When should cows be inseminated?
If cows are inseminated too early the sperm will no longer be
viable when they meet the egg; if they're inseminated too late then
the reverse will occur. However, there is more leeway with the
former than the latter. The golden rule of timing of insemination
remains: inseminate before the cow releases the egg ('ovulation') -
we want the sperm to be in the oviduct before the egg gets
This means that the timing of insemination is based on
predicting when ovulation is likely to occur not when it
has occurred. In the cow, ovulation occurs 24 to 32 hours after the
start of standing heat; this means that we can tie the timing of
insemination to observing heat, with initial research showing that
the best conception rates occurred when cows were inseminated
from mid oestrus until 2 to 3 hours after the end of
Figure 1: Accurate heat detection is crucial to the timing of
insemination. The use of devices such as Kamars better identify
heat and thus mean the timing of insemination can be more
The am/pm rule
This research led to the development of the 'am/pm rule' - where
a cow which was seen on heat in the evening was inseminated the
next morning, while a cow that was seen on heat in the morning was
inseminated the following evening.
However, the modern dairy cow shows heat less intensely (fewer
than 50% of cows in oestrus stand to be mounted compared to >70%
in the 1980s) and shows it for a shorter period of time (duration
of standing oestrus has decreased from an average of 15 hours to 5
hours), so the question is often asked as to whether the am/pm rule
needs to be changed.
Figure 2: Oestrus behaviour starts around 24-32 hours before
ovulation. The ideal time for mating is at the end of oestrus (or
about 12 hours after it starts). Picture courtesy of Jakob
When determining the answer to when to inseminate, the first
question to ask is: how good is heat detection? If there are two
periods of targeted observation for heat detection every day AND
the proportion of cows submitted is relatively high, then the am/pm
rule is likely to still be optimum, with recent research showing
that insemination 12 hours after the start of oestrus activity
still results in the best conception rates.
If the proportion of cows submitted is low, because of
relatively poor heat detection or cows not showing heat intensely,
or if two periods of dedicated heat observation per day are not
feasible, then the 'am/am and pm/am rule' may produce the best
results. That is cows recorded as being on heat the previous
evening and the current morning all get inseminated that morning,
as this will limit the proportion of cows which are inseminated too
Although it is often suggested as an option when fertility is
poor, inseminating once-a-day is not just effective when you have
poor heat detection and low conception rates. Inseminating all cows
once a day in the morning is the standard system used in New
Zealand, which has an average 3-week submission rate of 82% and a
first service conception rate of 48%, figures that are much higher
than the average in the UK. So inseminating everything in the
morning is not just a solution for poor submission rates but can
work on farms with good fertility too.
Figure 3: Timing of AI is crucial. Too early and the sperm are
no longer capable of effectively fertilising the egg; too late and
the aged egg will not be capable of producing a viable embryo.
Picture courtesy of Jakob Malmo.
The optimum figure of 12 hours between onset of oestrus and
insemination is based on cows ovulating between 24 and 32 hours
after the start of oestrus. A significant proportion of
cows (up to 20% in some studies) have longer intervals, such cows
will have lower conception rates but be otherwise normal. They are
often referred to as 'repeat breeder' cows, as they come into heat
at the right intervals but don't get pregnant. In such cows the use
of GnRH at the time of AI ('holding injection') can improve their
conception rate, by bringing forward ovulation and, therefore,
reducing the risk that the interval between insemination and
ovulation is too long.
Fixed time AI
There are a multitude of programmes designed to allow producers
to use fixed time AI (FTAI) and avoid the need to detect heat. The
timing of FTAI is programme specific, as different programmes
induce ovulation at different times.
Progesterone-driven programmes, such as a 7-day protocol with
the injection of prostaglandin (PG) on day 6, 24 hours before the
removal of the progesterone device, can be followed by double fixed
time AI 48 and 72 hours after device removal; in contrast PG-driven
programmes should be given double fixed time (FT) AI 72 and 96
hours after the last PG injection. For both these programmes single
FTAI can be used at 56 and 80 hours, respectively, but, because
they don't tightly control the timing of ovulation, conception
rates are likely to be reduced, particularly for PG. The choice of
single FTAI or double FTAI will be dependent on farm circumstances
and the cost of the semen being used.
GPG programmes (programmes which combine gonadotrophin-releasing
hormone (GnRH) and PG) are designed to more tightly control the
timing of ovulation and thus produce reasonable conception rates
with a single FTAI. As such, timing of insemination after the last
injection (the second GnRH [G]) is more critical. Ideally, cows
should be inseminated 16-20 hours (and definitely no more than 24
hours) after the last injection. For large numbers of cows this may
need careful planning to ensure that insemination occurs at the
right time. If necessary, varying the interval between the PG and
the second G over a range of 48-56 hours may ensure that the timing
of the FTAI is optimal (see Figure 4).
Figure 4: Critical timings for a GPG programme. The key
interval is the interval between the second G (GnRH) and the AI.
Conception rates will be reduced outside of the 16-20 hour window
(particularly if it is longer than 24 hours). The gap between the P
(PG) and the second G is less critical and can be manipulated to
ensure AI is optimal.