15 / 24
AUTUMN 2015 ISSUE
LIVESTOCK MATTERS
14
HERD FERT I L I TY
However, through veterinary intervention, a
variety of strategies can be employed to
ensure cows do return to cycling again, and
hormonal levels can be re-balanced to allow
the production of a healthy oocyte (egg) and
ultimately a healthy full term pregnancy.
Pregnancy rate
Pregnancy rate is a key performance indicator
when measuring herd fertility. It is driven by
heat detection/submission rates, and
conception rate. Improving these parameters
will directly improve the efficiency and
profitability of the herd.
Vet Owen Tunney of Willows Vet Group in
Cheshire has helped one dairy farmer to
improve the herd’s pregnancy rate by using a
synchronisation and fixed time AI protocol to
drive up submission and conception rates.
But first, a recap on the reproduction system
of the cow and the hormonal influences that
affect fertility.
Hormone interactions
The reproductive system of female mammals
involves a complex series of inter-related
hormonal feedback mechanisms.
Taking fertility back to the basics: in a healthy
cycling cow ovulation occurs every 18-23
days, when the ovary releases an egg from
a follicle.
Once an egg has been released, the follicle
becomes a corpus luteum (CL) - and starts to
release the hormone progesterone. If the egg
is not fertilised, then more follicles continue to
develop in waves, (hence the term ‘follicular
waves’), so that at any one time they are in
different stages of maturity.
Healthy follicular development is reliant on a
delicate hormonal balance within the ovary.
Follicles develop under the influence of Follicle
Stimulating Hormone (FSH) and waves of
Luteinising Hormone (LH) which is, in turn,
reliant on adequate progesterone being
released from the CL.
The CL will disappear approximately 16 days
after the previous heat, allowing a new
dominant follicle to develop. This follicle will
produce oestrogens, which then cause the
oestrus activity (bulling activity). When the LH
pulses peak, this causes ovulation of the next
egg, and the cycle continues with another
follicle then becoming dominant and, 21 days
later, another egg being released.
This cycle continues until the cow conceives.
When she does, the corpus luteum does not
disappear, but instead, continues to produce
progesterone, which allows the pregnancy to
be maintained.
Conception rate factors
Owen explains: ‘Aside from a good AI
technique, conception rate is affected by the
health and quality of the follicles and eggs.
‘Good fertility requires healthy follicles which
will then release good quality eggs so that
once fertilised, they can result in a full term
pregnancy.
‘The health of a follicle is dependent upon
adequate levels of progesterone being
produced by the corpus luteum, allowing an
appropriate pulsatile release of luteinising
hormone through the oestrus cycle. However,
LH production is sensitive to stress factors such
as inadequate nutrition, a difficult calving,
uterine disease, lameness, extremes of
temperature, or a poor environment.
‘So cows under stress can produce less LH
which will reduce the quality of the follicles
and the eggs they release.‘
Submission rate challenges
‘A key factor affecting submission rates is
heat detection,‘ says Owen. ‘This has
become much more difficult in today’s herds
due to the extra production stresses and
modern genetics which have decreased the
period of oestrus.‘
In fact, a comparison of oestrus behaviour of
dairy cows in 1970 shows that, back then,
a cow was likely to be in oestrus for 15
hours and would mount another animal 20
times. But modern dairy cows average only
8.5 mounts and seven hours of oestrus.
Moreover, these heats are more likely to
occur at night.
Intervention strategies to
improve pregnancy rates
The healthy functioning of a cow’s reproductive system is fundamental to good
fertility. However, stress factors can unbalance the system and lead to silent heats,
delays in cows returning to cycling, or holding to conception. This extends the
calving interval, increases average days in milk, and reduces the annual milk cheque.
Veterinary surgeon
Owen Tunney
XLVets practice
Willows Veterinary
Group
WILLOWS
FARM ANIMAL
VETERINARY PRACTICE
OWEN TUNNEY, WILLOWS VETERINARY GROUP