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Replacement Females
Many of the concepts involved in the selection of sires are equally appropriate to selection of replacement females. In general, female selection is less intense than selection of males and accuracy is usually greater for proved bulls. Therefore, most genetic progress results from sire selection. From an economic perspective, Selected replacement females should calve first at two years of age, reproduce annually thereafter, and remain in the herd for an extended period of time.
Because postpartum interval for first calf heifers is longer than for mature cows, it is important that heifers calve early in their first calving season. Thus, virgin heifers must have reached puberty and be exhibiting regular estrous cycles before the start of their first breeding season. Age at puberty is primarily a function of age and weight. It is generally recommended that replacements be selected from among the older available heifers and developed to reach 55 to 60% of their anticipated mature body weight by the start of their first breeding season. Pelvic area measurements may be warranted in herds with a history of dystocia problems or when high birth weight/low calving ease sires are used.
Failure to become pregnant is the leading cause for females to leave the breeding herd. Sustained reproduction is compromised when genetic potential for production of the female is mismatched with the nutritional environment in which she is expected to function. Thus, the desirable EPD profile for a replacement female is dependent upon the resource base of the farm or ranch. Optimum EPDs for growth and milk production under harsh conditions are lower than in better environments. Also, the ability to deposit fat under good nutritive conditions may be valuable for females that must endure seasonal periods of energy deficit.
Beyond the ability to reproduce annually, replacement females should remain functionally sound to advanced ages. Proper foot, leg, and udder structure is important. A cow's udder should be well attached, level across the bottom, and have small to moderate sized teats that are not excessively long. In general, soundness of the udder deteriorates with age. At present, there is no EPD for udder quality. In selection of sires producing replacement daughters, perhaps some emphasis should be given to sons of cows that remained sound to advanced ages.
