During long days the inhibitory action of melatonin
on the mare’s reproductive axis is lifted and increased
gonadotropin releasing hormone pulse frequency
stimulates the anterior pituitary to release follicle
stimulating hormone and luteinising hormone, which
act in concert to promote the growth, development
and ovulation of ovarian follicles [9–11]. The northern
hemisphere specifies a universal birthday for
Thoroughbred racehorses of 1 January. Breeders
therefore desire foals born early in the year to
produce mature yearlings and precocious 2-yearold racehorses. Studies show that annual earnings
are significantly higher for horses born in January–
February than for those born in April–June [12,13]. For
breeders to satisfy industry timelines, it is necessary
that they manipulate the mares’ reproductively active
period to meet the official start date of the breeding
season on 15 February.
Beginning 1 December, it has become standard
industry practice to extend day length for 8–10
weeks by exposing mares to artificial light until 23.00
h, allowing natural dawn to occur during the most
sensitive phase of the 24 h cycle [4]. An artificial
photoperiod of 16 h light: 8 h dark is facilitated using
light from a 100 W light bulb in a 3.6 °— 3.6 m stall
[3,14], loosely described as ‘enough light to read a
newspaper’. Light therapy such as this can advance
the breeding season by as much as 3 months [15].
Recent research indicates that light within the
short-wavelength spectrum (465–485 nm) is most
effective at inhibiting melatonin [16] as melanopsin is
particularly sensitive to short wavelength, blue light
[17,18].
A recent study in Thoroughbred mares has shown
that melatonin can be successfully inhibited using
low-level blue light (465 nm) from a light emitting
diode (LED) source directed at a single eye [19]. We
aim to determine if timed, low-intensity, blue light
administered to a single eye from head worn light
masks is as effective as indoor maintenance under
white light at advancing the breeding season in
mares.
Materials and methods
Animals
All experimental procedures were approved by
the University College Dublin Animal Research
Ethics Committee and the University of Kentucky
Institutional Animal Care and Use Committee protocol
number 2012-0928.
Healthy, Thoroughbred mares (n = 59), all aged
between 5 and 24 years, were used for this experiment.
All mares included in the study had a body condition
18
score between 4 and 6 using the Henneke body
condition scoring system [20] and there was no
difference in average body condition score between
groups (Group 1, 5.1 °” 0.7; Group 2, 5.1 °” 0.5; Group
3, 4.8°” 0.5, P>0.05). The experiment took place on 2
farms located in Fayette County, Lexington, Kentucky.
Group 1 (n = 16) was kept in a large pasture during the
day and housed indoors in individual stalls at night.
Group 2 (n = 25) was maintained in 2 groups in large
pastures during the day and night.
Group 3 (n = 19) was maintained on a separate farm,
within an 8 km radius of the first, as a group in a large
pasture during both day and night. There was no
difference between the age ranges of mares within
the different groups (Group 1, 12.6 °” 4.9; Group 2, 13.1
°” 4.4; Group 3, 11.4 °” 4, P>0.05).
The experiment took place from 20 November 2011 to
10 February 2012 at longitude W -84.4° and latitude N
38.1°, which experienced temperatures that ranged
from -10.6 to 20.6°C during this period. During the
experiment Group 1 grazed ad libitum during the day
and had ad libitum access to hay at night. In addition,
this groupwas supplemented with either 2.7 kg/day
of a low-starch diet; Purina Wellsolve Lo