CSIRO PUBLISHING Reproduction, Fertility and Development, 2010, 22, 126–131 www.publish.csiro.au/journals/rfd Use of equine chorionic gonadotrophin in synchronised AI of seasonal-breeding, pasture-based, anoestrous dairy cattle M. A. Bryan A,E , G. A. Bó B , C. Heuer C and F. R. Emslie D A VetSouth Ltd, PO Box 12, Winton, Southland, New Zealand. B Instituto de Reproducción Animal Córdoba (IRAC), 2 Univ. Católica de Córdoba, 3UNICEN, 4Ag. Cba. Ciencia, Córdoba, Argentina. C Epicentre, Institute ofVeterinary Animal and Biomedical Sciences, Massey University, Private Bag 11-222, Palmerston North, New Zealand. D Bomac Laboratories Ltd, PO Box 76-369, Manakau City, Auckland, New Zealand. E Corresponding author. Email: markb@thevets.co.nz Abstract. In seasonally calving dairy areas, a critical index of efficiency and profitability is the ability to maintain a short annual calving spread. In essence, this means that all cows need to conceive within 90 days of calving. Anovulatory anoestrus (AA) is a major barrier to reproductive performance under these conditions and the successful treatment of AA is an important aspect of reproductive intervention. The present paper reviews regimens used to treat AA in New Zealand and reports on a large-scale field trial investigating the effects of equine chorionic gonadotrophin (eCG) within a proven efficacious treatment protocol. The administration of eCG at the time of removal of a progesterone-releasing device in an 8-day, oestradiol benzoate (EB)-based treatment protocol increased the percentage of cows in calf after 7 and 28 days of breeding. The interaction between cow age and treatment with eCG was significant, with cows over 5 years of age having higher conception rates (P = 0.002) and 7 day in-calf rates (P = 0.002) than cows on a standard 8-day treatment protocol without eCG. The addition of eCG to a standard 8-day EB/progesterone protocol can improve reproductive success in seasonally calving AA dairy cows. Additional keywords: anovulatory anoestrus, oestradiol benzoate, progesterone. Introduction In seasonally calving, pasture-based dairy herds, maintaining an annual calving pattern requires all cows to conceive within a short time frame. To minimise both culling and the induction of parturition to manage late-calving cows, the window for concep- tion is 90 days post partum. Because cows in larger herds spend a large part of the day outside on pasture, the traditional approach to reproductive management in New Zealand is to use tailpaint as a marker of oestrous activity. Cows displaying oestrus will have a loss of tailpaint and this has been shown to be an effec- tive means of detecting oestrus when performed regularly by an experienced operator (Smith and Macmillan 1978). Normally, a date is chosen for the commencement of breed- ing of all cows and this ‘planned start of mating’ (PSM) relates to the optimal time for the herd to begin calving in the fol- lowing season, determined by pasture growth and other factors. The PSM is typically 90 days after the start of calving and so determination of the oestrous activity of cows before this date is an important factor in herd management. Cows deemed to be anoestrus by PSM are generally examined by a veterinarian to determine ovarian status, with classification and treatment initiated accordingly (Rhodes et al. 2003). Cows deemed to be in anovulatory anoestrus (AA) are then treated with an appropriate protocol to initiate oestrus, which is followed by AI. Fielden et al. (1973) first described AA in New Zealand. AA is defined clinically by determination of no palpable corpus luteum (CL) by rectal palpation following an absence of signs of oestrus (McDougall 2001). This clinical finding may be complemented by estimating ovary size and the absence of a follicle >10 mm in diameter (Morris and Day 1994). This method of detection of AA is effective (Macmillan 2002a; McDougall 2003) and inter- vention is performed typically as early as practical in the mating season. Progesterone-releasing devices have been used to treat AA dairy cows in New Zealand for nearly 20 years (Macmillan and Peterson 1993).The original basic protocol was described by Rhodes et al. (1998) and has been refined over the years by the addition of 2 mg oestradiol benzoate (EB) at the time of insertion of the progesterone (P4)-releasing device (Verkerk et al. 1998) and by extending the protocol to 8 days with 1 mg EB 24 h after device removal (McDougall 2001). The economics of treatment of AA cows have been well demonstrated (Macmillan 2002b), with cows on the 8-day P4/EB protocol conceiving significantly © IETS 2010 10.1071/RD09225 1031-3613/10/010126