Heifers On Pasture

Animal Reproductive Technologies

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There are two major divisions of A.I.  

1)         Fresh semen can be used for insemination after it has been collected and diluted with a “semen extender” that helps preserve viability.  Semen extenders consist of isotonic solutions of sugars, salts and macromolecule suspensions such as milk or egg yolk.  The semen can be used while still warm or it can be slowly chilled and used at a later time.  Chilled fresh semen allows for storage of semen for 2 to 5 days (depending on the species).  Chilled semen can then be shipped to remote destinations.  Chilled semen is used extensively in species where freezing (cryo-preservation) semen is not very effective, or on individual sire where semen freezing is ineffective.  Chilled semen is used most commonly in horses, dogs and swine.

2)         Frozen semen is commonly used in cattle and sheep, and less commonly in horses, dogs and rarely (experimental only) in swine.  Frozen semen is used almost exclusively in the cattle industry, due to the high success rate and efficiency with which it can be used.  Processing of semen for cryo-preservation includes extension, as with fresh semen, but in addition a cryoprotectant, such as glycerol, is added to the extended semen prior to freezing.  The semen is frozen in liquid nitrogen vapor and stored in liquid nitrogen.  The timing of frozen insemination and the placement of the semen is the same. 


Advantages and disadvantages of artificial insemination
Increased genetic influence of superior sires or popular sires, a corollary disadvantage is that rapid dissemination of faulty genetic traits is also possible.

Increased opportunity to obtain production data on offspring and make genetic evaluation of young sire prospects used to breed large numbers of females.  In dairy and beef cattle, young sire genetic evaluation programs are used extensively.

Ability to ship semen, either frozen or chilled, to anywhere in the world.

Disease control can be implemented via testing of sires for contagious and venereal diseases and by addition of antibiotics to semen for the control of specific bacterial diseases and to prevent proliferation of bacterial contamination.

Financial gain can be realized from semen sales or stud fees by the owner of a popular sires.

Cost of AI is frequently less than owning a service sire for small breeders.

The most obvious disadvantage is the amount of planning and labor required.


Timing of insemination
Ideally, identification of the LH surge would be the best method to determine the timing of AI. 

Ovulation occurs 24 to 72 hours after the LH surge (depending on species).  In practice it is necessary to estimate the occurrence of the LH surge and impending ovulation from physical signs of estrus and simple diagnostic tests. 

In all species except the canine the optimum time for insemination is 6 to 12 hours prior to ovulation since matured oocytes, that are ready to be fertilized, are ovulated.  The ovulated oocyte may only retain its capacity to develop for 6 to 12 hours after ovulation. 

In the canine the optimum time for insemination is 24 hours after ovulation since the oocyte requires this time to finish maturation. 

When using frozen semen it is more important to determine the proper time of insemination due lower total sperm cell numbers and shorter time of viability of frozen semen.  Following are recommendations for farm animals:

Duration of estrus is 4 - 7 days.  The LH surge occurs during the last half of estrus.  Ovulation occurs 30 hours after the LH surge and 6 to 24 hours prior to the end of heat.  In mares it is difficult to determine the ideal time for breeding due to variability in the long period of estrus.  The end of estrus and receptivity occurs at ovulation or shortly after.

Typically mares are breed every other day, starting on the second or third day of estrus.  Rectal palpation or ultrasonography can be used to help determine the time to commence breeding.  Timing of insemination is based on the size of the dominant follicle.  Mares usually ovulate when the follicle reaches 40 mm.  Breeding can commence when a follicle reaches 35 mm.

Duration of estrus is 8 to 18 hours.  LH surge occurs at the start of estrus.  Ovulation occurs at 24 to 30 hours after the LH surge and the start of estrus.
Breed 12 to 24 hours after the start of estrus.

Ewe and doe:
Duration of estrus is 18 to 36 hours.  LH surge occurs just prior to, or at the start of estrus.  Ovulation occurs at 20 to 30 hours after the start of estrus.
Breed 18 to 24 hours after the start of estrus.

Duration of estrus is 4 - 7 days.  Breed twice, first insemination at 12 to 18 hours after the start of estrus and a second insemination 24 hours later.  If only one insemination is used, breed at 24 hours after onset of estrus.  Ovulation occurs at 24 to 42 hours after the start of estrus

Duration of proestrus is 7 to 9 days and the duration of estrus is 9 to 10 days.  The LH surge occurs immediately prior to the start of true estrus. Ovulation is variable, occurring 36 to 72 hours after the LH surge and onset of true estrus.  Breed every other day starting on the second day of true estrus. The canine is unique among the domestic species in that primary oocytes are ovulated and must complete the first meiotic division before they are ready for fertilization.  Due to this the optimal time for insemination is 24 hours after ovulation. 

Thankfully, the timing of insemination in the bitch is not as critical as in other species.  Primary oocytes are capable of being fertilized although incorporation of the sperm cell chromosomes and formation of the male pronucleus must wait until completion of the oocytes second meiotic division.  Also, oocytes in the bitch are somewhat longer lived after completion of maturation (2 to 3 days compared to 12 hours in other species).

In the canine there are two practical methods for determining the start of true estrus or the occurrence of the LH surge. 
1)         The latest technology involves a semi-quantitative progesterone assay for detection of the first significant rise in serum progesterone to over 2 ng/ml.  This test can be run in the kennel by the inseminator.  When this test is used daily, a progesterone rises above 2 ng/ml indicates that the LH surge and the start of true estrus has occurred 12 to 36 hours prior to the sample tested.  Ovulations can begin to occur at any time after the elevation in progesterone is detected. Breeding can commence in 1 day.

2)         Vaginal cytology is the traditional method for determining the start of true estrus.  Although not as accurate as the progesterone assay it is still adequate for determining the timing of insemination.  Determination of the degree of cornification of vaginal epithelial cells is the basis for the vaginal cytology test.  Cornification of the epithelial cells is under the influence of estradiol and reaches a maximum at about 2 to 3 days prior to the start of true estrus.  Once maximum cornification is observed (90 to 98% cornified epithelial cells) breeding can commence in 2 to 3 days.


Method and location of insemination
Mare: Intrauterine transcervical.

Cow: Intrauterine transcervical.

Ewe and doe: Intrauterine transcervical with oxytocin used prior to insemination to relax the cervix and assist with intracervical deposition.  Also intrauterine laparoscopic methods are used.

Bitch: Several methods have been used with variable results; Intrauterine transcervical (very difficult), Intravaginal or intracervical, Intrauterine laparoscopic and Intrauterine laparotomy.

Intrauterine transcervical


Inseminating Dose of Motile Sperm Cells


Motile cells per inseminate

Motile cells per inseminate

Volume typically used




Fresh, split & extended: 10ml , Frozen 1ml




Frozen semen, 0.5ml straw




Fresh, entire ejaculate, Chilled 5ml, Frozen .5ml

Ovine, Caprine



Fresh or Frozen, 0.25 or 0.5ml straws

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