AVS 466 Horse Science and Management

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AVS 466 Horse Science and Management

• Dirk K. Vanderwall
• March 26, 2008
• Mare Reproductive Physiology

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Female Reproductive Physiology
Hypothalamus
GnRH
LH
FSH
Progesterone
Estrogen
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Hypothalamus
GnRH
Pituitary Gland
LH
FSH
Ovaries
Progesterone
Estrogen
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Reproductive Seasonality

• Physiological breeding season
• Ovulatory season
• April through September
• Physiological non breeding season
• Anovulatory season (anestrus)
• December and January
• Transitional period
• Spring February and March
• Fall October and November

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Reproductive Seasonality
Adapted from Ginther, 1974
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Reproductive seasonality is regulated
primarily by changes in day length
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Photoperiod Effect

• Reproductive activity in the spring is
  stimulated by increasing day length
• Mechanism involves alteration of hormone
  secretion by the pineal gland and
  hypothalamus
• Pineal gland melatonin
• Hypothalamus GnRH
• Artificial lighting can be used to hasten
  the onset of the ovulatory season

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Artificial Lighting

• Type of light
• Brightness of the light
• Stall vs. paddock lighting
• Timing of the artificial lighting
• When to start providing the artificial
  light (date) ?
• How much artificial light to provide each
  day and when during the day ?
• Step wise method
• All at once method
• Skeleton method

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Physiological Non-Breeding Season(Winter
Anestrus / Anovulatory Season)

• Minimal follicular development
• Secretion of GnRH, FSH and LH are low
• Estrogen and progesterone secretion are low

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Physiological Non-Breeding Season(Winter
Anestrus / Anovulatory Season)

• Minimal follicular development
• small, firm ovaries
• Follicles lt 15 mm in diameter
• Mares may exhibit estrous behavior (heat)

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Transitional Period

• Increased day length stimulates GnRH
  secretion from the hypothalamus
• GnRH stimulates FSH and LH secretion
• FSH induces follicular development
• Estrogen produced by developing follicles
  causes the mare to exhibit irregular and /
  or prolonged estrus
• When LH levels have increased, the first
  ovulation of the year occurs

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Transitional Period

• 1 to 3 waves of follicular growth and
  regression
• Irregular or prolonged estrus
• Enlarged ovaries with multiple clustered
  follicles

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Transitional PeriodFollicular Dynamics
from Dr. Pat McCue
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Physiological Breeding Season

• 21 day inter-ovulatory interval (estrous
  cycle)
• Estrus (heat) 5 7 days
• Ovulation 24 to 48 hours prior to end of
  heat
• Diestrus 14 16 days

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Follicular Dynamics

• 1 or 2 waves of follicles emerge during
  each estrous cycle
• Each wave consists of a dominant follicle
  and several subordinate follicles

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Follicular Dynamics
Single primary wave
Primary wave and non ovulatory secondary
wave
Primary wave and ovulatory secondary wave
Ginther, 1995
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Ovulation

• LH surge induces final maturation of the
  follicle and ovulation

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Corpus Luteum Formation

• Two ultrasonographic luteal morphologies have
  been described
• uniformly echogenic
• centrally non echogenic
• corpus hemorrhagicum (CH)
• Granulosa and theca cells luteinize, and
  begin secreting progesterone
• There is no functional difference between
  the two luteal morphologies

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Corpus Luteum Function

• The primary function of the CL is the
  secretion of progesterone, which is
  necessary for the maintenance of pregnancy

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Corpus Luteum Regression

• Nonpregnant mares must regress their CL in
  order to initiate another estrous cycle
• The duration of CL function in the
  nonpregnant mare is 14 to 16 days

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Prostaglandin Secretion

• In nonpregnant mares, prostaglandin F2alpha
  is secreted in a pulsatile pattern from
  the uterus on days 14 to 16 after
  ovulation, which eliminates function of the
  CL

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Hormonal Profile of the Estrous Cycle
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