PHYSIOLOGY AND ENDOCRINOLOGY SYMPOSIUM: Harnessing basic knowledge of factors controlling puberty to improve synchronization of estrus and fertility in heifers1

Abstract

The development of replacement heifers is a major economic investment for all beef and dairy operations. The costs associated with heifer development cannot be recovered if heifers do not conceive and remain productive in the herd; therefore, heifers need to conceive early in the breeding season or risk being culled. Previous research has reported up to a 21% increase in fertility from pubertal estrus to the third estrus of a heifer. The use of reproductive tract scores to determine pubertal status has demonstrated that peripubertal and pubertal heifers have increased pregnancy success to estrous synchronization compared with heifers that were prepubertal. The development of RIA has allowed accurate measurement of peripheral blood hormone concentrations associated with the pubertal process and luteal formation. This basic knowledge has increased our understanding of the mechanisms that control puberty in heifers. In addition, understanding the hormonal changes that occur during the estrous cycle has allowed for the development of estrous synchronization protocols that result in increased control of follicular growth, regression of luteal tissue, and ovulation. Transrectal ultrasonography has increased our understanding of follicular waves; this understanding led to research investigating the endocrine regulation of follicular waves and development of methods to synchronize follicular waves for purposes of fixed-time AI. Current topics of research include the effect of antral follicle count on fertility and the effect of maternal nutrition (on the fetus in utero) on subsequent reproductive potential of a heifer (i.e., fetal programming). Advancements in genomic technologies will likely provide a powerful tool for selecting heifers at birth that will have a greater probability of being reproductively successful if managed correctly. Therefore, knowledge gained through basic research on factors that control puberty has improved and will continue to improve heifer development and fertility.

Heifer fertility and carry over consequences for life time production in dairy and beef cattle

“…This finally triggers the switch to a positive feedback of oestradiol driving an LH surge (Day et al, 1987). A heifer reaches puberty when she ovulates a potentially fertile oocyte and some definitions also stipulate that this should be accompanied by visual signs of oestrus (Perry, 2012). In practice the two events of first oestrus and first ovulation are often separated.…”

Optimising reproductive performance of beef cows and replacement heifers

“…In agreement, Perry et al (1991) observed that the conception rate of heifers bred at the first observed post-pubertal oestrus was 36 percentage points lowered than their counterparts bred at later oestrus events. Thus, the timing of puberty and the proportion of heifers that are pubertal before the planned onset of their first breeding season is critically important to overall herd reproductive efficiency (Perry, 2012).…”

“…Despite this it is clear that differences exist between breeds and breed types values of 60% typical for European-derived beef breeds, 55% for dual purpose beef/dairy breeds and 65% for Bos indicus cattle (Larson, 2007). Traditionally the recommended target weights for beef heifers at puberty (pre-breeding) was 0.60 to 0.65 (Whittier et al, 2005;Engelken, 2008), and at first breeding, 0.65 to 0.70 (Whittier et al, 2005;Perry, 2012) of estimated mature weight. These target weights, though frequently challenging to attain in practice, ensure that a very high proportion of heifers are post-pubertal at the onset of the breeding season, can easily attain 85% of mature weight at time of first calving and are sufficiently physically well developed to minimise dystocia, thus facilitating earlier recommencement of cyclicity post-calving and successful rebreeding.…”

“…This suggests that adiposity, or at least the achievement of a certain threshold level of adiposity may be a prerequisite for occurrence of puberty and resumption of postpartum ovarian cyclicity. Although the precise chemical or hormonal signals that link BW and adiposity to pubertal onset have not been clearly defined, an increase in circulating concentrations of leptin, an adipose-derived hormone that regulates a wide variety of physiological processes, has been shown to precede the onset of puberty in several species, including cattle (Perry, 2012) leading to postulation that leptin acts as a permissive signal to the occurrence of puberty. Furthermore, administration of recombinant ovine leptin stimulated secretion of GnRH and LH in undernourished heifers, but was not capable of accelerating puberty onset in well-nourished heifers Zieba et al, 2004).…”

Relationship of post-weaning growth and age at puberty in crossbred beef heifers

“…The average heifer weight at yearling suggests that the main limitation for breeding yearling is body weight at the start of the breeding season (Bolze and Corah, 1993;Perry, 2012). The age of first mating in beef cattle is an important event that has an economic impact on the production system efficiency (Hugh et al, 2011;Laske et al, 2012).…”