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Dry July

July 8, 2021

Its July already and time to put your dry mares “under lights” to advance the breeding season.
Mares are long day, seasonal breeders. Meaning their regular oestrous cycle occurs in response to increasing day length.
Over the winter (shortened day length) the pineal gland in the brain releases the hormone melatonin. At the same time the mare secretes less gonadotrophin-releasing hormone (GnRH) and luteinising hormone (LH). The change in these hormones results in a period of anoestrus (stops cycling). As days lengthen, melatonin production decreases and GnRH secretion increases causing the anterior pituitary gland to release LH. These changes allow the mare to transition back to normal cyclicity.
The transitional period (between winter anoestrus and oestrus) can last 60-80 days. It is characterized by multiple waves of follicular growth and regression and can be associated with erratic oestrus behaviour. These transitional heats are not fertile and can be frustrating for broodmare managers and vets.
Factors other than day length, that affect duration of anestrous and onset of spring transition include; latitude, climate, age, body condition and nutrition. Mares in good body condition begin cycling on average a month earlier than mares in poor condition. Similarly, an increasing plane of nutrition or grazing green pasture is associated with earlier returns to cycling. Younger mares cycle earlier than older mares. Ambient temperature is also a factor.
A New Zealand study looking at reproductive performance of Thoroughbred mares in the Waikato region, showed light exposure significantly increased the odds of dry mares conceiving by the end of the season. This occurred due to a combination of an increased first cycle pregnancy rate and increased number of service opportunities in a season. The start of mating to conception interval was also shorter in dry mares under lights.
Light protocol
An artificial light protocol should begin 60 days prior to the start of the mating as mares still need to transition. It is generally agreed that a minimum of 14.5 hours of continuous light, at an intensity of 107 lux (100-watt bulb) is required.
A common regime is to extend light exposure at the end of the day giving 15-16 hours of light followed by 8-9 hours of dark. This is easily implemented by housing or yarding mares
in the evening with lights coming on before dusk and with a timer to turn them off a few hours later.
Alternatively, breeders may choose to use a light mask such as an Equilume, which directs light to a single eye. These masks are programmed to turn on before dusk and off 5-6 hours later. The masks enable mares to be managed at pasture, reducing labour, bedding and feed costs associated with indoor housing. Under New Zealand conditions this can be a good alternative to housing and yarding mares for light exposure.
Providing artificial light to simulate increased day length before the start of breeding in dry mares is relatively easy and can significantly improve reproductive performance.
A filly that starts her breeding career under lights and is placed under lights when dry (not in foal) has on average 3 more foals over the span of her breeding career.
Rochelle Kerr BVS, BAgSci, DipAgSci CVA