Food Magazine - The Food Commission

farming
“This milk’ll have them all getting broody!”
significant part of the cycle. After calving the
cow is ‘open’ and available for a new
conception, which will usually take place within
three months (a cow’s menstrual cycle is 21
days).
About two months before she gives birth, the
milking is stopped and she is ‘dried off’. Within
days of giving birth the milking routine starts
again. Within three months of giving birth, the
cow is made pregnant again. As the diagram of
the cow’s year shows (below), the cow is
pregnant for about seven out of ten months
lactation, from which we can deduce that at least
two thirds of our milk is extracted from pregnant
cows.
The cow’s year
Open
Milked 10 months
Pregnant 9 months
Dry
In order to maximise yields still further, a farmer
may be tempted to shorten the open period and
the drying off period, so that there are fewer
months when the cow is not lactating. The effect
of this would be to increase the proportion of
time that the milk is being collected while the
cow is pregnant.
Taking milk while a cow is pregnant, and
especially during the last few weeks of her
pregnancy, raises questions about hormones in
milk.
During pregnancy, the cow’s ovaries secrete
high levels of progesterone. Her placenta
secretes high levels of oestrogen. These
hormones, plus others, including corticosteroids,
growth hormones and prolactin, target the
mammary gland to stimulate lactation.
The figure below shows the hormone cycles
in cattle during a menstrual cycle (around 21
days) followed by pregnancy and calving. The
curves show blood levels of these hormones,
but milk can be expected to follow the pattern.
Indeed, one of the tests for whether a cow is
pregnant is to examine the progesterone levels in
milk. The answer is ‘yes’ if the progesterone
exceeds 10 microgram per litre of milk 1 , which is
typically higher than the blood levels.
New techniques are being explored to reduce
the ‘dry’ period before she gives birth. There are
moves towards using a one-month dry period, 2
and researchers are now demonstrating that a
zero dry period is possible. 3 Yet these last few
weeks of pregnancy are when circulating
hormones can rise to their highest levels.
The quantities of hormones in our milk supply
are not equivalent to pharmceutical amounts,
and their absorption from the alimentary tract
may be poor. But there is a surprising lack of
evidence about the possible effects on our health
of consuming cattle hormones in small
quantities, every day for decades. All we know is
that a string of research papers has highlighted
various concerns that appear to be linked to milk
and dairy food consumption (see box).
Infants will be especially vulnerable, partly
because they drink a lot of milk, partly because
they are still growing and may be more
vulnerable to hormonal interference, and partly
because, in the first few months especially, their
gut walls are more permeable to larger
molecules such as hormones.
Milk is a food that is rapidly expanding its
market base as diets in the Eastern and Southern
hemispheres become increasingly westernised.
Its production has become intensified and its
hormone content increased, yet we know next to
nothing about its potential impact on health. And
we haven’t even started to look at the other
components of milk, such as its enzymes,
antibodies or nucleotides.
1) P Rioux, D Rajotte. Progesterone in milk: a
simple experiment illustrating estrous cycle
and enzyme immunoassay. Adv Physiol
Educ 2004, 28:64-67.
2) KC Bachman. Milk production of dairy cows
treated with oestrogen at the onset of a
short dry period. J Dairy Sci 2002, 85:797-
803.
3) Washington State University: Dry Period –
Does the Cow Need One See
http://www.puyallup.wsu.edu/dairy/shortcou
rse/shortcourseArticles.asp
Blood levels of hormones during menstrual cycle and pregnancy of cattle
Source: Virginia State University, Dept Dairy Science.