By Margaret Evans

Traditionally, horse breeders look to the stallion for pedigree lines to produce the superior performance offspring with the desired characteristics of speed, conformation, and health. But the lineage of mares plays an equally important role not only in the genetic quality of the foal but potentially in the breeder’s selection for fillies or colts. In addition, a mare’s maternal heritage influences the length of gestation.

At the Graf Lehndorff Institute for Equine Science, a joint research institution of the Vetmeduni Vienna and the Brandenburg State Stud in Germany, researcher Juliane Kuhl investigated the degree to which the maternal lineage (family lines) influences gestation length and foal characteristics.

“When I started my work at the Brandenburg State Stud in 2009, part of my duty was to supervise parturition in the mares,” says Kuhl. “As horse breeders may well know, gestation length in mares is highly variable and early signs of impending parturition can be subtle impeding accurate prediction of foaling. However, the experienced staff at the stud often told me something like: “Oh, this is a P-mare. She will probably take more time until foaling.” Or the other way round: “Mon Plaisir - belonging to the M-mare lineage - will probably come earlier with the foal.” And often they were right. On the other hand, every morning when the stud director would be looking at the mares and the newborn foals, I got to hear: “Again a filly. You have to produce more colts.” I wanted to know more and started to look at the breeding data of these mares.”

Broodmares and their foals at the Brandenburg State Stud, Germany. Photo courtesy of Juliane Kuhl.

Working with a team of researchers, Kuhl found that the length of gestation varied significantly between the different lineages. In addition, certain family lines will produce more fillies than colts.

As breeders know, the gestation period for horses ranges from 335 to 345 days and sometimes more widely from 320 to 360 days. Some maternal lines will average 10 days longer than other family lines. The researchers also noted in their study that the gestation length for male foals tended to be longer than for female foals which added to the complex variation between the mare families in the focus broodmare group.

“In general, when you want to investigate genetic effects on a trait influenced by a variety of factors which, in the horse, is gestation length as well as foal sex ratio, environmental factors have to be reduced to a minimum to be as constant as possible,” says Kuhl. “The broodmare herd at the Brandenburg State Stud offers ideal conditions. A peculiarity of the stud is the large broodmare herd where breeding selection from distinct mare families - maternal lineages - takes place over many generations in a nearly invariable environment. The mare lineages have been recorded continuously from 1946 onwards and, for some mares, can be traced back until the foundation of the stud in 1788. Hence, with the support of my supervisor Christine Aurich, head of the Graf Lehndorff Institute for Equine Science, and the staff at the Brandenburg State Stud, we were able to retrieve and analyze records of 142 mares giving birth to 640 foals at the stud in a 20 year period.”

Juliane Kuhl with a broodmare friend. Photo courtesy of Juliane Kuhl.

The Brandenburg State Stud in Neustadt (Dosse) in Germany is one of the biggest in Europe and is at the centre of sustainable rural development. In addition to horse breeding, activities include equestrian training in the FN-approved riding school, riding with the Neustadt Secondary School where riding is part of the school curriculum, and tourism which flourishes in the union of history and horses. To protect it from privatization, the stud became a public foundation in 2001.

The facility is located on 400 hectares where its cultural and historic buildings were built during the reign of Prussian King Friedrich Wilhelm II in 1788. The stud maintains a broodmare herd of about 40 animals with the aim of each producing a foal annually. Maiden mares are entered into the herd at age three. 

Data from 142 warmblood mares for the study spanned the breeding seasons from 1992 to 2011. In 1992, ultrasound examination of the genital tract was introduced into the facility’s breeding management program and that allowed the exact day of ovulation to be reliably determined.

Based on the historical data, the mares in the study represented 13 mare families. Of the 640 foals born in the study period, 48 percent were colts and 52 percent were fillies. A total of 786 cases were included in the study and, from the 142 mares, 119 were assigned to six maternal lineages with more than 10 mares per lineage, and 23 mares belonged to smaller maternal lineages. 

The live foal rate was 83.5 percent. The mean gestation rate was 338.5 days with a considerable range of 313 to 370 days. The gestation length was calculated from the day of ovulation until the day of parturition of a viable foal. In case of pregnancy loss, abortion or stillbirth, the data were excluded.

The study showed that gestation length was not only affected by maternal lineage but also was significantly influenced by the individual mare, her age, the year of breeding, month of breeding, and the sex of the foal. The two influencers of the sex ratio of the foals were the mare age group and the mare’s maternal lineage.

The researchers found that the mean gestation length for mares was longer in their first pregnancy compared to subsequent pregnancies. First time broodmares were bred at three years of age. The same tendency – longer gestation times - was also seen in aging broodmares, those over 12 years of age.

Why longer gestation shows up in younger and older broodmares is thought to be influenced by differences in endometrial and placental function. The endometrium is the innermost glandular membrane in the uterus, an environment that is perfect for a fertilized egg to embed itself and begin development. Kuhl says that the endometrium and the placenta are essential for nourishing the fetus in the uterus. Research, though, has shown that in maiden mares the contact area of the placenta with the endometrium is smaller and less developed. Curiously, female embryos appear to have the ability to cope and survive in this environment better than male embryos.

The Brandenburg State Stud in Germany. Photo courtesy of Juliane Kuhl.

“In the horse, the uterine environment at conception and during early pregnancy has a pronounced effect on sex ratio of the offspring,” says Kuhl. “Findings of the present study are thus in agreement with the hypothesis that the early female embryo has mechanisms that counteract its death under detrimental uterine conditions. In further support of this suggestion, female pregnancies seem to have a better placental adaptation to adverse conditions than male ones. Age related alterations in endometrial quality and function - and consequently fertility - are well-known in mares. Together with the findings of studies on male/female differences [using] in vitro-cultured embryos, we can assume that female embryos are better at coping with suboptimal conditions as provided by the uterus/endometrium of maiden or aged mares leading to the female biased sex ratio in maiden and aged mares.”

Kuhl says that the bias toward having fillies in younger and aged mares as shown in their study dovetails with similar findings from different wild ass subspecies. However, for mares between four and twelve years of age, the foal sex ratio is balanced.

“These results are important for horse breeders,” Kuhl believes. “They could possibly choose their mares depending on the desired sex of a foal.”

What actually drives the coping mechanism of female embryos in the earliest days of development is still unclear.

“We suspect that these effects are due to the differences in mitochondrial DNA,” says study director Christine Aurich. “This specific DNA is inherited over the maternal line and influences cell metabolism and placenta function. We also know that female embryos are more resilient. As 20 to 30 percent of early pregnancies are lost spontaneously, it is possible that male embryos survive less frequently. This could be a reason for the observed shift in the sex ratio. But it is also possible that embryo survival is influenced by differences in placental function.”

Kuhl adds that horses are a polygynous species which means one male, the harem stallion, lives with a group of females. Past studies have suggested that where one sex has more variable reproductive success such as males in a polygynous species, dams in good condition compared to those in poor condition would be advantaged by producing more of the stable sex. Kuhl says that the nutritional state of the mare seems to be communicated to the embryo via the uterine environment.

“Studies on semi-feral horses have shown a female-biased sex ratio in years following breeding seasons with poor food availability or in mares with poorer body condition or losing weight around conception.”

The reason for the differences in gestation lengths of male foals being longer than females is also still unclear. 

“Most research available at the moment just focuses on the various factors accounting for the differences in duration of pregnancy but not on the underlying physiological mechanisms,” says Kuhl. “That is the reason why I cannot satisfyingly explain this finding. What we do know is that male and female are different from the very beginning. Interestingly – and to our surprise – the difference in mean gestation length for fillies and colts was one to two days whereas differences up to a week were seen between the mare families.”

She says that the heritability of gestation length has been shown for different horse breeds and is considered high enough to respond to selection. Their finding that mare family lineage contributes substantially to the variation in gestation length led them to the conclusion that differences in the mitochondrial DNA is key to this genetic predisposition.

“Mitochondria are responsible for the cellular respiration and the only organelles that possess their own genetic information,” says Kuhl. “In horses as in most mammals, mitochondria are exclusively transferred from the mother to the offspring via the oocyte [egg cell] while paternal mitochondria disintegrate soon after fertilization.”

But whether a longer pregnancy is good for a foal is still open for debate. On the one hand, she says, a longer gestation length may not be advantageous for wild horses when optimal food availability may be lost or, for intensively-managed performance horses the window for breeding shrinks when a longer gestation delays conception for the timely birth of next year’s offspring. Loss of synchronization between foaling and the best environmental conditions could impair survival and development of the offspring in wild and feral horses. 

Breeders generally like to see births early in the year for horses competing in age-related events such as horse racing. On the other hand, she adds, at the Brandenburg State Stud the offspring of the mare lineage with the longest gestation length is also the most successful one. “Belantis – son of a P-mare – gained Silver at the World Championships for six-year-old dressage horses in Verden, Germany in 2015,” she says. “But there are a lot of questions to be answered in further research studies.”

Juliane Kuhl with Belantis, son of a mare from the P-lineage. Belantis won silver at the 2015 Six-Year-Old World Young Horse Championships in Verden/Aller, Germany. Photo courtesy of Juliane Kuhl.

A fascinating aspect of the research showed that the effect of the breeding month on gestation length was most obvious for mares bred very late in the breeding season.

“Despite a very concise management with most mares bred in three months’ time, March to May, our results confirm previous reports that gestation length decreases towards the end of the breeding season,” says Kuhl. “To date I can only speculate on the underlying physiological mechanisms. Alterations in day length – we do know that day length influences reproductive activity in horses - as well as nutrition around conception may contribute to this finding. An advantage of a shorter duration of pregnancy towards the end of the breeding season in non-domesticated horses might also be a longer availability of a high energy diet for the lactating mare.”

Kuhl believes that, while their study was focused on warmblood broodmares, the implications of their results would apply to other breeds. 

“In our study, we could confirm the factors already known to have an impact on gestation length, like the mare as an individual, as well as year or month of breeding and sex of the foal. A high variability in foal sex ratio in horses has also been shown for different breeds including feral horses in New Zealand as well as Camargue horses in France.”

The article “Maternal Lineage of Warmblood Mares Contributes to Variation of Gestation Length and Bias of Foal Sex Ratio” by Juliane Kuhl, K.F. Stock, M. Wulf, and Christine Aurich was published in the open-access journal PLOS ONE. 

Main photo courtesy of Juliane Kuhl.