Season of birth can affect foal size; stress influences performance
Ahorse’s metabolism can slow down when the weather gets cold. It’s one way the horse’s body can realign resources to stay warm. With modern breeding practices often manipulating mares’ natural reproductive cycles to deliver winter foals (born in February to early March), a group of researchers wondered if that metabolic reduction could affect fetal development.
The researchers, led by Christine Aurich, DVM, PhD, Dipl. ECAR, of the University of Veterinary Medicine Vienna, and Elisabeth Beythien, Tierarzt (a veterinarian from a German university) of the Graf Lehndorff-Institute for Equine Science, studied 27 broodmares and their foals. All the mares had similar diets and a similar number of previous foalings. Gestation length and the sex ratio of foals did not differ.
The mare–foal pairs were grouped by foaling date: Group 1 foals were born between February and early March; Group 2 between early March and early April; and Group 3 from mid-April to May.
The researchers measured the size and weight of each mare’s placenta at birth. The measurements were smaller in the winterfoaling mares than mares delivering later. This indicates a reduction in nutrient transfer to the fetus, which could influence fetal growth.
A number of size parameters were also recorded for each foal weekly from birth to 12 weeks of age. Researchers found that most parameters—including height at withers, distance from fetlock to carpal joint and length from poll to nose— were lower in the earliest foaling group compared to the other groups. Foal weight did not differ among the groups.
“The size difference of the early-born foals was present over the whole observation period, i.e., until 12 weeks of age,” says Dr. Beythien. After that time, the mares and foals were sent out to pasture and could not be easily measured.
Despite the initial size disparity, Dr. Beythien notes that earlyborn foals will be taller and larger than later-born foals during the first few months of life simply due to the difference in age. “If you imagine a foal presentation scheduled in June, foals born in March will be taller than foals born in May because they are approximately eight weeks older at this time,” she explains.
For this reason, breeders often feel early-born foals have an advantage in young-horse competitions at 3 to 4 years of age. Ultimately, though, breeding mares too early in the year could be detrimental to the foals’ mature size, says Dr. Beythien. Based on other studies, she adds, the lower energy supplied to the winter foals during gestation may also contribute to health problems during adult life. Therefore, the team concluded, breeders may be better off aiming for foals to be born during the natural foaling season (typically between April and September) when they’ll “experience a healthier environment during gestation,” says Dr. Beythien.
Yes, Stress Influences Performance
You may have wondered whether stress can really detract from your horse’s performance. Now there’s scientific proof that it can, according to a study conducted by Aleksandra Gorecka-Bruzda, PhD, DSc, and colleagues at the Institute of Genetics and Animal Breeding of the Polish Academy of Sciences.
The scientists set out to objectively determine if there was a relationship between sporthorse performance and behavior and physiological stress indicators. The study included a test group of 19 equine show-jumping competitors, six of whom were housed at the institute and 13 of whom were transported. Another five horses remained in their home training routine and served as controls.
The test horses were grouped according to jump height: “light,” with obstacles equal to or less than 100 cm (about 3-foot-3) and “difficult,” with obstacles over 100 cm. The researchers measured salivary cortisol concentration, a physiological indicator of stress, before a first jumping round as well as 20 minutes and one hour after a second jumping round.
For both rounds, they also evaluated conflict behaviors—signs of resistance in-
dicating mental or physical discomfort. These included headshaking, yanking on the reins and tail-swishing. Faults, i.e. refusals and knockdowns, of each horse were totaled for both rounds and the animals were classified in “less faults” class if they made less than or equal to 1 (median value) fault, or as “more faults” if they exceeded this value (more than one fault).
The researchers found that salivary cortisol concentration at the 20-minute post-jumping mark was higher in the “more faults” group and among horses who had been transported. Conflict behavior was observed more frequently in horses with more faults and among those jumping the taller fences. Also, horses who waited longer to jump their second round showed more conflict behaviors.
In short, horses showing more stress responses were less successful performers. Thus, the team concluded, finding more effective ways to reduce the stress of transport and competition itself could both improve your horse’s welfare and his competitive performance. —— Sushil Dulai Wenholz
Researchers have found that winter foals (those born in February to early March) tend to be smaller at birth than those born during the natural foaling season (between April and September).
In a recent study, scientists measured signs of resistance in horses, including headshaking, yanking on the reins and tail-swishing, to determine the extent that stress has on their performance.