Selection and evaluation of beef heifers (Proceedings)

Article

Productivity for beef cattle herds has been shown to be increased when a high percentage of heifers become pregnant early in the first breeding season. A producer's heifer selection and development program should result in most heifers in the replacement pool reaching puberty at least 42 days prior to the start of breeding because the conception success to first service is lower on the puberal estrus compared to the third estrus.

Productivity for beef cattle herds has been shown to be increased when a high percentage of heifers become pregnant early in the first breeding season. A producer's heifer selection and development program should result in most heifers in the replacement pool reaching puberty at least 42 days prior to the start of breeding because the conception success to first service is lower on the puberal estrus compared to the third estrus. In addition, profitability is improved in herds with excellent cow longevity because of lower costs due to fewer replacement heifers maintained and less dystocia, as well as increased income (heavier weaning weights) due to fewer young cows. Both onset of puberty and cow longevity are largely determined by which females are selected as replacement heifers.

Conformation

Selection begins at birth. Heifer calves from early maturing cows requiring minimal nutritional supplementation to conceive early in the calving season should be identified as possible replacements. These heifers should be from dams that have excellent udder, foot, and leg conformation.

Determining that udder conformation will be acceptable as an adult is difficult, if not impossible when examining a heifer. Therefore, heifers should be selected from dams that have udders with a wide rear attachment and a fore attachment that extends as far forward as possible. Teats should be small enough to be easily grasped by a newborn calf.

Pigeon toes, long toes, and straight hocks are examples of some common structural problems in adult beef cows. Heifers identified as potential replacements should have feet of adequate size relative to her frame, with evenly sized toes. The heel should have adequate depth and the pastern and foot should have a correct degree of slope as viewed from the side. The hind leg should have an acceptable set to the hock as viewed from the side. The range of acceptability is 120° to 155° with the ideal of 140°. When evaluating front limb structure, heifers should have an adequate slope to the shoulder (45°-50°) and the legs should be acceptably straight when viewed from the side and front. A straight shoulder reduces the shock-absorbing function of the front limb and reduces the ability of the mature animal to move long distances in range situations, and front legs that bow outward even slightly make a heifer unacceptable. If a heifer or her dam fails any of these criteria, the calf is identified as unsuitable as a replacement. In addition, heifer calves from bulls that have been identified as producing early-maturing, high fertility females with low nutritional maintenance requirement should be identified as potential replacements. If, however, a bull's offspring have a tendency toward any structural unsoundness or delay in reaching puberty, heifers from those individuals are unacceptable.

A valuable objective measurement of a bull's ability to produce daughters that reach puberty at a young age is the bull's yearling scrotal circumference. Scrotal circumference is a moderately to highly heritable trait. Scrotal circumference at 1 year of age has a high correlation with puberty in bulls and in the percentage of that bull's daughters that are pubertal by 1 year of age. A minimum standard for the sires of replacement heifers is a yearling scrotal circumference of 32 cm.

Breed / Type

Whether examining within-breed or across-breed differences, producers should concentrate on matching the metabolic demands of the mature cows with available forage and environment. Mature cow size and milking ability are important considerations in matching breed and type to production environment. Producers should choose breeds and biological types that will have high reproductive efficiency without increasing nutritional requirements above that provided by available grazed forages.

Cowherd records that allow assessment of body condition score at weaning and prebreeding, and how early in the breeding season that females become pregnant should be used to identify cows as potential dams of replacement heifers. Females with higher levels of milk production have higher production efficiency up to the level of maintenance requirement supplied by grazed forage. However, if increased milk production raises the maintenance requirements above that which can be obtained from grazing available forage, the herd will either experience lower pregnancy success or will need to be supplemented with high quality harvested forages or grains, thereby raising production costs above returns. Others have questioned the advisability of using high milk producing breeds or selecting for higher milking ability in breeds with adequate milk production because of studies that show the highest biologic and economic efficiency in low milking ability cows versus medium or high milking cows.

In general, faster-gaining breeds that mature at a larger size reach puberty at an older age than slower-gaining breeds with a smaller mature size. Researchers have also shown that breeds selected for milk production (e.g., Gelbvieh, Brown Swiss, Simmental, Braunvieh, Gelbvieh, Red Poll, Pinzgauer) reach puberty at younger ages than do breeds of similar size not selected for milk production. Researchers have also found that Bos indicus (Brahman-derivative) breeds and breed crosses are older at puberty and heavier than British-bred heifers. However, once Bos indicus heifers reached puberty, conception success is not different from Bos taurus heifers. Also, Bos indicus cows have been shown to have longevity that is greater compared to purebred Bos taurus cows. Therefore, the slow onset of puberty seen in Bos indicus heifers does not extend to decreased fertility as cows. For commercial operators, crossbred heifers should be preferred because of their inherent hybrid vigor and greater fertility, longevity, and lifetime production.

Expected Progeny Differences (EPD)

Expected Progeny Differences are a prediction of the transmitting ability of a parent animal; or how a bull's or cow's progeny will compare to other animal's progeny for various traits. They allow producers to make valid comparisons between purebred animals of the same breed raised in different herds, even under differing environmental and management conditions. The traits measured vary slightly between breeds but generally include birth weight, weaning weight, yearling weight, and a prediction of daughter's milking ability. Some breeds include a measurement for calving ease. Expected progeny differences are expressed in units of the trait of interest. For example, birth weight EPD is expressed in pounds of birth weight, and milk EPD is expressed in pounds of weaning weight due to milk production of the dam. Some breeds have expanded their evaluation programs to include traits such as scrotal circumference, mature size, and carcass characteristics.

Calculation of EPDs considers the performance data of the animal, its relatives, and offspring compared to other members of those animals' contemporary groups. As the amount of information on an animal and its relatives increases, the accuracy of these predictions also increases.

Producers should use EPD to select heifers from sires that will add the optimum level of growth, milk production, and other economically important traits. Scrotal circumference EPD, in particular, may be of special interest when selecting sires of replacement heifers. Heifers sired by bulls with high scrotal circumference EPD have been shown to reach puberty at significantly earlier ages than daughters of bulls with low scrotal circumference EPD.27

The use of herd records, Expected Progeny Differences (EPD) and unbiased visual appraisal of a calf's dam and sire will allow the veterinarian and producer to begin the selection process as soon as a heifer is born. The calf's own performance and structural soundness are used for further culling at weaning, pre-breeding, and at pregnancy detection shortly after the end of the breeding season.

Selection Criteria at Weaning

A rigorous selection standard should be set at weaning time for prospective replacements based on available records and visual appraisal. Complete records of calf, dam, and sire performance are ideal; however, selection pressure can be applied to the herd simply by knowing a potential replacement's weaning weight, week of birth, and dam's identity. Heifers identified at birth as unsuitable replacements because of either sire or dam shortcomings should not be allowed in the selection pool. Producers should select heifers born early in the calving season as older females are more likely to have reached puberty by the start of the breeding season and consequently, be more likely to conceive early in the breeding season than heifers born late in the calving season. The rate of gain needed to reach the target weight that coincides with puberty by the start of the breeding season is less for older heifers compared to younger calves in the same herd. These older calves will then allow greater feeding and management flexibility than lighter, younger heifers.

Evaluation of Reproudctive Soundnesss of Yearling Heifers

Reproductive Tract Scores

Age at puberty can be determined fairly closely in a laboratory setting by measuring blood progesterone levels from samples taken every 10 days (or more frequently). Of course, this method is not practical for production herds and another method of determining onset of puberty was needed. The reproductive tract scoring (RTS) system was developed to subjectively classify puberal status using size of the uterus and ovaries estimated by palpation per rectum.29 The system assigns a score to each heifer using a 5-point scale where a score of 1 is considered an immature tract and scores of 4 and 5 are considered a cycling tract (Table 1).

Table 1. Reproductive Tract Scores29

A RTS of 1 is used to describe heifers with infantile reproductive tracts that are not near the time of puberty when palpated. These heifers have small, flaccid tracts and small ovaries with no significant structures. Heifers may be assigned a RTS of 1 because: 1) they are simply too young to fit into the breeding season being planned, 2) they are too light to reach their target weight and are not able to express their genetic potential for reaching puberty, 3) they were implanted with a growth-promotant near the time of birth. Heifers assigned a RTS of 2 have slightly larger uterine diameter but tone is still lacking and the ovaries have very small follicles. Heifers described as having a RTS of 3 have some uterine tone and larger uterine diameter than heifers with more immature scores. These heifers are subjectively evaluated as being close to cycling (within 6 weeks). Heifers assigned either a score of 4 or 5 are considered cycling as indicated by good uterine tone and size, and easily palpable ovarian structures. RTS 4 is assigned to heifers that although they have large follicles present, do not have a palpable corpus luteum (CL) either because they are in their puberal cycle or the CL is not detected by palpation. Heifers with a RTS of 5 are similar in uterine and ovarian size, tone, and structure when palpated per rectum as compared to RTS 4 heifers except that a CL is identified.

The scores assigned with the RTS system are able to predict the reproductive performance of yearling heifers, especially for pregnancy percentage following synchronized breeding and to pregnancy percentage at the end of the breeding season. Heifers with more mature reproductive tracts had higher pregnancy percentages and calved earlier.

Heifers should be evaluated for reproductive tract score about six to eight weeks prior to the breeding season. If deficiencies are found, management changes instituted this far ahead of the breeding season can result in an increased number of heifers reaching puberty by the start of the breeding season. If the heifers are evaluated too far ahead of the breeding season (> 8 weeks), the heifers are likely to be young and to have lower tract scores than is a true reflection of their potential to reach puberty before the breeding season.

A reasonable goal is to have at least 80% of replacement heifers cycling before the start of the breeding season. A group is considered to be properly developed to reach this goal if at least 60% of the heifers are scored as a RTS 4 or 5 and most of the remainder of the heifers are RTS 3 when evaluated six to eight weeks before breeding. Because progesterone or melengestrol acetate (MGA) will induce puberty in some heifers that are near puberty, a lower percentage (50%) of heifers with RTS 4 or 5 when evaluated six to eight weeks prior to breeding is adequate to meet the 80% goal at breeding if using MGA® or CIDR®. In order to reach the goal of at least 80% of heifers in a replacement pool cycling at the start of the breeding season, nutrition must remain adequate for continued growth from the time of RTS evaluation until breeding.

If a low percentage of heifers are cycling at the time of RTS evaluation and many of the heifers are scored less than a 3, management changes must be instituted immediately. These changes may include: 1) increasing the plane of nutrition so that increased weight gain will allow the heifers to reach target weight by the start of the breeding season, 2) increasing the plane of nutrition and delaying the start of the breeding season by several weeks, 3) holding the heifers over to bred six months later to calve in the fall (for spring-calving herds), 4) marketing the heifers for feeder cattle and finding another source of replacements.

Pelvic Area Measurement

The use of pelvic area measurement at one year of age as a tool to decrease the incidence of dystocia has been described extensively since the late 1970s. Veterinarians have used pelvic area measurements of yearlings because the major cause of dystocia is a disproportionately large calf compared to the heifer's pelvic area. The correlation between yearling and 2-year-old pelvic areas is 0.70; therefore, measuring heifer's pelvic area as a yearling is beneficial for predicting pelvic size at the time of parturition. Pelvic area is moderately to highly heritable (.44 to .61), so after a few years of measuring replacement heifers and bulls used to produce replacements, producers can increase average pelvic size of the herd.

Critics of using pelvic area measurements to decrease dystocia point out that pelvic area is also positively correlated to mature cow size and calf birth weight. If producers place selection pressure on heifers for pelvic area by selecting for increasingly larger pelvic area, calf birth weight will also increase and the rate of dystocia is not likely to decrease. A number of researchers have shown that selection based on pelvic area alone did not significantly reduce the incidence of dystocia in groups of heifers.

Rather than using pelvic area measurement to select for maximum pelvic size, this tool should be used to set a minimum pelvic size as a culling criterion (such as 130-150 cm2 at a year of age) without assigning preference for heifers that exceed the minimum. In addition, by including mature weight as a selection criterion, heifers with a genetic predisposition for small pelvic area are culled without increasing mature size.

Using weight, RTS, and pelvic area to describe the reproductive soundness of heifers

An effective way to evaluate the reproductive soundness of yearling heifers in a ranch setting is by using yearling weights, RTS, and pelvic area measurements together to describe the maturity and reproductive soundness of the heifer group. These three criteria are closely correlated, in that, within a set of heifers with similar genetic makeup, one should expect higher tract scores in heifers that have heavier yearling weights and these heifers should also have greater pelvic areas than lighter-weight heifers.

Because we expect yearling weight, RTS and pelvic area to all be related, one should make note of heifers or groups of heifers where that relationship is not strong. Heifers that have reached their target weight and have a high RTS but that have small pelvic areas may have a genetic predisposition for a small pelvis. This genetic input may have come from the male or the female side of the genetic makeup. Another example where heifers do not perform as expected is the case where heifers are implanted with a growth promotant near the time of birth. Many times these heifers have very adequate yearling weights and pelvic areas, but RTS indicate tract immaturity.

Pelvic area tends to increase more rapidly near the time of puberty than during the pre-pubertal period. This knowledge is useful when examining pelvic area data in that a heifer that has a RTS of 5 and is of adequate yearling weight but who has a small pelvis has a high probability of having a small pelvis at the time of calving as a two-year-old. Whereas, a heifer with the same pelvic area that has a RTS of 2 and has not reached her target weight may very well have an adequate pelvis at calving if management changes are made so that she reaches puberty and becomes pregnant.

Selection Criteria After the Breeding Season

The final culling of prospective replacement heifers is done once pregnancy status is determined soon after the end of the breeding season. By selecting only those heifers that conceive to a proven AI sire or to natural service during a short breeding season, producers can be assured of selecting for females that reach puberty at a young age and conceive early in the breeding season. Lesmeister et al. showed that heifers that conceive early in their first breeding season have greater lifetime productivity than do their counterparts that conceive later in their first breeding season.1

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