Thursday, November 15, 2012
FULL TEXT PAPER
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Artificial Lighting: Preparing for Early Breeding
by Bradford W. Daigneault, M.S.,
Horses are long day breeders that depend on natural sun light
for reproductive function. Mares are
seasonal polyestrous animals that have multiple estrous cycles throughout the
warmer months of the year. During periods of shorter daylight and colder
weather, reproductive cyclicity in the mare becomes more irregular, eventually
leading to a period of anestrus, or lack of estrus or heat. Stallions also experience a decrease in the
number of sperm produced and variable shrinkage of the testis in response to
shorter daylight. Consequently, reproduction
in both stallions and mares slows down in the northern hemisphere during this
time of year and mares will not resume predictive and continuous estrous cycles
until around April.
The horse industry has determined that January 1st
of every year is the day at which a horse gains a year in age. As a result, foals born later in the year are
at a distinct competitive disadvantage if their biological age is less than
their registered age. This is especially
true for racing horses born in late spring or summer that find themselves
competing against other two year olds that are in fact several months more
mature both physically and mentally. For
these reasons, the optimal time to breed a mare is usually mid-February in
order to ensure that foals are born as close to January 1st as
possible. Breeding earlier than February
15th can be risky if the mare has an average gestation of 343 days
but foals out several weeks early. The
result would be a December foal that actually turns 1 year of age on January 1st
according to registry rules, but is in fact only a few days or weeks old. Mares exposed to natural day length during the
fall through winter do not typically begin to cycle normally until April. Therefore, to achieve early foals mares need
to be induced to cycle in order to selectively breed in February.
Artificial lighting is an effective method that can be used
to stimulate a mare to resume estrous earlier than what is biologically normal. Light enters through the retina and
suppresses the release of melatonin from the pineal gland thus allowing the
resumption of hormones to be released from the hypothalamus of the brain and
the anterior pituitary. GnRH (gonadotropin
releasing hormone) is released from the hypothalamus and stimulates the
production of FSH and LH (follicle stimulating hormone and luteinizing hormone)
to act on the ovary to stimulate follicular development and subsequent
ovulation of oocytes or eggs.
Mares need approximately 16 hours of continuous light
and 8 hours of darkness in order to maintain reproductive function. Unfortunately, daylight savings time renders
this part of the northern hemisphere with roughly ten or less hours of
daylight. The use of artificial lighting
in stalls or pastures can be employed to prevent the release of melatonin and
stimulate release of hormones. The light
emitted from 100 watt bulbs or 10 foot candles is sufficient for photostimulation. A general rule of thumb is that one should be
able to read a newspaper in a stall and avoid creating dark spots where the
mare can escape lighting. You can also
use a light meter for a more specific measurement.
Timers are often used to employ this tactic and can be set
to automatically turn on for 6 hours after sunset each day and then turn off at
the end of the artificial lighting period.
An alternative approach would be to keep horses in stalls from
7:00-10:00am under lights (3hrs) and then turn them out into pasture from 10:00
am to 4:00pm (6 hrs). Lights can then be
set to turn on automatically at 4:00 pm and go off at 11:00 pm (7 hrs) for a
combined total of 16 hours. Additionally,
timers can be set to activate light three hours before daylight and then three
hours after sunset. Any number of combinations
can be effective as long as adequate light is supplied. It should be noted that leaving mares under
lights for more than 16 hours in one day is of no additional benefit.
Mares generally require 60 to 90 days of photostimulation
before resumption of the first estrous cycle of the season. Therefore, lighting programs should be implemented
around December 1st to ensure the ideal situation in which a mare has a normal
estrous cycle before the first breeding attempt and that any pre-breeding
practices are accomplished in a timely manner.
Furthermore, mares will often go through a "transitional phase" as
winter ends and spring begins which can result in the development of multiple,
small anovulatory follicles and unpredictable estrous cycles. Starting mares under lights in December helps
to control the time of the spring transition and ensure a more predictable
ovulation in an environment that is suitable for maintaining pregnancy.
Although the horse industry dictates the age of foals, not
every person is concerned about when their mare will foal out. Some prefer to allow mares to cycle during
more natural times of the year (April through July) and allow their foals to be
born into warmer weather. The choice is
ultimately at the discretion of the owner.
Using artificial lighting can be an effective method for manipulating
breeding for those who require earlier foals.
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