Grass greenness flush can influence breeding phenology and fertility in equatorial thoroughbred mares in the absence of photoperiod variation

Reproductive phenology is an important trait subjected to natural selection. Current horses in America belong to the Palearctic original populations after being introduced by European colonizers. Photoperiod variation is the main environmental factor for the adjustment of reproductive timing in horses, but is absent in equatorial areas. Here we hypothesize that seasonality of green‐grass availability may influence breeding phenology in equatorial regions. We used data of 929 services to mares from 2006 to 2011 in a thoroughbred equine exploitation in Ecuador that experienced strong grass seasonality. Actual births could not be used to infer natural phenology because they were influenced by management decisions. Instead, we used variations in the probability of pregnancy after a service as a measure of the natural tendency of mares to show breeding phenology. We found that although managers tended to schedule pregnancies in two periods within the year, mares were more prone to become pregnant after the increase in grass greenness that takes place at the beginning of the year (February). Our finding has potential applications to improve the success of services and the welfare of animals, by providing green‐grass stimuli in the appropriate season.     

A field study on reproductive efficiency of mares maintained predominately on native pasture

A field study utilizing 145 mares of various age and breeding status was conducted to determine reproductive performance under native range conditions with only limited supplemental feeding. All mares had an average initial body condition score of 4.5 and a final score of 5.1 by the time breeding was completed and mares returned to pasture. Average foaling rate was 80%, and mares that had not conceived during the previous breeding season had a foaling rate of 94%, which was higher (P<.05) than 74% for lactating mares. Mares 16 years and older had a significantly lower foaling rate (P<.05) than younger mares. Old mares that were lactating at time of breeding had only a 37% foaling rate, which was less (P<.05) than for young lactating mares. The 94 mares bred by natural cover or artificial insemination that actually foaled required 1.43 cycles per conception. Lactating mares in the oldest age group required more cycles per conception (P<.05) than open mares, and these older, lactating mares also required more cycles per conception than younger mares with foals at side. Those mares diagnosed as pregnant or open at 45 days post breeding had a pregnancy rate of 97%. Average pregnancy loss for all mares was 7.7%. These data indicate that lactating mares in moderate body condition tended to skip a breeding season and that a body condition score of 5 was only marginally acceptable, especially in the case of lactating mares. Authors' address: Equine Science Program, Department of Animal Science, Texas Agricultural Extension Service, Texas A&M University, College Station, TX 77843. Technical Article Number 30023, Texas Agr. Exp. Sta.     

Injuries and blemishes in a semi-feral herd of ponies

With the goal of characterizing the type and degree of injuries and blemishes incurred by horses living under natural social conditions, all members of a semi-feral herd of ponies were inspected on 4 occasions over a period of 28 months. Two occasions were during breeding/foaling season and 2 during non–breeding/foaling season months. On each occasion, each animal was examined (N = 47-65) and all injuries or blemish marks were recorded. Based on the types and number of injuries or blemishes, an injury/blemish grade was assigned for each inspection, ranging from 1 for no blemishes to 6 for more than 1 open wound. Almost all injuries and blemishes recorded were extremely mild. Only 12 of the 213 inspections and 14 of the 858 total injuries and blemishes involved wounds affecting tissues other than hair and skin.Males had significantly greater mean injury/blemish grade than females (P < .0001). For all social categories and ages, the mean injury/blemish grade was significantly greater in the breeding/foaling season months than during the non–breeding/foaling season (P < .0001).All foals were blemish-free at the non–breeding/foaling season inspections (n = 24) and relatively blemish-free (9 of 14) at the breeding/foaling season inspections. The rump and the barrel areas were the most common site of injuries or blemishes, both as a percentage of the total injury count and as a percentage of inspections involving those areas. These findings are consistent with the seasonal and gender patterns of aggressive behavior seen in this herd. Compared with reports of truly feral horse populations, it appears that injuries are fewer and less severe in this semi-feral herd. In addition, there appear to be fewer leg injuries in this semi-feral herd than has been described for truly feral horse populations.     

The Effect of PGF2α-Induction of Estrus on Pregnancy Rates in Mares

In mares, the onset of estrus is routinely induced after a luteolytic dose of a prostaglandin F2alpha analogue. Mares in diestrus with a mature, functional corpus luteum will respond by coming into estrus 3 to 4 days after induction¹. Shortening the interestrous interval in mares has an important economic impact on the equine breeding industry. Because the breeding season of the mare is short, timing of insemination and appropriate coordination of endocrine events is critical to pregnancy success and ultimate foaling rates. Several recent studies have reported that the use of prostaglandin analogues is associated with lower pregnancy rates. In this study the induction of estrus with cloprostonol (125-250 ug IM) did not affect pregnancy rates in mares that have ovulatory cycles (n = 461).     

Reflections on mare behavior: Social and sexual perspectives

Among horse owners “mare-ishness” is easily recognized, if poorly defined, as generally awkward or recalcitrant behavior. In common with other mammals, however, horses do show a range of female-specific behavior patterns concerned particularly with sexual and maternal functions but also encompassing aspects of social peer interaction. Many of these behavior patterns are observed more easily in free living feral populations and are repressed at least partially under standard domestic management regimes. Feral horses form small stable breeding groups within which mares create linear dominance hierarchies based on the outcomes of aggressive encounters and positive social bonding. Rank within the social hierarchy may also influence reproductive success, with dominant mares having shorter foaling intervals and earlier foaling than lower ranking subordinate mares. Advantageous early foaling may reflect preferential mating by the group stallion based either simply on enhanced physical condition leading to earlier reproductive cycling in dominant mares or on an increased attractiveness to dominant mares by the stallion contingent on some aspect of dominance characteristics. The endocrine axis responsible for regulation of sexual behavior in the mare shows a degree of commonality with systems underlying social dominance. Both these behavior patterns are driven by a complex interaction of endocrine and neuro-endocrine factors sensitive to physical and social environmental cues and in feral free-living horses these are interlinked to an extent that may be under appreciated in domestic breeding populations.     

Effect of environmental factors and changes in the body condition score on the onset of the breeding season in mares

Several methods have been proposed to advance the onset of the breeding season in horses. Most of them are based on the exposure to an artificial lighting period combined with hormonal treatments. Mares exposed to an artificial photoperiod are most often housed indoors where the ambient temperature is often higher than the outside temperature. Mares held in barns are also exposed to different daylight intensities than horses kept outside, depending on the architecture. In the current study, we evaluated the impact of ambient temperature, daylight intensity and changes in body condition score (BCS) on the timing of first ovulation after winter anestrus in mares exposed to an artificial photoperiod. Mares (n = 211) were housed in barns with different ambient temperature and daylight exposure but with the same artificial photoperiod exposure (except for a natural photoperiod control group). Artificial photoperiod as well as an increase in BCS over the winter significantly advanced the first spring ovulation. The BCS at the start and end of the anestrus period did not have an effect on the interval to first ovulation and neither did the modest increase in ambient temperature in the barn. However, a higher light intensity during the daytime significantly advanced the first spring ovulation. The results of this study suggest that exposure to more sunlight advances the onset of the breeding season. This effect is likely mediated through the biological effect of short wavelength blue light and its impact on melatonin suppression and biological rhythms. We suggest that greater/direct exposure to the blue light component of daylight improves the response to the artificial photoperiod. The results of the present study can further assist to optimize the conditions that lead to an efficient spring transition of breeding mares.     

Surgical results and fertility following correction of vesicovaginal reflux in mares

Objective To evaluate the surgical results and fertility of mares bred at various intervals relative to surgical management of urovagina. Design Sixty-one mares underwent surgery to correct vesicovaginal reflux. Surgery was performed at varying times relative to breeding and postoperative fertility was evaluated. Results Five mares were lost to follow-up and a further four were electively not bred. Of the remaining 52 mares, seasonal pregnancy and live foaling rates were 67% (35/52) and 54% (26/48), respectively. When mares were bred in the same cycle as the surgery, the first cycle following surgery, the second cycle following surgery or the following breeding season after surgery the seasonal pregnancy rates were 89% (8/9), 63% (10/16), 67% (2/3) and 63% (15/24), respectively. After censoring four mares that died of natural causes pre-foaling, the foaling rates were 88% (7/8), 50% (7/14), 0% (0/3), 52% (12/23), respectively. All mares bred in the same cycle as surgery or in the next cycle were bred once only that season, so the pregnancy rate per cycle of 72% (18/25) was identical to the seasonal pregnancy rate. Conclusions Good fertility (comparable to a normal population of mares) was achieved following surgery and the results suggest that delaying breeding until the following breeding season is not necessary. In addition, breeding in the same cycle as surgical repair is a previously unreported technique that should be considered to maintain a yearly foaling interval.     

Vestibuloplasty for Persistent Pneumovagina in Mares

Occasionally Caslick vulvoplasty does not resolve the condition of pneumovagina. This study aimed at evaluating the validity of surgical construction of artificial vestibular valve in standing mares. For 10 sedated mares with history of chronic pneumovagina, upside down U-form incision was made on the vestibular mucosa and sutured together to form a large valve after the cover. The surgical site was opened for another cover or foaling and sutured again. The condition of air aspiration into the vagina was monitored ultrasonographically after the surgery. In 9 of the 10 mares, the signs of pneumovagina improved after the surgery and 8 mares had normal foaling the next year. In addition, all mares which had the second surgery after the surgical site was incised for the second cover or foaling were conceived and had normal foaling in the following breeding season. The results revealed that construction of the vestibular valve can be a potential surgical option for the treatment of persistent pneumovagina.     

Retained fetal membranes in the mare: A retrospective study

A retrospective study of 3456 deliveries was conducted from the records of four Standardbred broodmare farms where mares were bred by artificial insemination and maintained under close veterinary supervision. Retained fetal membranes (RFM) were observed in 10.6% of the deliveries. Retained fetal membranes occurred more frequently (p < 0.05) after dystocia and in mares which had RFM the previous year. Retained fetal membranes after normal foaling had no significant effect on the reproductive performance (pregnancy rate, pregnancy loss rate, or foaling rate), nor on the general health of the mares, regardless of the duration of RFM (3 to 144 hours). Postfoaling laminitis was not observed. Oxytocin therapy of mares with RFM starting at two hours postpartum significantly reduced the incidence of RFM ≥ 8 hours. Mares with RFM which had received intrauterine antimicrobials between foaling and first breeding had a foaling rate similar to mares with RFM which had not received intrauterine therapy.     

Reproductive performance and factors that decrease pregnancy rate in heavy draft horses bred at the foal heat

Reproductive performance and factors that decrease the pregnancy rate in heavy draft horses bred at the foal heat were investigated. The study was based on a total of 422 mares varying from 4 to 20 years in age, which had foaled during the period 1991 to 1994. Over the four years, the percentage of the mares bred at the foal heat was 62.6% and the pregnancy rate was 60.2%. The pregnancy rate following breeding at the foal heat in 1991 (67.7%) was significantly higher than that in 1992 (44.1%). Significant correlation was observed between the pregnancy rate and the mean placental retention time and also between the pregnancy rate and the mean temperature during the winter season. When compared on a monthly basis, the pregnancy rate following breeding at the foal heat in February was 53.3% and in May, 48.2%. The mean number of covers per mare at the foal heat was 2.3; this figure decreased to 1.8 for those bred in May. The mean number of days from parturition to the first breeding date was 11.3 days and the number of days from parturition to the last breeding day at the foal heat was 14.0 days. The number of days from parturition to the last breeding day at the foal heat decreased to 12.5 days in May. The latter may have been influenced by factors involving the stallion. When compared on a monthly basis, the mean placental retention time was longest in January (219 min.) following which it decreased gradually up to mid-summer. The mean placental retention time of mares which were not bred at the foal heat was significantly longer (197 min.) than in mares bred at the foaling heat (143 min.) and also in mares that conceived following breeding at the foal heat (135 min.). The pregnancy rate was less in mares bred shortly after parturition. The findings of this study would suggest that certain factors decrease the pregnancy rate in mares bred at the foal heat. The stress of low temperature in winter, prolonged placental retention time, breeding too early after parturition and factors pertaining to individual stallions may be involved. Expulsion of the placenta as soon as possible after foaling and appropriate breeding timing are significant factors that can influence the pregnancy rate following breeding at the foal heat.     

Genital injuries from mating in the mare

Minor accidents during natural mating are common occurrences during the breeding of horses. Mares may suffer from a variety of genital injuries including vulval separations, vaginal lacerations and, less commonly, vaginal rupture. Possible sequelae to, and management of, these injuries are described, with additional mention of false entry by the stallion and persistent hymen in maiden mares. Vaginal rupture during natural mating can lead to death of the mare. The implications for litigation involving veterinary surgeons and horse breeders are discussed.     

Management Strategies Aiming to Improve Horse Welfare Reduce Embryonic Death Rates in Mares

The objective of this retrospective study was to evaluate the effect of management strategies aiming to improve animal well‐being on pregnancy and embryonic death (ED) rates. Breeding records of a cohort of 1206 Thoroughbred mares brought to a stallion station facility, to be bred with the stallions housed there, were evaluated during ten breeding seasons. Mares were blocked according to management strategies in two groups: Stress and Relax. Strategies used to improve animal well‐being (Relax group) were as follows: stopping the teasing routine, reducing or eliminating stall confinement, reducing the number of mares per group and maintaining herd stability during the breeding season. In barren mares, the pregnancy rate was higher in the Relax group (91.8%) when compared to the observed in Stress group (84.7%). However, no difference in pregnancy rates were observed (Stress = 85.2% vs. Relax = 86.2) in foaling mares. ED rate was higher in barren and foaling mares of the Stress group mares (25.5% and 26.8%, respectively) compared with the Relax group (16.1% and 14.7%, respectively). No significant differences were observed on foal heat pregnancy rate between groups; yet, the embryo loss on foal heat was significant reduced in Relax mares (Relax = 8.7% vs Stress = 24.5%). In conclusion, management strategies aimed to reduce social stress can reduce early pregnancy losses and the average cycles per pregnancy, improving reproductive performance in mares.     

Season Does Not Influence Embryo Recovery Rate and Conceptus Size Until Day 14 After Ovulation in the Horse

Although the horse is a seasonal breeding species, a considerable number of mares continue to cycle throughout autumn and winter. Slower equine embryo growth during the non‐breeding season has been hypothesized, and because smaller embryo size is beneficial for cryopreservation, embryo collection outside the breeding season could be an interesting approach for the production of frozen horse embryos. In the present retrospective study, we have therefore analysed embryo recovery rates and conceptus size in mares (n = 30) throughout the year. Conceptus diameter was either size determined after collection with a microscopic scale (day 7–10 after ovulation) or determined by transrectal ultrasound immediately before collection (day 11–14 after ovulation). In 19 of the 30 mares (63%), ovulatory cycles were detected throughout the year. A total of 352 embryo collections with a mean recovery rate of 64.2% were performed and not affected by season. The size was analysed in a total of 165 conceptuses. Conceptus diameter significantly increased (p < 0.001) with day of pregnancy (e.g. day 7: 0.3 ± 0.04, day 10: 4.1 ± 0.2, day 12: 10.1 ± 0.5, day 14: 17.4 ± 0.9 mm), but was not influenced by season. In conclusion, successful embryo collection is possible throughout the year in spontaneously cyclic mares. Under these conditions, neither collection rates nor embryo growth appeared to be affected by season.     

Surgical correction of rectovaginal fistula in mares and subsequent fertility

Objective To evaluate the fertility of mares bred at various intervals relative to surgical management of rectovaginal fistula (RVF). Materials and Methods Surgical repair of RVF was performed in 28 mares at variable times relative to foaling (30 days to 24 months) and also relative to rebreeding (same cycle or delayed). Postoperative fertility was then evaluated. Results Two mares were already pregnant at the time of surgery and 20 of 23 mares (87%) that were bred immediately prior to or following surgery conceived from their first service. When mares were bred in the same cycle as surgery, the next cycle following surgery or in the following breeding season after surgery the pregnancy rate was 5/5, 5/6 and 10/12, respectively, and the foaling rates were 4/5, 4/6 and 7/12. The two mares already pregnant at the time of surgery foaled successfully. Conclusions Excellent fertility can be achieved following surgical repair of RVF and our results suggest that delaying breeding until the following breeding season is not necessary. In addition, breeding in the same cycle as the surgical repair is a previously unreported technique that should be considered to maintain normal fertility and a yearly foaling interval.     

A cross-sectional survey of Thoroughbred stud farm management in the North Island of New Zealand

AIM: To obtain initial baseline data on the management of Thoroughbred stud farms in the North Island of New Zealand.

METHODS: Data on the management of Thoroughbred stud farms were collected from a sample of 22 stud farms located in the south Auckland/Waikato region (n=15) and lower North Island (n=7) of New Zealand, using a face-to-face survey. The studmaster provided information on the size, scope and management of the farms during the 2004/2005 breeding season. Analysis was based on the location of the farm and size of the breeding operation (number of resident mares).

RESULTS: Effective farm size ranged from 20 to 526 ha and averaged 167 (standard error (SE) 36) and 88 (SE 49) ha in the south Auckland/Waikato and lower North Island areas, respectively. Some farms in the Auckland/Waikato region stood shuttle stallions. The median number of stallions per farm was three (range 0.9), and the median mare-to-stallion ratio was 43 (range 10.250). The farms had a mean of 50 (range 7.180) wet mares and 21 (range 0.100) dry mares. The number of mares per breeding stallion increased with increasing size of breeding operation (p=0.04), being 28 (95% confidence interval (CI) = 10.56) vs 40 (95% CI=16.74) vs 74 (95% CI=44.113) for moderate (≤70 mares), medium (90–199 mares) and large (≥200 mares in total) operations, respectively. Seventy-one percent of farms aimed to breed dry mares early in the breeding season, and used a combination of lights, hormone therapy, and rising plane of nutrition to achieve this.

Foaling took place in foaling paddocks monitored using a night foaling attendant (17/22) or with foaling alarms (5/22). At birth, 17/22 studmasters routinely administered antibiotics, 14/22 administered tetanus antitoxin, 9/22 administered an enema to foals, and 2/22 did not routinely administer prophylactic treatments. Weaning occurred at 5 (range 3.7–7) months of age, and foals were confined to a box for 1–2 weeks on 16/22 farms. Weaned foals were drenched with anthelmintics every 7 (range 4–9) weeks, and were fed 2.9 (range 1–6) kg of concentrate feed while at pasture until intensive management associated with preparation of the horses for auction began 13 (range 6–20) weeks before the yearling sales. Eight farms weighed the weanlings, at least monthly, to monitor growth.

CONCLUSIONS AND CLINICAL RELEVANCE: The management of Thoroughbred horses was relatively consistent throughout the regions surveyed. Utilisation of breeding stallions tended to be more efficient on the larger stud farms in the south Auckland/Waikato region. Even though foals are grown at pasture they are often provided with large quantities of concentrate feed.     

A cross-sectional survey of Thoroughbred stud farm management in the North Island of New Zealand

AIM: To obtain initial baseline data on the management of Thoroughbred stud farms in the North Island of New Zealand. METHODS: Data on the management of Thoroughbred stud farms were collected from a sample of 22 stud farms located in the south Auckland/Waikato region (n=15) and lower North Island (n=7) of New Zealand, using a face-to-face survey. The studmaster provided information on the size, scope and management of the farms during the 2004/2005 breeding season. Analysis was based on the location of the farm and size of the breeding operation (number of resident mares). RESULTS: Effective farm size ranged from 20 to 526 ha and averaged 167 (standard error (SE) 36) and 88 (SE 49) ha in the south Auckland/Waikato and lower North Island areas, respectively. Some farms in the Auckland/Waikato region stood shuttle stallions. The median number of stallions per farm was three (range 0-9), and the median mare-to-stallion ratio was 43 (range 10-250). The farms had a mean of 50 (range 7-180) wet mares and 21 (range 0-100) dry mares. The number of mares per breeding stallion increased with increasing size of breeding operation (p=0.04), being 28 (95% confidence interval (CI) = 10-56) vs 40 (95% CI=16-74) vs 74 (95% CI=44-113) for moderate (or=200 mares in total) operations, respectively. Seventy-one percent of farms aimed to breed dry mares early in the breeding season, and used a combination of lights, hormone therapy, and rising plane of nutrition to achieve this. Foaling took place in foaling paddocks monitored using a night foaling attendant (17/22) or with foaling alarms (5/22). At birth, 17/22 studmasters routinely administered antibiotics, 14/22 administered tetanus antitoxin, 9/22 administered an enema to foals, and 2/22 did not routinely administer prophylactic treatments. Weaning occurred at 5 (range 3.7-7) months of age, and foals were confined to a box for 1-2 weeks on 16/22 farms. Weaned foals were drenched with anthelmintics every 7 (range 4-9) weeks, and were fed 2.9 (range 1-6) kg of concentrate feed while at pasture until intensive management associated with preparation of the horses for auction began 13 (range 6-20) weeks before the yearling sales. Eight farms weighed the weanlings, at least monthly, to monitor growth. CONCLUSIONS AND CLINICAL RELEVANCE: The management of Thoroughbred horses was relatively consistent throughout the regions surveyed. Utilisation of breeding stallions tended to be more efficient on the larger stud farms in the south Auckland/Waikato region. Even though foals are grown at pasture they are often provided with large quantities of concentrate feed.     

Male, female and management risk factors for non-return to service in Dutch mares

The "effect" of stallion, mare and management-related factors on the odds of pregnancy per cycle in the horse were identified and quantified from the breeding records of Dutch Warmblood (n=4491), Friesian (n=1467) and Shetland-pony mares (n=3267) mated either naturally or by artificial insemination to one of the 88 stallions between 1992 and 1996. A mare was considered to be pregnant when she did not return to oestrous within 28 days of the last insemination. For Dutch Warmblood horses, the percentage of mares that did not return for service within 28 days (NR28) varied between studfarms and ranged from 61 to 82%. The NR28 for mares inseminated with fresh semen ranged from 67 to 74% and for mares inseminated with frozen/thawed semen this percentage was 59. Mares served at a second cycle had lower odds not to return than mares served at the third or subsequent cycle (OR=0.84). For Friesian horses, the NR28 for young mares was higher than that for older mares. Mares served before 1 May in any year had lower odds of non-return than mares served after 1 July (OR=0.69). The NR28 of mares inseminated once per cycle was 6% lower than that of mares inseminated three times or more per cycle. For Shetland ponies, the NR28 also varied between studfarms and ranged from 62 to 78%. Stallions < or =3 years old had lower odds of non-return compared to older stallion (> or =11) (OR=0.57). Mares served before 1 July had lower odds of non-return. Other significant factors for this breed were age of the mare, cycle number and insemination frequency. Stallion factors accounted for 5.9, 2.0 and 14.7% of the variation in the NR28 for Dutch Warmblood, Friesian horses and the Shetland ponies, respectively.     

Prevalence of gastric ulcers in Thoroughbred broodmares in pasture: a preliminary report

Gastroscopic examinations were performed in 62 Thoroughbred broodmares (33 pregnant, 29 non-pregnant) at one breeding farm to investigate the prevalence of gastric ulceration. Age, pregnancy status, race earnings, last race start, herd size, medical history, number of live foals, breeding years, feed type and number of feedings were recorded, plus coat condition and body condition score were determined. Twenty-one mares were re-evaluated after foaling, and the foaling date, foal weight at birth and placenta weight were recorded.The overall prevalence of gastric ulcers was 70.9%, with a median ulcer score of 3.0 (range: 2–5). Most ulcers were present on the squamous portion of the stomach, while two mares had glandular ulcers. There were no differences in the presence, location and severity of gastric ulcers between pregnant and non-pregnant mares. Furthermore, there were no significant associations between the variables measured and the presence of gastric ulceration. The prevalence of gastric ulceration in this specific population of horses was higher than expected and further investigation is warranted to determine the factors that contributed to this finding.     

Typing of rabbit haemorrhagic disease virus from New Zealand wild rabbits

Abstract

AIMS: To examine the gestation lengths and occurrence of daytime foaling of Standardbred mares foaling outdoors at stud farms in Southland, New Zealand (latitude 45?S).

METHODS: Data were collected prospectively at two commercial Standardbred stud farms (Farms A and B), during the 2008/9 and 2009/10 breeding seasons (October to February). For each foaling, the identity of the mare, date and time of foaling, gender of foal, time the mare passed the fetal membranes, time the foal stood, and foaling problems including dystocias, were recorded. The effect of farm, season, gender of foal, month of artificial insemination (AI) or foaling, age and parity of mare on gestation length, percentage of mares foaling during daylight hours, and percentage of foalings recorded as dystocia, were examined.

RESULTS: A total of 614 foaling records were obtained from 507 mares. For 594 foalings with complete records, mean gestation length was 349 (SE 0.5) days. Mean gestation length was shorter for fillies (347.8 (SE 0.6) days) than colts (350.3 (SE 0.6) days) p = 0.021) and decreased with month of AI, from October to February (p = 0.001). The time of foaling was bimodally distributed with the primary peak around 0200 hours and a secondary smaller peak around 1300 hours. The percentage of mares foaling in daylight was lower on Farm A (69/285 (24%)) than Farm B (128/313 (41%)) (p = 0.001). Colt foals were less likely to be born during the day than fillies (OR = 0.63; (95% CI = 0.44–0.88); p = 0.008), but there was no effect of age or parity of mare or month of foaling (p>0.05).

CONCLUSIONS AND CLINICAL RELEVANCE: Mean gestation length of mares in this study was longer than that previously reported from other countries. Longer gestation length decreases the time available for these mares to get back into foal in the same season. It is important that managers take this into account and ensure optimum conditions for conception; breeding at the first postpartum oestrus may be essential. There were more mares foaling in daylight hours on one stud than has previously been reported. Under these conditions mares should be monitored for foaling during daytime as well as at night.