing and passing meconium were noted, together with placental weight and gross placental abnormalities. Each foal underwent 4 routine veterinary examinations: once a day for the first 3 days of life all foals were given trimethoprim sulphonamide i.m. (Wohlfender et al. 2009); and foals were examined at 25–30 days for registration. Any additional veterinary examinations were recorded. Diagnoses in the first month of life were classified as failure of passive transfer of colostral antibody (defined as serum immunoglobulin G concentration ≤4 g/l at 12–40 h), perinatal asphyxia syndrome (PAS), neonatal isoerythrolysis, meconium impaction, nonseptic musculoskeletal conditions, conformational abnormalities, diarrhoea, pneumonia, umbilical infection and septicaemia, which was further classified as major or minor by subjective assessments made by one experienced clinician (C.M.M.) based on the severity of clinical and laboratory signs and intensity of treatment necessary for a positive outcome. Other veterinary problems, such as nonseptic ocular and umbilical conditions, were classified as miscellaneous.

Farms were asked to provide foals' registered names and those that were named were assumed to have entered race training. Foals did not typically remain in one geographical location from birth to entering training. However, the vast majority of these foals continued to be owned by their original breeders throughout their early life and racing careers. Race records were obtained from a free-access websitea and lifetime earnings were extracted when the youngest crop had completed their 3-year-old racing season. Outcome measures based on horses' achievements at the end of their 3-year-old racing season were having raced at least once, won or been placed at least once, and won or been placed in listed or higher graded races at least once.

Data analysis

Data were analysed using Minitab 12.1.2b. All continuous data were tested with the Anderson-Darling normality test and for all analyses P values of ≤0.05 were regarded as significant. Summary statistics for parametric data are presented as mean ± s.d. and range, and nonparametric data are presented as median ± 95% confidence interval (CI). Foalings where birthweight was not recorded were excluded from analysis. Birthweight categories were developed from the lightest and heaviest deciles: light <48.3 kg, average 48.3–63.2 kg and heavy >63.2 kg. Two sample t tests were used to compare normally distributed continuous data and the Mann-Whitney test was used for nonparametric data. Chi-squared analysis and Fischer's exact tests were used to compare categorical data and, where appropriate, relative risk (RR) and 95% CI were calculated. Birthweight and distribution among birthweight groups by the foals' gender were compared using 2 sample t test and Chi-squared analysis, respectively. Light and heavy foal groups were each compared to the average foal group using the tests described above for continuous and categorical data.


There were 230 mares: 99 (43%) foaled once, 83 (36%) foaled twice and 48 (21%) foaled 3 times, giving a total of 409 foalings. Birthweight data were missing for 7 foals (6 stillborn, one live) and these were excluded from further analysis: 361 foalings produced light/average foals and 41 produced heavy foals.

The nature and prevalence of mares' veterinary problems in the first month post partum are listed in Table S1. There were no differences in the prevalence of veterinary problems during parturition, in the first 48 h or on Days 3–7. Mares with average foals had a higher prevalence of minor nonreproductive problems on Days 8–30 days post partum (light/average = 3%, heavy = 10%; RR = 2.93, 95% CI = 1.0–8.8, P = 0.042). Three mares (0.8%) in the light/average group and one mare in the heavy group (2.4%) died due to rupture of a uterine artery; however, this difference was not statistically significant.

Whether the mare was covered the following season (light/average: 92%; heavy: 90%), whether she conceived, and the times from foaling to first cover (light/average 31.3 ± 11.4, 4–145 days; heavy: 27.51 ± 7.12, 10–47 days), foaling to final cover (light/average: 43.80 ± 25.5, 9–185 days; heavy: 35.81 ± 17.3, 21–94 days) and number of covers (light/average: 1.94 ± 1.33, 1–8; heavy: 1.54 ± 0.96, 1–5) were not significantly different. The proportion foaling the following season was lower in the heavy group (58.5%) than the light/average group (72.6%) but this was not statistically significant (RR = 0.8, 95% CI = 0.62–1.05, P = 0.055).

Nine foals (2.2%) were stillborn and excluded from further analysis. One colt born alive and immediately hospitalised and successfully treated for PAS also lacked birthweight data and was excluded; thus 399 foals were analysed. There were 193 colts and 206 fillies, 43 were light, 315 were average and 41 were heavy (Table 1). Although colts (56.3 ± 6.19, 36.4–71.4 kg) were not significantly different from fillies (55.9 ± 5.42, 34.1–71.8 kg), and the gender distribution between the light group: 20 colts (46.5%) and 23 fillies (53.4%) and the average group: 143 colts (45.3%) and 172 fillies (54.65%) was not significantly different, fillies were less likely to be heavy: 30 colts (73.2%) and 11 fillies (26.8%, RR = 0.62, 95% CI 0.497–0.7742, P<0.001). Placental weights were different among groups and heavy foals had longer gestation than average foals, whereas light and average foals were not different (Table 1). No foals born alive died within the first month. The nature and prevalence of foals' veterinary problems in the first month post partum are listed in Table S2. Eighteen foals (4.5%) were not named, 381 (95%) entered race training, 246 (64.5%) raced at least once, 184 (48.3%) were placed or won at least once and 51 (14.1%) achieved win or place in a listed or higher class race (Table S3).

Table 1. Observations made at birth in 399 Thoroughbred foals
 Light foals n = 43Comparison of light and average foalsAverage foals n = 315Comparison of heavy and average foalsHeavy foals n = 41
  1. NA = not applicable, ns = no significant difference.

Birthweight45.6 ± 3.05, 34.1–48.2 kgNA56.0 ± 2.14, 48.3–62 kgNA65.7 ± 2.11 kg, 63.2–72 kg
Placental weight5.46 ± 0.9, 3.2–6.82 kgP<0.0016.6 ± 1.5, 3.2–15.9 kgP<0.0017.75 ± 1.14, 5.91–10.00 kg
Gestation period344.1 ± 12, 320–390 daysns342.9 ± 9.9, 322–402 daysP = 0.03347 ± 12.77, 331–397 days
Duration of Stage 2 parturition16 ± 6.6, 4–30 minns17.20 ± 8.68, 3–85 minP = 0.0120.95 ± 9.6, 6–40 min
Duration of Stage 3 parturition48.6 ± 61.5, 5–330 minns46.5 ± 60.35, 35–560 minns51.6 ± 57.9, 10–300 min
Time from birth to standing89.62 ± 119.94, 13–780 minP<0.00160.38 ± 135.88, 15–2328 minns48.8 ± 21.1, 14–133 min
Time from birth to nursing139.7 ± 73.3, 45–340 minP<0.001103.2 ± 56.67, 25–485 minns88.24 ± 32.9, 25–195 min
Time from birth to passing meconium77.76 ± 49.4 18–220 minP = 0.0169.83 ± 68.5, 1–390 minns83.51 ± 72.6, 15–335 min

Compared to average foals, there was no difference in duration of Stages 2 or 3 of labour but light foals took significantly longer to stand, nurse and pass meconium (Table 1). Light foals did not have a higher prevalence of veterinary problems in the first month (Table S2). Three light foals (6.9%) and 13 average foals were unnamed. This difference was not significant and there was no difference in the 3-year-old racing outcomes or lifetime earnings between light and average foals (Table S3).

Compared to average foals, Stage 2 of labour took longer in heavy foals (Table 1). Duration of Stage 3 of labour, and times to stand, nurse and pass meconium were not different. Twelve heavy (29%) and 52 (16%) average foals had nonseptic musculoskeletal problems, thus heavy foals were at more risk (RR = 1.77, 95% CI 1.0–3.0, P = 0.045). Conformational defects were more prevalent in the heavy (8, 19%) than the average (29, 9%) foals (RR = 2.12, 95% CI 1.0–4.3, P = 0.042). Two heavy foals (4.8%) were not named, and this was not significantly different from average foals. There was no difference in the 3-year-old racing outcomes or in lifetime earnings between average and heavy foals (Table S3).


Forty years ago, the birthweight of full-term Thoroughbred foals was 49.6 ± 0.45 kg (Rossdale 1976). Recently, higher birthweights have been documented (Galvin and Corley 2010), and managers and veterinarians working on Thoroughbred farms began to suspect that increasing birthweights might be detrimental to mare and foal health despite possibly being beneficial to racing performance. These observations and concerns motivated this study. The range in birthweight in this population was remarkably wide, with the heaviest at 72 kg being more than twice that of the lightest at 34.1 kg, but averages were similar to those of Irish Thoroughbreds born from 2004 to 2008 (Galvin and Corley 2010).

Overall, 3.5% of these foalings were assisted, 1.2% by veterinarians, comparing favourably with a previous study (Morley and Townsend 1997). Excessive foal size can cause dystocia (Freeman et al. 1999; Byron et al. 2003), but in the current population the heavy group did not require more foaling assistance than the light/average group. Heavy foals did have longer duration of Stage 2 of labour. All 4 mares that died had rupture of a uterine artery, which, with severe haemoperitoneum, often leads to death (Dechant et al. 2006). This can occur at any age or parity (Arnold et al. 2008), but cases are frequently multiparous and middle-aged or older (Rooney 1964; Arnold et al. 2008). A significant association between birthweight and maternal death was not demonstrated, although a higher proportion of mares died following a heavy foal. This observation could relate to the higher parity in the heavy group rather than birthweight per se. The increased prevalence of minor nonreproductive problems was probably incidental as there were a wide range of episodes, including superficial injuries, detached retina, transient abdominal pain and shipping fever. Birthweight did not influence the indices of reproductive efficiency. The number of covers following foaling was slightly higher than previously reported (Morris and Allen 2002). Overall, 65% of mares produced foals the following season, similar to some previous reports (Jeffcott et al. 1982; Byron et al. 2003; Katila et al. 2010) but lower than in Thoroughbred mares in Newmarket, UK (Morris and Allen 2002) and in Sweden (Hemberg et al. 2004). Although this study did not identify a significant difference in subsequent foaling rates, the heavy group had lower subsequent foaling rates and thus warrants further investigation in a larger prospective study that includes more detailed assessment of the mares' uterine health status.

Low birthweight is a feature of immaturity (Rossdale et al. 1984). Although major problems associated with immaturity in these foals were not documented and low birthweight was not associated with an increased prevalence of first month illness in this group, some of these foals were extremely small and light foals took longer to stand, nurse and pass meconium, all indications that the foals affected should be monitored carefully. This is the first study to examine the impact of high birthweight on foal health and it was associated with a higher prevalence of nonseptic musculoskeletal conditions and conformational defects, but these were not characterised in detail in this study. Stillbirths occurred in 2% of foalings, similar to other reports (Cohen 1994; Morley and Townsend 1997). There were no deaths within the first month of life, whereas previous mortality rates have ranged from 22% in the first 10 days in extensively managed mares (Haas et al. 1996) to 3% in the first week, 1.7% in the first month in Texan foals of a variety of breeds (Cohen 1994) and 4.6% in the first 14 days in Canadian Thoroughbreds (Morley and Townsend 1997). If a foal received more than the normal 4 veterinary visits, this was considered as a proxy for illness in this population, and at 40% this is a higher disease prevalence reported within the first 2 weeks (Morley and Townsend 1997) and 6 months (Cohen 1994) of life. However, this may reflect management policy rather than the true prevalence of disease. Equally, the prevalence of foals with haematological, but no clinical, evidence of infection is high with a total of 65 foals affected (16%). It is important to acknowledge that these foals had minimal to no clinical signs and treatment and the diagnosis was based primarily on laboratory data obtained from routine screening, specifically abnormal leucocyte counts and increased concentrations of plasma fibrinogen and serum amyloid A. Eight (2%) foals had overt clinical signs of septicaemia and a further 32 (8%) had other problems relating to infectious disease (umbilical infection, pneumonia and diarrhoea). These prevalences are similar to those reported in foals born in Newmarket, UK, in 2005 (Wohlfender et al. 2009), higher than in Canadian Thoroughbreds (Morley and Townsend 1997), but lower than in Texan foals (Cohen 1994). The prevalence of PAS (1%) was similar to that reported in Irish Thoroughbreds (Galvin and Corley 2010) but slightly lower than that reported in Texan foals (Cohen 1994).

The proportion that were named and entered race training compared favourably with previous reports (Jeffcott et al. 1982) but probably reflected the farms' policies, and overall the proportion that raced is almost identical to that observed in another recent report (Smith et al. 2004). Being unnamed can be taken as a proxy for mortality from age 1–18 months of age, and was similar to a previous report of mortality within the first year of life Thoroughbreds in Canada (Morley and Townsend 1997) and to that in the first 6 months of life foals in Texas (Cohen 1994). Birthweight was not associated with racing performance in this population.

This study has some significant limitations. It relied on retrospective examination of veterinary records made by a large number of individuals, including both veterinarians and farm personnel, and thus there is potential that some of the subjective disease classifications may introduce significant bias. The most significant flaw is that birthweight data were not available for 6 of the 9 stillborn foals, and thus this important and interesting subset of foalings/foals had to be excluded from analysis. Birth weight was also not documented in one further foal that was hospitalised immediately after birth for treatment of severe PAS, where it was not weighed for several hours. Hospital records indicated that it weighed 62 kg on its first day of life but because this was near the cut-off between average and heavy groups, the exact time of weighing was not clear, and by the time it was weighed the foal had received fluid therapy, it was excluded. This foal did not race within his first 3 years. For interest, the impact of birthweight on the prevalence of PAS and racing outcomes was recalculated with this foal assigned to the average and heavy groups in turn, and the results were unchanged. Although this study population included 399 foals and 409 foalings from 230 mares, the low prevalence of maternal and foal illnesses meant that the study lacked power to detect differences between the birthweight groups. Nevertheless, some significant differences and trends were observed. These findings add to the understanding of the impact of birthweight on short- and long-term health outcomes in mares and foals and indicate that the hypothesis that extremely high birthweight can be detrimental warrants further investigation.

Conflicts of interests

No conflicts of interest have been declared.

Source of funding

S. Whittaker received a Summer Student Scholarship from the Beaufort Cottage Educational Trust. At the time the work was conducted Tim Parkin was a Defra and Scottish Funding Council Veterinary Training and Research Initiative Fellow.

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