by: Paul Beck
University of Arkansas
Replacement heifers should be bred at 15 months of age in order to calve for the first time as a two-year-old so that they can be a contributing (and profitable) part of our cowherd. Additionally, heifers that give birth early in their first calving season tend to calve early in subsequent calving seasons. Heifers that breed early and subsequently calve early are usually the most reproductively efficient females and will transfer this improved fertility to their offspring, increasing the reproductive efficiency of the entire cowherd. Infertile or hard breeding cows that do not conceive early (or do not conceive at all) cost producers through reduced weaning weight and increased cost per calf produced.
The most fertile heifers are those that have their first estrus before the breeding season, so age at puberty is an important factor in getting heifers bred their first time. The rule of thumb is for heifers to reach at least 65 percent of mature bodyweight prior to breeding. Mature bodyweight can be determined for the individual heifer by measuring hip height and calculating frame size with an equation that factors in heifer age. Research in Nebraska indicates that age of puberty was not affected by differing rates of gain as long as bodyweight goals were reached by the start of breeding.
In Arkansas, many times the fall and winter are periods of limited forage availability and gains of heifers are too slow on hay based diets for heifers to reach puberty by 15 months of age. Cool-season annual grasses (such as wheat, cereal rye, or annual ryegrass) provide high quality forage and support potential for excellent animal performance, if adequate forage allowance is maintained. The growth of these forages is much less in the fall and winter than during the spring causing issues with setting a stocking rate low enough during the winter to provide adequate forage for animal growth and having adequate grazing pressure for full utilization of spring forage growth. So, producers will either have to utilize low stocking rates of cool-season annual pasture during the fall and winter or feed large amounts of hay and supplement during the fall and winter and rely on better animal performance during the spring.
Research conducted at the Southwest Research & Extension Center managed heifers to gain 1.5 pound per day from weaning until breeding in early April on wheat and rye grass pasture or fed hay and supplement from weaning until early February to gain � pound per day before grazing wheat and ryegrass pasture from February until breeding in early April. Heifers that grazed wheat and rye grass pasture from weaning to breeding were similar in bodyweight, body condition and overall average daily gain to heifers that were fed a restricted diet from weaning until February and then allowed to graze. Even though heifers were similar in size and condition, more heifers that had continuous moderate performance were cycling by the end of the breeding season compared with heifers that had early nutrient restriction, yet total pregnancy, conception date and calving dates were similar between groups.
As long as bodyweight goals are reached, there are multiple avenues available to producers to develop replacement heifers. Care must be exercised by producers to ensure that heifer's dietary restriction is not too severe or too long, making it impossible for bodyweight benchmarks to be reached.
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