VN November 2021

Vetnews | November 2021 37 Regulars I Zoetis Livestock Column programmes usually consist of two injections of PGF2 α 11-14 days apart, so that any refractory CL’s present when the first injection of PGF2 α was administered will be responsive by the time the second injection is given. 6 GnRH-based programmes include both Ovsynch and Cosynch. Using Ovsynch as an example, administration of a GnRH at the beginning of an Ovsynch programme would aim to ovulate a susceptible (> 10 mm in diameter) dominant follicle. 2,8 This then inadvertently leads to the emergence of a new follicular wave ~2 days later and increased progesterone as a CL forms. 2,8 Administration of PGF2 α on day 6 or 7 after the first GnRH will lead to luteolysis, and the second GnRH would aim to synchronise the LH surge and ovulation. 2,8 An LH peak can be expected within 2 to 4 hours, and ovulation will most likely occur 24 – 36 hours after GnRH administration, depending on the size of the follicle. 1,3 While this programme usually employs fixed-time artificial insemination (FTAI), some authors have applied modifications such as insertion of a progesterone device between the first GnRH injection and the PGF2 α administration as well as heat detection before FTAI, in an effort to improve pregnancy rates. 8 Pre-synchronisation with PGF2 α could also assist with ensuring more animals are at the appropriate stage of their cycle (between days 5 and 12) at the initiation of the Ovsynch programme. 8 This would entail the administration of two PGF2α injections 14 days apart and initiation of the Ovsynch programme 12-14 days after the second injection. 8 For Oestradiol-based programmes, the type and concentration of Oestradiol will also have different effects when given either at the beginning or at the end of a synchronisation programme. These programmes almost always include a progesterone device. 7 Oestradiol benzoate (EB), given at the beginning of a synchronisation programme, will suppress FSH and LH and lead to regression of the dominant follicle. 6,8 As soon as the dominant follicle has regressed, oestradiol levels in the body fall, FSH levels rise, and a new follicular wave can emerge. 8 A PGF2 injection given on day 7 or 8 will induce luteolysis. 8 Given at the end of the synchronisation programme, EB or Oestradiol Cypionate (EC) aims to stimulate maturation of the follicle, which will lead to a synchronised LH surge and ovulation. 7 It is important to note that EB will be administered a day after removing the progesterone device, whereas EC on the day of removal. 7 This difference in timing of administration of EB vs EC is mainly due to the differences in their half-life values and the expected timing of an LH surge after administration (EB ~20 hours vs EC ~50 hours. 7 Two other hormones that can play an essential role in oestrus synchronisationisprogesteroneandeCG.Progesteronesupplementation forms part of some traditional programs or has been added as a modification to other programmes. In some instances, progesterone supplementation can induce cyclicity and/or increase the pregnancy per AI rate. 5,6 High producing lactating dairy cows have lower circulating progesterone levels as a result of higher catabolism. 5 This had led to an increased growth rate of the dominant follicle, but reduced embryo quality and pregnancy rates per AI; hence these animals benefit from progesterone supplementation during an oestrous synchronisation programme. 5 eCG has FSH-like properties and can assist in the final growth and maturation of the dominant follicle. 7 The use of eCG is usually reserved for previously anoestrous animals or those in a poorer body condition and will result in a larger and more mature follicle. 3,7 Conclusion With any synchronisation programme, it is crucial to managing the expectations of the producer. Successful synchronisation depends on numerous factors. Individual animal response rates are influenced by factors such as the parity of the animal, body condition score, nutritional status etc. The animal’s cycle stage during a programme will also influence the diameter of a dominant follicle, the age of a CL and the expression of oestrus before insemination. 6 Further to that, effective heat detection, inseminator efficiency, and other factors that affect fertility can all play a role in the success of the programme. 7 The success with which all these factors are managed will influence the outcome of the synchronisation programme. A special thank you to Dr Morné De La Rey for always taking time to answer all my questions and sharing his knowledge and expertise with me, it is very much appreciated. References: 1. Kanitz W. Follicular dynamic and ovulation in cattle – a review. Arch. Tierz . 2003;46:187-198. 2. Mart í nez MF, Kastelic JP, Adams GP, Janzen E, McCartney DH, Mapletoft RJ. Estrus synchronisation and pregnancy rates in beef cattle given CIDR-B, prostaglandin and estradiol, or GnRH. Can Vet J . 2000;41:786-790. 3. Demeterco D et al. The effects of intramuscular or intravenous injections of Gonadotropin releasing hormone at fixed-time artificial insemination on pregnancy rates of Bos Indicus beef cows. Braz. Arch. Biol. Technol . 2014;57:361-366. 4. Smith MF, Pohler KG, Perry GA, Patterson D. Physiological factors that affect pregnancy rate to artificial insemination in beef cattle. Proceedings Applied Reproductive Strategies in beef cattle , December 3-4, 2012, Sioux falls, South Dakota, p.33-51. 5. Bisinotto RS, Lean IJ, Thatcher WW, Santos JEP. Meta-analysis of progesterone supplementation during timed artificial insemination programs in dairy cows. J. Dairy Sci . 2014;98:2472- 2487. 6. Madureira G et al. Progesterone-based time AI protocols for Bos indicus cattle II: Reproductive outcomes of either EB or GnRH-type protocol, using or not GnRH at AI. Theriogenology . 2020; 145: 86- 93. 7. Hughes S. Managing the oestrous cycle in cattle: a guide to protocol selection. Zoetis CPD 2015. 8. Colazo MG, Mapletoft RJ. A review of current timed-AI (TAI) programs for beef and dairy cattle. CVJ . August 2014;55:p.772-780. More information available from Zoetis South Africa (Pty) Ltd., Co. Reg. No.: 2012/001825/07, 6th Floor, North Wing, 90 Rivonia Road, Sandton, 2196. PostNet Suite 53, Private Bag 9976, Sandton, 2146. South Africa. Tel.: +27 11 245 3300 or 0860 ZOETIS (0860 963847). www.zoetis.co.za. Reference No.: RU/CE/10/21/13 v