Evaluation of a GnRH agonist bioimplant for oestrous suppression and pregnancy prevention in heifers

Summary

Controlling pregnancies in heifers that are surplus to breeding requirements is important for extensive beef production systems in Northern Australia. Traditionally, fertility control has been achieved by surgical procedures, identifying a contraceptive technology that is non-invasive, practical, and satisfies animal production imperatives whilst recognising the importance of animal welfare to consumers. One such technology is the use of a gonadotrophin releasing hormone (GnRH) agonist bioimplant. A different project, NAP3.105, conducted preliminary testing of a prototype and found that a contraceptive response was induced. In later studies, a return to normal pasture availability and accelerated rate of live weight gain, raising the question of it a GnRH agonist bioimplant can maintain suppressed ovarian function in heifers and cows that are undergoing a relatively fast rate of live weight gain. This project addressed that issue, and sought to identify a dose of GnRH agonist that would induce a contraceptive effect in heifers for 12 months or longer.
It was found that the low dose treatment suppressed conception for approximately 3 months, while the medium dose treatment prevented conception for 6 months. Approximately 90% of heifers treated with a high dose had suppressed ovarian activity after 12 months. This trial established the contraceptive efficacy of a GnRH agonist bioimplant in heifers, and can suppress ovarian function in heifers undergoing a relatively fast rate of live weight gain.

Objectives

There were two main objectives:
1.Establish that the GnRH agonist bioimplant suppresses oestrous behaviour and prevents conception in pubertal (2-year-old) heifers that are undergoing a relatively fast rate of live weight gain during the wet season in northern Australia.
2.Identify the cost/benefit of using the technology to prevent pregnancies in heifers.

Key findings

The greatest rate of live weight gain occurred during the latter part of the trial, which allowed testing of the primary objective concerning the relationship between rate of live weight gain and capacity of the GnRH agonist bioimplant to suppress ovarian function. Control heifers conceived throughout the trial demonstrating that there were no environmental limitations to pregnancy in heifers showing regular ovarian cycles. The GnRH agonist Low Dose treatment suppressed conception for approximately 3 months while the Medium Dose treatment prevented pregnancy for around 6 months. Pregnancies at 12 months were: Low Dose, 36/48 (75%); Medium Dose, 26/48 (54%). Approximately 90% of heifers in the GnRH agonist High Dose treatment continued to have suppressed ovarian activity at 12 months of treatment, and only 3/45 (6%) had conceived at this time.
The results of this trial have established the contraceptive efficacy of a GnRH agonist bioimplant in heifers. Furthermore, the GnRH agonist High Dose suppressed ovarian function in heifers undergoing a relatively fast rate of live weight gain.

Benefits to industry

The GnRH agonist bioimplant evaluated in this project represents the next generation, non-surgical, technology for the control of fertility in heifers and cows maintained under extensive management in northern Australia. The agonist bioimplant will be a future preferred technology as it is practical and non-invasive. With regard to the latter, the beef industry in northern Australia will increasingly implement husbandry practices which recognise the importance of consumer and community awareness of animal welfare.

A further outstanding advantage of the GnRH agonist bioimplant, relative to all surgical procedures, is the reversibility of GnRH agonist treatment. The GnRH agonist bioimplant technology should therefore be considered within a whole-enterprise framework of reproductive management of both breeder and non-breeder heifers and cows. This allows for a paradigm shift in the conceptual framework for managing reproduction in extensive beef herds. In future, conceptions patterns in extensive herds can be determined by selective treatment of heifers and cows with GnRH agonist, with a decreased concern over the control of bulls.