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.

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