Reproductive performance and the transition period of thoroughbred mares in New Zealand : evidence and implications for future alternative management strategies : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University, Palmerston North, New Zealand
The aims of this research were to investigate the reproductive performance of
Thoroughbred mares in New Zealand, to examine the use of intravaginal progesterone to
manage transitional mares and to develop a potential model for haemorrhagic
anovulatory follicle (HAF) development in the mare.
Firstly, a prospective cohort study was performed involving five stud farms in the
Waikato region of New Zealand during three consecutive breeding seasons (2006-2008).
A total of 1482 individual mares contributed 2007 mare years and 3402 oestrous cycles
over the three breeding seasons. The mean first-cycle pregnancy rate was 53.6%, the
end-of-season pregnancy rate was 85.3% and the foaling rate was 80.2%. The length of
the breeding season was relatively short with 87% of services occurring in the 91 day
period between 1st September and 30th November. Multivariable analyses revealed that
reproductive performance was influenced by two main mare-related factors; the age of
the mare and her reproductive status (dry or foaling). Increasing mare age significantly
reduced the first-cycle pregnancy rate, reduced the end-of-season pregnancy rate and
increased the interval from the start of the breeding season to conception. In terms of
reproductive status, dry mares had a significantly higher first-cycle pregnancy rate and
end-of-season pregnancy rate compared with foaling mares. The majority of variation in
reproductive performance was associated with mare-level factors and the contribution of
the stallion and stud farm was relatively minor.
Transitional mares treated with intravaginal progesterone at the start of the breeding
season were served two weeks earlier than untreated control mares. In the first 21 days
of the season, 95% of treated mares were served compared with 43% of control mares.
Treated mares also conceived earlier and had a higher end-of-season pregnancy rate than
control mares. Follicle development in response to intravaginal progesterone treatment
appeared to be mediated through a close temporal association between progesterone,
FSH and LH during treatment.
In the last study, transplantation of chorionic girdle into non-pregnant mares resulted in
elevated eCG concentrations in all mares and the development of multiple HAFs in the
treated mares over a prolonged time-span.
In conclusion, this research has, i) identified the most important factors associated with
the reproductive performance of New Zealand Thoroughbred mares, ii) determined that
intravaginal progesterone treatment is a suitable management tool for transitional mares
and iii) created a model for HAF development in the mare.