Substantial research has been conducted contributing to the traditional guidelines of developing heifers to 60 to 65% of mature body weight at time of breeding. In general, studies evaluating different postweaning rates of gain or target weights have used either different amounts of feed, or different types of feeds varying in energy and/or protein content to obtain differences in rates of growth. A review of these studies conducted over the last several decades along with new research indicates the association among BW, puberty and heifer pregnancy rate appears to be changing over time. In general, research reports published through the late 1980s have shown much greater negative effects of limited postweaning growth on age of puberty and subsequent pregnancy, where as more recent studies indicate less of a negative impact of delayed puberty on pregnancy response. Several factors likely contribute to this change over time. Initial research in this area of interest corresponds to the industry shift from calving heifers at 3 years of age to calving at 2 years of age.

Thus, selection pressure for age of puberty was probably minimal in the animals used in the early studies. While selection intensity would have increased with the reduction in calving age of heifers, genetic progress would take time due to the long generation interval in cattle. In the mid 1980s, researchers identified the association between scrotal circumference in bulls and age of puberty in their female offspring. Since then, scrotal circumference has been used as an indicator trait for puberty. The change occurring in scrotal circumference from 1985 to the present indicates substantial progress has been made, and a similar response in age of puberty would be expected. Indeed, the inability of heifers to attain puberty prior to breeding may not be as problematic as heifers reaching puberty before weaning.

Another factor that seems to have changed is the association between timing of puberty and subsequent pregnancy rate. Early research indicated heifers should experience two or three estrous cycles before the onset of the breeding season because fertility of the first estrus is lower than subsequent estrous cycles. Thus it was expected delayed onset of puberty would be associated with lower pregnancy rates. However, several studies have not shown strong associations between nutritionally related changes in age of puberty and final pregnancy rates. Evidence for a genetic basis for these differences is provided by Freetly and coworkers (1997), who reported pregnancy rates were greater in heifers AI sired by bulls born after 1988 than bulls born between 1982 and 1984, but age and weight at puberty were not. These changes, combined with the continued increase in cost of harvested feedstuffs indicate the need for alternative development systems which allow heifers the opportunity to conceive early as yearlings at reduced cost.

Feeding replacement heifers to a traditional target weight increases development costs relative to more extensive heifer development . Funston and Deutscher (2004) reported similar pregnancy rates from the initial through fourth breeding season for spring calving heifers developed to reach either 53 or 58% of mature weight prior to breeding as yearlings. This demonstrated heifers developed to only 53% of mature weight could achieve similar initial pregnancy rates and retention compared to heifers developed to 58% of mature weight. Further research using the same herd found pre-breeding weights as low as 51% of mature weight was more cost effective than development to 57% of mature weight when lighter heifers were allowed 60 d to become pregnant. Extending the breeding season by 15 d for lighter heifers resulted in first-calf conception rates being similar between systems (45 vs. 60 d breeding season).

Current research involves low input heifer development systems utilizing grazed winter forages. A 2 yr study is in progress and preliminary data indicate heifer development using corn residue is a suitable alternative to winter range or a dry lot following weaning. However, development on corn residue reduces average daily gain post-weaning, delays puberty and may reduce first service conception rate without sacrificing final pregnancy rates in a 45 d breeding season.

Postweaning management of heifers to achieve traditional target weights, particularly by feeding high-energy diets, is not supported by available research. Heifers developed on forage, however, generally require some protein supplementation to achieve even modest gains. One reason reproductive performance has not been drastically impaired by feeding to lower weights may relate to genetic changes in age of puberty.

Support Materials:

Heifer Development - Then and Now

Effect of Pre-breeding Weight and MGA Supplementation on Heifer Performance

Developing Replacement Heifers: Recent Research Findings on Target Weights and Fat Supplementation