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   Limousin Foundation
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Traditionally, many cattlemen have selected replacement heifers based largely on which heifers look the best. While this selection practice emphasizes genetics for growth and tends to pick heifers from the earliest calving cows, it may not result in sufficient focus on genetics for expressed fertility, calving ease, optimum levels of milk production, sensible maintenance requirements and adequate longevity. Present day genetic information affords producers the ability to produce and select replacements with a higher likelihood of success as compared to traditional approaches.

What does a replacement heifer need to do in order to achieve production success? Most producers would agree that successful replacements:

• reach puberty and become pregnant early in the first breeding season,
• calve unassisted each year,
• breed back early in the second breeding season as wet two-year-olds, with high levels of sustained fertility thereafter,
• have optimum milk and fleshing ability, so as to wean profitable calves, yet maintain sufficient body condition for early annual re-breeding,
• have sensible maintenance requirements as a result of mature size and milk production levels which are matched to available feed resources,
• have sound teats, udders, feet and legs as well as calm temperament and as much longevity as possible to minimize annual replacement rates,
• represent genetics for optimum levels of growth, efficiency and carcass merit for the purpose of producing profitable feeder and fed cattle as well as replacements, and
• possess high salvage value so as to offset replacement costs.

Given the demanding list of attributes required of replacement heifers, it is useful to consider how genetic information might be used to select for success. Opportunities to take advantage of genetic information exist at three points of heifer production:

1. When sires are selected to produce replacement heifers
2. When heifers are selected from the pool of replacements
3. When sires are selected to breed replacements.

The &qupt;better&qupt; the pool of potential heifers from which to select, the better the odds of selecting a productive group of replacements. For commercial cattlemen, there are at least two keys to producing a superior pool of replacements: 1) a well designed crossbreeding program which generates maternal heterosis and uses breeds that are well adapted to the production environment, and 2) use of EPDs to select sires with genetics for the combination of items listed above.

Data from the U.S. Meat Animal Research Center underscores the economic importance of crossbreeding. "Heterosis (hybrid vigor) for major bioeconomic traits, including reproduction, calf survival, maternal ability, growth rate, and longevity of beef cattle is important. Heterosis can be used to increase weight of calf weaned per cow exposed to breeding by 20 percent. Crossbred cows remain in the herd 1.3 years longer and have a 30% greater lifetime production than straight-bred cows (MARC, October 1999).&qupt;

Choosing which breeds to use in crossbreeding starts with an evaluation of your present cowherd and feed resources, as well as a determination of the targeted end-product market. Selecting breeds from which to produce replacements should also go beyond evaluation of only puberty and level of milk production, and should include consideration of breed comparison data related to maternal calving ease, calf vigor and survivability, cow maintenance energy requirements, mature cow size, optimum milk production, convenience traits such as teat and udder soundness, longevity, and traits contributing to lifecycle production efficiency (see Table 1.)

Table 1. Breed Differences in Maternal Traits

Breed	Cow Weight (lb)	Cow Hip Height (in)	Calf Birth Weight (lb)	Calving Difficulty (%)	12-hr Milk Yield (lb)	Biological Efficiency* At 3,500 DMI	Biological Efficiency* At 7,000 DMI
Limousin	1,213	52.4	86.4	15.7	10.2	33	42
Angus	1,094	49.6	74.7	8.8	9.3	39	17
Charolais	1,349	53.9	98.1	19.5	10.5	27	45
Gelbvieh	1,266	53.9	95.2	21.5	12.7	29	36
Hereford	1,091	50.0	79.4	16.5	6.7	30	13
Simmental	1,272	54.3	94.2	23.9	13.1	26	42

* Grams of calf weaned per kilogram of dry matter intake per cow exposed

Sources: USDA-ARS Technical Bulletin, 1999 and Jenkins and Ferrell, JAS 72:2787, 1994

When selecting specific sires, or sons of sires within a given breed for producing replacements, producers are encouraged to utilize EPDs for traits related to calving ease, early growth, optimum milk production, early puberty (scrotal circumference) and high heifer pregnancy, calm docility, sensible mature size and high stayability. Additionally, although it is more difficult to obtain from most sires, genetic information is available from several semen companies as to the quality of teats and udders as well as other soundness traits of daughters. Use of estrus synchronization and artificial insemination (A.I.), using sires proven (EPDs with high accuracy) for their ability to produce sound, productive females, can take most of the guesswork out of producing a large pool of superior replacements.

In addition to known pedigree and performance information from the sires and dams of replacements, performance data can be collected from yearling age heifers when they are processed prior to breeding as part of the herd health program. Collection of the following information can help guide selection of heifers toward genetics for early puberty, maternal calving ease, growth, as well as moderate mature size and adequate fleshing ability:

Reproductive Tract Scores - The reproductive tract scoring system estimates a heifers pubertal status via rectal palpation of the uterine horns and ovaries. Tract scores range from one to five, with heifers scored as 4 and 5 believed to be cycling based on the presence of a corpus luteum or large follicle on the ovaries and good uterine tone. Heifers scored as three are thought to be on the verge of cycling, while those scored as one and two have more infantile, less developed reproductive tracts and are not yet cycling. Research has indicated that heifers scored as three, four and five generally have higher pregnancy rates and earlier conception dates as compared to heifers scored as one or two. In addition to serving as a heifer selection tool, reproductive tract scores can be used to determine if a group of heifers are sexually mature enough to respond favorably to a synchronization and A. I. program. Palpation of the reproductive tract can identify freemartins or heifers which might already be pregnant prior to the start of the breeding season. Many large animal vets can evaluate reproductive tracts of replacement heifers.

Pelvic Measurements - Calving difficulty primarily occurs among first-calf heifers as a result of "heavy" calf birth weight in relation to the size of the dam's pelvic area. While birth weight is unquestionably the key direct factor related to calving ease, pelvic area appears to be the highest ranking maternal variable. Colorado State University research has shown that as birth weight of calves increase and pelvic area of replacement heifers decrease, the incidence of assisted births also increase. Depending upon the average pelvic size of the group, culling heifers with the smallest pelvic areas (i.e. smallest 5 to 10%), has been shown to help reduce the incidence of assisted births.

Yearling Weight , Hip Height and Condition Score - Weight, height and condition information can be used to select heifers with adequate early growth and fleshing ability but help avoid selecting heifers which might be too big and hard fleshing later in life. Avoiding heifers which are too extreme, either excessively big or small is expected to contribute to greater cowherd uniformity.

The accuracy of replacement heifer selection can be improved by combining these yearling data with knowledge of pedigree information from sires and dams for appropriate levels of maternal traits (i.e. milk) and longevity, with incorporation of a visual evaluation of soundness and doing ability.

While the first test of a replacement heifer is becoming pregnant early in the first breeding season, the next major hurdle is the heifers ability to give unassisted birth to a live, vigorous calf. In addition to avoiding production and selection of heifers with genetics for high birth weights themselves, selection of service sires on the basis of their EPDs for low birth weight, high expected calving ease, and short gestation length is the most reliable way to minimize calving problems.

Calving ease in first-calf heifers is also related to reproductive performance during the second breeding season. Research has demonstrated that heifers which do not require assistance at calving tend to have higher pregnancy rates following the second breeding season by three to five percent, and are about a week earlier in terms of day of second calving as compared to heifers which require assistance. This is most meaningful when you consider that the number one reason cows are prematurely culled at young ages is because of reproductive failure (open at the end of the breeding season).

Implementation of a synchronization and A.I. program, whereby sires can be reliably selected for calving ease in addition to adequate performance in other traits, is the most sure-fire way to genetically manage for a successful first calf from replacements. Through the use of A.I., unique service sires can be used which not only minimize calving difficulty, but also pass on genetics for the desired combination of maternal, growth and carcass merit. Considerable risk can be managed through the use of synchronization and A.I. in replacement heifers.

While the thought of using Limousin sires on replacement heifers is not as common as using low birth weight Angus or Red Angus, Limousin is the lowest birth weight and easiest calving of the widely used continental breeds (including Gelbvieh, Simmental and Charolais). From both a natural service and A.I. sire perspective, there is a sizeable population of low birth weight, short gestation length and high calving ease Limousin sires available. Through the use of EPD information generated by NALF's genetic evaluation program, it is possible to identify reliable Limousin heifer bulls which also represent superior genetics for either maternal traits or above average growth and carcass merit. NALF's sire selector program on the internet (www.nalf.org) is especially useful for identifying Limousin sires for specific situations.

Successful replacement heifer selection begins with wise selection of the sires from which heifers are produced. Once a superior pool of replacements is developed, use of performance information collected on yearling heifers, as well as information known about the sires (or group of sires) and dams of heifers, can be used in conjunction with visual evaluation to improve the odds of selecting the most productive heifers. Finally, smart selection of sires to breed to heifers can help insure calving ease and production of a profitable first calf from replacements. Through planned crossbreeding and disciplined use of available genetic information, heifers can be produced, selected and bred with more confidence and less risk as compared to traditional practices.

Limousin is the largest Continental breed registry in the United States. Limousin is the leader in Muscle Growth Efficiency and is the ideal complement to British cross cows. For more information, contact www.nalf.org.