Replacement Heifer Development: The Attainment of Puberty01 January 2013
Research has shown that age at puberty is an economically important trait to achieve optimal lifetime productivity of a replacement heifer, writes Kalyn Bischoff, South Dakota State University Extension.
Heifers that become pregnant as yearling, produce a calf by 24 months of age, and conceive earlier in the breeding season, continue to do so during subsequent breeding seasons, allowing for greater lifetime productivity. In addition, heifers that have experienced multiple estrous cycles prior to their first breeding season have an increased probability for early conception. The limiting factor in the attainment of these benchmarks is the attainment of puberty.
Puberty, in beef heifers, can be defined as the first fertile ovulation, resulting in the development of a fully functional corpus luteum (CL), followed by normal estrous cycles. In order to properly manage heifers to reach developmental benchmarks, producers must first understand what puberty is, and how it is attained.
The CL forms on the ovary from the tissue of the ovulated follicle. Therefore in order for puberty to be attained ovulation must take place. Ovulation is regulated by a symphony of hormones that feedback on one another to drive follicle development and growth. These hormones include: estrogen, luteinizing hormone (LH), and gonadotropin-releasing hormone (GnRH). Centers in the brain are responsible for the coordination of these hormones.
The fundamental requirement for the attainment of puberty is the secretion of GnRH in the correct pulse frequency and amplitude to stimulate the secretion of LH to generate follicle development and result in ovulation. Prior to puberty, the surge of LH needed to cause ovulation cannot be reached. Research has shown that a negative feedback loop is in place: where estrogen, produced by the cells of the growing follicle, actually inhibits the secretion of GnRH. As puberty draws near that feedback is converted into a positive loop to allow for enough LH to drive follicle development and result in ovulation. This is due to a shift in the brains neurons ability to respond to the hormones being secreted.
Thus, as the follicle grows, there is an increase in the amount of estrogen being produced which drives GnRH secretion, resulting in more LH secretion to continue the development of the follicle. Follicles must grow to at least 10 mm in size before they are considered ovulatory. Once estrogen reaches a threshold level, GnRH causes the pre-ovulatory surge of LH, which results in ovulation, release of the oocyte from the follicle. This ovulation results in the formation of a CL and puberty is attained.
Puberty can be controlled by a number of factors such as diet, age, breed, genetics, and environment (Factors Affecting Puberty). Therefore understanding puberty will allow producers to better manage the factors that affect the attainment of puberty.