Reproductive Biology of Mares Flashcards
Winter anestrus
-ovaries are small and hard
-absence of ovulation
-low levels of reproduction hormones (may occasionally show signs of heat though)
Phases of reproductive rhythm of non-pregnant mares:
-winter anestrus
-spring transition
-estrous cyclicity
-fall transition
When does winter anestrus begin?
late fall (about November)
Spring transition
-some ovarian activity
-small follicles (may partially develop, then recede)
-no ovulation
-inconsistent estrus behavior
Estrous cyclicity (the breeding season)
-begins with the first ovulation
-regular estrous cycles (estrus and diestrus)
-large ovaries
When does estrous cyclicity end?
with pregnancy or with the onset of anestrus
Fall transition
-changes in ovarian activity
-hormonal changes (less estrogen from follicles, less progesterone from CL)
-regular cyclicity deminishes
-ovulation ceases, anestrus begins
When does the spring transition begin?
early-mid February (about 6-8 weeks after Dec 20th)
When does the spring transition end?
late March-early April (lasts about 6-8 weeks)
When does regular cyclicity begin?
with the first ovulation after the pring transition
When does regular cyclicity end?
mid August to early September
What is the approximate time of the “natural breeding season”
regular cycles from late March - early April until mid August - early September
When does the 1st normal ovulation occur?
late March - early April
(will then foal in late Feb/early March)
What is the 1st normal ovulation influenced by?
natural day length
Natural breeding season is:
April into August
Commerical breeding season is:
about Feb 15 - mid JuneA
Why start the breeding season sooner?
-foals are born earlier (older at yearling age)
-mare has more cycles, more opportunity to breed if cycle is missed
-increases season length/stallions can breed more mares
-spreads out foaling season (reduce crowding?)
Disadvantages to starting the breeding season sooner:
-poor weather, mare and foal may not get to go outside enough
-not as high quality pasture
-a spread out foaling season requires longer foal watch (more labor needed for longer)
Seasonality is primarily governed by what?
day length (and mare body condition)
What does day length affect?
melatonin production by the pineal gland
Melatonin increases during ______
the dark
Melatonin decreases during ______
the light
During the winter melatonin concentrations are…
elevated for long periods during longer nights
What does melatonin do?
it suppresses gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus
-low GnRH, cyclicity is inhibited in mares
Low melatonin levels allows the hypothalamus to…
increase GnRH secretions
GnRH stimulates the anterior pituitary to release…
follicle stimulating hormone (FSH) and luteinizing hormone (LH) which act on the ovary
Advancing the breeding season
increase “day-length” (hours of daylight) ahead of natural season
What is the goal of advancing the breeding season?
to suppress melatonin, allowing GnRH to increase ahead of natural season
Increasing day length can be used to:
-end winter anestrus sooner
-begin spring transition sooner (may/may not shorten the spring transition period)
-regular cyclicity begins sooner
-total length of breeding season is extended
What are the normal hours of daylight in Central KY in late December?
about 9 hours
What are the normal hours of daylight in Central KY in late March?
about 12 hours
What is the method to manipulating day length?
Impose 16 hours light/8 hours dark
-may increase gradually or just start 16:8
-usually add hours at the end of the day
What is the timing to manipulating day length during the year?
Initiate 16h light:8h dark in late November-mid December
-allows breeding season to begin mid Feb. (about 120 days)
-instead of early April (natural breeding season) (about 75 days)
Mares being kept in a barn for imposed day length:
16 hours inside, then outside during the day
Advantages of mares being kept in a barn:
-highly controlled (cannot access any dark areas in the stall)
Disadvantages of mares being kept in a barn:
-labor (horse handling and cleaning)
-lots of stalls and lots of bedding
-expense of lights, electric, timer, etc.
-must bring in stall before dark
Group lighting for imposed day length:
mares are kept in a group outside, in a smaller paddock or dry lot where there are lights surrounding it
Advantages of group lighting
-less labor; stall cleaning
-less horse handling
-ventilated and social
-don’t need stalls
Disadvantages of group lighting:
-expensive to build (lights)
-condition of lot (can get muddy)
Light mask (Equilume) for imposed day length:
mares where a mask that has the left eye open, and the right eye is in a cup with an LED blue light shining
Advantages of the light mask:
-mares can stay in pastures
-low labor
-cost? (upfront cost could end up equaling cost of bedding/stalls for other methods)
Disadvantages of the light masks:
-has to stay on the mare
-cost? ($700-1000 each)
-vision? (may not be able to fully see out of the right eye, but no permanent damage that we know of)
Imposed lighting, lengthening daylight can also be used for:
-stallions (sperm production may be lower in general in the darker months)
-pregnant mares (to try to shorten their gestation length)
-performance horses (some people think it affects their performance and they shed in response to daylight so coat length)
