Effects of Longer Day Length on Pregnant Mares

By Mark Andrews

Artificial light has been used to mimic longer day length and hasten the onset of seasonal reproductive activity in mares. Extending the day length to about 16 hours is now a common management tool on horse breeding farms.

Research by Dr. Barbara Anne Murphy and colleagues showed that shorter wavelength (blue) light is particularly effective at suppressing melatonin secretion in the horse and initiating ovarian activity. This led to the development of Equilume, a device that provides low-intensity blue light (468 nm) from light-emitting diodes (LED) directed at a single eye.

Further research has now been published that shows benefits from the use of the system in mares in late pregnancy.

Foals born during the natural breeding season for the horse, which is April, May, and June in the northern hemisphere, tend to weigh more at birth than foals born in January, February, and March. The increase in day length as the natural foaling season progresses is also associated with a reduction in the duration of pregnancy.

A series of studies in Kentucky and Ireland compared mares wearing Equilume light masks in their final months of gestation with untreated mares.

The researchers found a significant difference in gestation length between mares that received light therapy and those that did not. Treated mares had pregnancies 11 days shorter on average.

A separate study found that foals from light-treated mares were, on average, 3.6 kg heavier than foals from untreated mares.

The researchers also report a significant effect on foal birth coat. The third study, conducted in Ireland, compared the weight and length of hair taken from the mane of foals within 48 hours of birth. Foals born to light-treated mares produced mane hair that was significantly lighter and shorter than that of foals from untreated mares.

The researchers comment: “Collectively, these studies serve to highlight the influential role of the circa-annual changes in photoperiod length on the pre-partum mare for normal foetal development during the natural breeding season.”

They add, “It also emphasizes the potential that exists to improve breeding efficiency parameters by artificially simulating this important environmental cue in the latter stages of gestation against the backdrop of an economically driven early breeding season.”

Printed with permission of Mark Andrews, Equine Science Update.

This article was originally published in Canada’s Equine Guide 2018, a publication of Canadian Horse Journal.