Proper development of replacement heifers is critical. Heifers should be managed to reach puberty early, conceive early in the first breeding season, calve unassisted, and breed back early for their second calf. This development needs to be accomplished at low costs without sacrificing performance. Previous studies (Patterson et al., 1992) indicate puberty can be expected to occur at a genetically predetermined size among individual animals; and only when heifers reach predetermined target weights can high pregnancy rates be obtained. Recommended guidelines generally have been 60 to 66% of mature weight in beef heifers, depending on frame size (Patterson et al., 1992). Some studies do not support a critical body weight or composition hypothesis in heifer development (Brooks et al., 1985). Inconclusive results not only exist regarding the appropriate target weight, but also the appropriate time heifers should be placed with bulls in relation to the mature cowherd. Common practice is to breed heifers before the cowherd so they have a longer rebreeding period. This development program requires additional resources if heifers are developed to the same target weight, because an accelerated rate of gain is needed to reach the target weight earlier. As summer calving has gained interest in Nebraska, heifer development programs that allow heifers to conceive early as yearlings and rebreed for a second calf at the lowest cost possible are needed. The objectives of this study were: 1) to compare development of spring-born heifers at two prebreeding target weights (55% or 60% of mature weight) and determine effects on reproduction and cow and calf productivity, and 2) to develop summer-born heifers to similar target weights, but different breeding dates (30 days before or same date as mature cowherd) and determine effects on reproduction and subsequent cow and calf productivity
