Reproductive Biology of Mares

Question 1

Winter anestrus

Answer 1

-ovaries are small and hard
-absence of ovulation
-low levels of reproduction hormones (may occasionally show signs of heat though)

Question 2

Phases of reproductive rhythm of non-pregnant mares:

Answer 2

-winter anestrus
-spring transition
-estrous cyclicity
-fall transition

Question 3

When does winter anestrus begin?

Answer 3

late fall (about November)

Question 4

Spring transition

Answer 4

-some ovarian activity
-small follicles (may partially develop, then recede)
-no ovulation
-inconsistent estrus behavior

Question 5

Estrous cyclicity (the breeding season)

Answer 5

-begins with the first ovulation
-regular estrous cycles (estrus and diestrus)
-large ovaries

Question 6

When does estrous cyclicity end?

Answer 6

with pregnancy or with the onset of anestrus

Question 7

Fall transition

Answer 7

-changes in ovarian activity
-hormonal changes (less estrogen from follicles, less progesterone from CL)
-regular cyclicity deminishes
-ovulation ceases, anestrus begins

Question 8

When does the spring transition begin?

Answer 8

early-mid February (about 6-8 weeks after Dec 20th)

Question 9

When does the spring transition end?

Answer 9

late March-early April (lasts about 6-8 weeks)

Question 10

When does regular cyclicity begin?

Answer 10

with the first ovulation after the pring transition

Question 11

When does regular cyclicity end?

Answer 11

mid August to early September

Question 12

What is the approximate time of the “natural breeding season”

Answer 12

regular cycles from late March - early April until mid August - early September

Question 13

When does the 1st normal ovulation occur?

Answer 13

late March - early April
(will then foal in late Feb/early March)

Question 14

What is the 1st normal ovulation influenced by?

Answer 14

natural day length

Question 15

Natural breeding season is:

Answer 15

April into August

Question 16

Commerical breeding season is:

Answer 16

about Feb 15 - mid JuneA

Question 17

Why start the breeding season sooner?

Answer 17

-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?)

Question 18

Disadvantages to starting the breeding season sooner:

Answer 18

-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)

Question 19

Seasonality is primarily governed by what?

Answer 19

day length (and mare body condition)

Question 20

What does day length affect?

Answer 20

melatonin production by the pineal gland

Question 21

Melatonin increases during ______

Answer 21

the dark

Question 22

Melatonin decreases during ______

Answer 22

the light

Question 23

During the winter melatonin concentrations are…

Answer 23

elevated for long periods during longer nights

Question 24

What does melatonin do?

Answer 24

it suppresses gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus
-low GnRH, cyclicity is inhibited in mares

Question 25

Low melatonin levels allows the hypothalamus to...

Answer 25

increase GnRH secretions

Question 26

GnRH stimulates the anterior pituitary to release...

Answer 26

follicle stimulating hormone (FSH) and luteinizing hormone (LH) which act on the ovary

Question 27

Advancing the breeding season

Answer 27

increase "day-length" (hours of daylight) ahead of natural season

Question 28

What is the goal of advancing the breeding season?

Answer 28

to suppress melatonin, allowing GnRH to increase ahead of natural season

Question 29

Increasing day length can be used to:

Answer 29

-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

Question 30

What are the normal hours of daylight in Central KY in late December?

Answer 30

about 9 hours

Question 31

What are the normal hours of daylight in Central KY in late March?

Answer 31

about 12 hours

Question 32

What is the method to manipulating day length?

Answer 32

Impose 16 hours light/8 hours dark -may increase gradually or just start 16:8 -usually add hours at the end of the day

Question 33

What is the timing to manipulating day length during the year?

Answer 33

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)

Question 34

Mares being kept in a barn for imposed day length:

Answer 34

16 hours inside, then outside during the day

Question 35

Advantages of mares being kept in a barn:

Answer 35

-highly controlled (cannot access any dark areas in the stall)

Question 36

Disadvantages of mares being kept in a barn:

Answer 36

-labor (horse handling and cleaning) -lots of stalls and lots of bedding -expense of lights, electric, timer, etc. -must bring in stall before dark

Question 37

Group lighting for imposed day length:

Answer 37

mares are kept in a group outside, in a smaller paddock or dry lot where there are lights surrounding it

Question 38

Advantages of group lighting

Answer 38

-less labor; stall cleaning -less horse handling -ventilated and social -don't need stalls

Question 39

Disadvantages of group lighting:

Answer 39

-expensive to build (lights) -condition of lot (can get muddy)

Question 40

Light mask (Equilume) for imposed day length:

Answer 40

mares where a mask that has the left eye open, and the right eye is in a cup with an LED blue light shining

Question 41

Advantages of the light mask:

Answer 41

-mares can stay in pastures -low labor -cost? (upfront cost could end up equaling cost of bedding/stalls for other methods)

Question 42

Disadvantages of the light masks:

Answer 42

-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)

Question 43

Imposed lighting, lengthening daylight can also be used for:

Answer 43

-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)