The Cow-Calf Manager
Livestock Update, November 2001
John Hall, Extension Animal Scientist, Beef, VA Tech
Understanding Reproductive Evaluations of Replacement Heifers
It's that time of year, when many producers are selecting heifers for replacements, decisions made now can have a big impact on the future of your herd. Whether you are raising your own replacement heifers or purchasing heifers, it is important that heifers have a proper reproductive evaluation and are bred properly. Understanding what the results from each of these procedures mean will make you a better heifer producer or buyer.
Reproductive evaluations give an indication of:
Before discussing results of reproductive exams, a quick nutrition and management reminder. Even heifers that pass their reproductive exams with flying colors can become reproductive problems if they are not fed or managed properly. Heifers must be fed to gain 1.5 to 1.75 lbs. per day from weaning to breeding so they will grow and develop properly. In order to be big enough to calve and breed back, bred heifers must also gain 1.5 lbs. per day from breeding until 60 days before calving and 2.5 lbs. per day for the last 60 days before calving.
A complete heifer reproductive exam or information should include:
Body Weight and Condition Score
The most important factors affecting reproductive performance in heifers are body weight and body condition. Many other measurements such as RTS and pelvic area are related to body weight. Heifers should weigh 65% of their mature weight at breeding and 85-90% of their mature weight at calving. At all times, heifers should be BCS 6 or 7. By reaching these body weights and condition, nutrition will not limit conception, calving ease, and re-breeding.
Reproductive Tract Scores
Reproductive tract scores are measured at 12-14 months of age, immediately before the start of the breeding season. Heifers are palpated and given a score from 1 to 5. A score of 1 indicates an infantile reproductive tract whereas a score of 5 indicates that the heifer is already cycling. Reproductive tract scoring will identify infertile or sub-fertile heifers that should be culled. It is an especially important procedure to perform on heifers of unknown origin, heifers that may have been implanted, and heifers that are to be sold as open heifers.
Even though RTS is highly related to body weight and age, heifers still a RTS because even big fast growing heifers can be infertile. Heifers that score a 4 or 5 will have higher pregnancy rates than heifers that score lower (Table 1). Heifers that score a 1 should not be retained for breeding, as they are infertile. Heifers that score 2 may be kept for breeding but will probably breed late in the breeding season. This is the reason that only heifers with RTS of 3 or better are allowed to sell in the VA Premium Assured Sales as open heifers. Heifers with a high RTS breed back sooner as 1st calf heifers.
Table 1. Description of Reproductive Tract Scores and Their Relationship to Fertility
|Score||Uterine Horns||Ovaries||% Pregnant to Estrus Synchronization and AI||Total Pregnant, %|
|1||Immature, <20 mm diameter, no tone||15 mm x 10 mm x 8 mm, no structures||2.6||26.2|
|2||20 - 25 mm diameter, no tone||18 mm x 12 mm x 10 mm, 8 mm follicles||22.6||74.2|
|3||20 - 25 mm diameter, slight tone||22 mm x 15 mm x 10 mm, 8 - 10 mm follicles||39.5||76.2|
|4||30 mm diameter, good tone||30 mm x 16 mm x 12 mm, > 10 mm follicles, possible corpus luteum||54.6||94.1|
|5||> 30 mm diameter, good tone||> 32 mm x 20 mm x 15 mm, corpus luteum present||55||85.0|
High reproductive tract scores are no guarantee that all heifers will get pregnant. Remember, pregnancy rates of 85 to 90% are the biological norm. So, about 1 in 10 or 1 in 15 heifers will be open at the end of the breeding season. If over 90% of your heifers get pregnant, then you should celebrate and give that bull a little extra grain!
Pelvic Area Measurement (PA)
Pelvic area of a heifer at calving will dictate the size of calf she can deliver. Extensive research from Nebraska and Montana indicates that a maximum deliverable calf weight can be predicted from pelvic area measurements taken at either 12 to 14 months of age (prebreeding) or 18 months of age (pregnancy exam). However, pelvic areas are of little use if the birth weight of the calf cannot be controlled or predicted. In addition, selecting heifers with larger pelvic areas result in retaining heifers with a larger overall frame size.
Pelvic areas should be used as a culling tool not a selection tool. Research indicates that heifers with PA smaller than 140 sq. cm. prebreeding or 180 sq. cm. at pregnancy exam have a high percentage of calving difficulty. I recommend that heifers with pelvic areas of < 150 sq. cm. (prebreeding) or < 190-200 sq. cm (preg. check) not be kept.
Selection of A Calving Ease Bull
The most important "examination" that a heifer can be given to reduce calving difficulty is to examine the birth weight EPD (BW EPD) of the bull she is (or will) be bred to. In general, BW EPD should be below the average for the breed for purebred heifers. For commercial heifers, BW EPD should be below the average for the breed of sire and the sire be a low birth weight (or calving ease) breed.
In the VA Premium Assured Heifer Program, commercial heifers are required to be bred to bulls in the top (lightest) 40% of their breed for birth weight. In addition, across breed EPDs are used to generate the desired maximum birth weight EPDs for breeds other than Angus. In reality, only Angus, Hereford and Red Angus bulls have EPDs that meet the VA PAH requirements for birth weight for commercial heifers (Table 2).
Table 2. Birth Weight EPD requirements for bulls used on VA Premium Assured Commercial Heifers.
|BW EPD||+ 2.3||- 9.7||- 5.5||- 2.3||- 1.4||- 5.5|
Accuracy is also very important. The accuracy of an EPD is an indication of how likely the EPD is to change. The higher the accuracy the less likely the EPD is going to change. Until a bull has a lot of calves, his EPD could change drastically. Table 3 shows the possible change associated with different accuracy levels for the Angus breed.
Table 3. Accuracy and Associated Possible Change in Birth Weight EPD
|Possible Change in BW EPD||2.6||2.45||2.35||2.2||2.0||1.8||1.6||1.4||1.2|
For example, commercial heifers are bred to two different Angus bulls with the same BW EPD of +1.0. One bull is an AI bull named Art. The other is a natural service bull named Nat. Art has an accuracy of .90 while Nat has an accuracy of .20. So what does this mean to the heifer and the producer that has to calve her?
Given his accuracy, Art's "true" BW EPD could be as low as -0.2 or as high as +2.2 or it could stay the same. On the other hand, given Nat's accuracy, his "true" BW EPD could be as low as -1.45 or as high as +3.45 or it could stay the same.
The heifers bred to Art are less likely to have large calves. At worst, Art would produce calves like a bull with a BW EPD of +2.2 which is OK for heifers. However, at worst Nat would produce calves like a bull with a BW EPD of +3.45, which could mean pulling quite a few calves.
The bottom line - heifers bred AI to low birth weight EPD bulls with high accuracy are more likely to have less problems with surprises in the form of high birth weight calves. So, when breeding heifers natural service with young bulls, select bulls with BW EPDs closer to 0 than +2.0. That should reduce unpleasant surprises.
For a comprehensive review of EPDs and how to use them see "Understanding EPDs" by Dr. Scott Greiner in Livestock Update Sept.-Nov. 2000 at http://www.ext.vt.edu/news/ or in past issues of the VA Cattleman.
Pregnancy Diagnosis With Fetal Aging
The final reproductive exam that most heifers receive is pregnancy diagnosis. Fetal aging or predicting calving dates also needs to be a part of this exam and it needs to be performed by an experience veterinarian. In order for fetal aging to be accurate, it should be performed when the cows are between 45 and 100 days of pregnancy. With ultrasound, a range of 30 to 90 days can be used. Fetal aging will give the predicted calving date as well as an indication of heifer fertility.
Heifers should conceive in the first 50 days of the calving season and the first 30 days is better. Heifers that conceive early are more fertile and will breed back more rapidly as 1st calf heifers. In addition, having heifers calve in a relatively short time frame allows them to be observed more closely for signs of calving difficulty. Heifers that calve in a tight group tend to receive more attention and produce more live calves.
Finally, if you are purchasing heifers, it is highly recommended that you have the heifers re-examined for pregnancy within 30 days of purchase. Natural fetal loss of about 1 to 2 percent (sometimes as high as 5 %) after pregnancy diagnosis is normal. Most replacement heifer producers may make some compensation for heifers that are determined to be open within 30 days of sale. However, because heifers are complex biological systems and management varies greatly from farm to farm, buyers of heifers need to assume some risk.
Proper use of reproductive evaluations of heifers can increase success and decrease disappointment with these future "good ol' cows" of your herd.