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Nutritional Development and the Target Weight Debate
Postnatal nutrition has a profound effect on reproductive efficiency and lifetime productivity of beef heifers. A nutritional management strategy that focuses on allowing a high percentage of heifers to achieve puberty before the breeding season while reducing feed costs is the goal of most operations. Both pre-weaning and post-weaning nutrition affect age at puberty. Pre-weaning nutrition may have a greater effect than post-weaning nutrition on onset of puberty, but the impact of management is limited pre-weaning. Creep feeding or creep grazing may increase the incidence of precocious puberty in heifers. Similarly, exposure to high concentrate diets during early weaning may increase precocious puberty, but early weaning has variable effects on pregnancy rate in heifers. To some extent, post-weaning gain can overcome pre-weaning nutrient deficiencies. However, it is currently unknown if post-weaning nutrition can alleviate negative prenatal nutritional effects on reproduction. Energy is the primary limiting nutrient in beef heifer diets; however, protein must be adequate and may be the limiting nutrient in certain circumstances. A few studies indicate that heifers may benefit from supplementation with rumen undegradable protein. In general, the source of nutrients does not appear to be important as long as animal requirements are met. The nutritional requirements listed in the Beef NRC appear to be adequate for heifer development, but more studies validating the Beef NRC requirements for growing heifers are needed. Altering pattern of growth during the post-weaning developmental period may offer opportunities for decreased development costs and perhaps enhanced cow longevity. The most practical benchmark for proper heifer development at the ranch level has been the target weight method. Recently, the traditional target weight of 65% of mature weight at breeding has been challenged. Achieving a target weight of 65% of mature weight by the beginning of the breeding season ensures nutrition does not limit reproductive success of heifers; however, it appears a target weight of 55% has application for some operations and may have positive effects on heifer longevity. At present, multiple nutritional tools are available to design systems or respond to environmental factors (i.e. drought) for developing replacement beef heifers. Proper analyses of the impact of these systems need to include the long-term effects on cow reproduction and longevity.
Keywords: Heifer, Nutrition, Reproduction