Heifers On Pasture

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The selection of donor cows is a process in which numerous factors will determine the ultimate success or failure of an embryo transfer program.  The most important factor is that the donor is truly a superior animal, phenotypically and genetically.  Although not essential, reproductive soundness of the donor is important to consider.  The factors to be considered are listed below.


Phenotypic selection

In beef cattle, donor selection can be based on several type and production traits.  The type traits can include general conformation, proportion, height (frame score), mature body weight, scrotal circumference, udder type, pelvic measurements and carcass traits such rib eye cross section area (muscle size), marbling, back fat thickness and yield grade or percent retail product. Dairy cattle, of coarse, are also selected based on milk production traits such as total milk production, butter fat content and protein content.


Many registered and commercial cattle breeders obtain individual animal data for production and body type.  This data is used to calculate a group index.  These indexes rank the cows or their calves, within the herd, or more specifically within a contemporary group, for many production traits.  A group index for a particular trait (such as weaning weight) will rank an individual as a percentage above or below the average within its contemporary group.  The trait average is assigned an index of 100.  An animal that performs at 110% of average will be assigned an index 110.  An inferior animal will have an index of 90, because it only performed at 90% of average.  Individual animal measurements and the trait indexes calculated from them are used for breeding animal selection within a herd.  Since a group index will rank all animals in a contemporary group from top to bottom, it is easy to select the top 10% of a male contemporary group as sire prospects, or the top 33% of a female contemporary group as replacement heifers.


Genetic selection

Cattle breeders can increase selection power by utilizing genetic statistics that incorporate production data not only for the individual in question, but also production data from its offspring, ancestors, siblings and other relatives.  Breed associations, Dairy Herd Improvement Associations (DHIA), the USDA, and other organizations that have access to data from large numbers of cattle, can calculate genetic statistics for the individual animal based on its own performance and also the performance of all its recorded relatives.  Genetic statistics are very helpful in making breeding decisions. Donor cows (and the bulls that are used for embryo production) are selected for calving ease traits, pre-weaning growth, yearling weight, carcass traits, maternal traits and milk production.


Individual animal data (phenotypic data) is submitted to breed associations, DHIA, etc. on thousands of animals from many different herds. This data, combined with data from related animals already submitted over time, is used in calculations that result in statistical predictors of an animals performance and more importantly the performance of those animals progeny.  These statistics are called “Expected Progeny Difference” (EPD) for beef cattle and “Predicted Transmitting Ability” (PTA) for dairy cattle.  EPD and PTA statistics are published in sire summaries for most popular sires.  Breeders can obtain EPD or PTA statistics for every animal in their herd.  EPD and PTA data tends to become more accurate as an animal gets older since the calculations are updated with production data from the individuals offspring.  The accuracy is expressed as a decimal percentage.  .98 is a very high accuracy score for an EPD. 

Embryo donor and sire candidates can be selected based on group indexes and also EPD scores for birth weight, weaning weight and yearling weight, maternal ability (milking ability) and carcass traits, or in dairy PTA scores for total milk, total fat, total protein, percent fat and percent protein.  In dairy cattle there is also a genetic statistic called a “Total Production Index” (TPI) which ranks a cow on overall genetic merit.


It is rare that an animal will excel in all production traits mentioned above.  When an animal is evaluated as a embryo donor (or AI sire) prospect, a breeder will usually choose a few traits that are of import within his herd for genetic improvement or for cattle marketing purposes.  Animals that excel in these traits (even if only average in other traits) are selected to effect genetic improvement for those traits.


Donor's reproductive status

Once genetically qualified cows have been selected as donor candidates, final selection of the animals should also include reproductive and general health exams, and health test results.  Other criteria may become apparent on an individual animal basis.  It is essential to have good restraint and adequate help when performing this physical exam.  It is helpful to have help for sampling, control of the animal during examination and for thorough record keeping, etc.  Each donor should pass a physical exam of all organ systems and the reproductive tract to assure general health and sound reproductive status.  Some particular aspects of the reproductive system exam follow.  Some of the conditions mentioned below do not effect the cows ability to conceive but do affect her usability and success as an embryo donor.  Virtually none of the conditions listed below are not absolute reasons to exclude a cow as a donor but they should still be considered as donor selection is occurring.  Some of the defects listed below, such as urine pooling, short cervix and excessively long vagina / deep uterus may affect fertility and can be considered genetic traits that should not be propagated.

A. Crooked cervix, if extreme, may be a hindrance to flushing. This condition is seen frequently in Holsteins and Zebu breeds.

B. Prolapsed cervical rings.  Appears as a caudal enlargement of the cervix.  May be reason to cull a cow as a donor if it is also associated with a shortening of the cervix.  Can act as a source of chronic infection of the cervix, hindrance to flushing and can also compromise the cervical seal and result in chronic metritis and early embryonic loss if severe.

C. Short cervix.  See above under prolapsed cervical rings

D. Cervicitis is detectable by irregularity in the size and consistency of the cervix and presence of vaginal discharge.

E. Vaginitis. From a myriad of specific and nonspecific causes such as IBR/IPV,

F. Ureoplasma, Mycoplasma and non-specific infections.  If present it is reason to expel an animal from the donor program until resolved.  The vulvar lips are always parted and the vulva and vaginal vestibule are examined during a reproductive exam in order to uncover these conditions.  A speculum may be used to perform a vaginal and cervical exam

G. Metritis, Pyometra, Mucometra are all conditions which interfere with fertility.  Cows that have fluid in the uterus, regardless of cause should be diagnosed and treated prior to use as a donor.

H. Pregnancy.  If pregnancy has not advanced beyond 90 days the donor candidate may be aborted using prostaglandin F2 alpha.  Time for at least one natural estrus cycle prior to synchronization of estrus for flushing should be allowed (30-40 days).  Pregnant cows beyond the first trimester should be skipped over as donors and used the following year.

I.          Urine Pooling.  Frequently seen in cows with: a tipped forward pelvic conformation, restricted vulva, sacroiliac luxation, pelvic ligament damage resulting in elongation of the vestibule and vagina (usually caused by calving related injuries and dystocia). These cows will pass excess yellow stained mucous when the vagina is expressed caudally or when mounting another cow.  It is a good idea to check for this condition if a cow with a very deep uterus is examined.  This condition may require surgery before successful embryo recoveries can be performed.  This may be adequate reason to expel a donor candidate.

J.          Deep Uterus.  Cows with deep uteri and long vaginas are sometimes difficult to flush embryos from.  This is not to say that they have any fertility problems or are undesirable for routine breeding purposes.  They are less than ideal donor but still may be used as long as the deep uterus is not caused by a pathological condition.  Problem can results from trauma to the endometrium and cervix resulting from excess manipulation required to flush embryos from a cow with this type of uterine conformation.


Donor selection schedule

At least 4 to 6 weeks before the anticipated date of the first embryo collection it is necessary to select the donor cows, whether embryos are to be transferred fresh or frozen.  As soon as possible all candidates should be examined, receive vaccinations, parasite treatments and export health testing to verify that the donor meets the importing country’s import requirements if the embryos are to be exported.  Vaccinations should be given at least 30 days prior to the estrus period in which the donor is bred for embryo production.  In addition, it is advisable to deliver the donor to the embryo collection center a month prior to embryo collection so that she has time to acclimate.



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