Agriculture Reference
In-Depth Information
that large amounts of hormones from livestock
farms are released into the environment each
year (Table 17.1).
These HGPs are primarily used in the beef
cattle industry, as exogenous androgens and
oestrogens have little efficacy in pigs (Chaudhary
and Price, 1987). In the dairy industry, proges-
terone-releasing implant devices are approved
in lactating cows for oestrus synchronization
(Price and Webb, 1988).
Exogenous hormonal growth
promoters
Naturally occurring HGPs
In both humans and animals, sex steroids such
as oestrogens, testosterone and progesterone
regulate growth and development. This has led to
the use of hormonal growth promoters (HGPs),
natural sex steroids or their synthetic counter-
parts, in meat animals to increase feed efficiency
and weight gain. Synthetic HGPs are widely
used by the largest cattle-producing countries
in the world, including the USA, Australia,
Argentina and Canada (Silence, 2004). The first
approved synthetic oestrogen was diethylstilbes-
trol (DES) in 1954. Because of its carcinogenic
potential, DES was banned for all use in cattle
production by the FDA in 1979 (Hayes, 2005).
Currently six different hormones are
approved for such use in the USA, including
three natural hormones (E2
The direct effects of exogenous E2b administra-
tion on farm animals (calves, heifers, steers,
lambs) include enhanced protein deposition in
skeletal muscle and reduced nitrogen excretion
(Meyer, 2001); growth performance is increased
by 5-15%. Testosterone or other androgens are
less active as compared to oestrogens in cattle or
lambs, probably because there are fewer androgen
receptors than oestrogen receptors (Sauerwein
and Meyer, 1989). The anabolic mode of action
of steroidal hormones is mediated directly by
several organs and tissues (liver, bone, skin and
other tissues) and indirectly via the somatotropic
axis involving growth hormone and insulin-like
growth factor I. The exogenous doses of these nat-
urally occurring HGPs are less than the amounts
naturally produced in mature bulls and pregnant
cows, so their environmental risk is considered
negligible (Avery and Avery, 2007).
, testosterone and
progesterone) and three synthetic compounds
that mimic the functions of these hormones
(trenbolone acetate (TBA), zeranol and melen-
goestrol acetate (MGA); Le Guevel and Pakdel,
2001). The chronology of the use of these hor-
mones in cattle in the USA is listed in Table 17.2
(Raun and Preston, 2002). The use of all of
these is banned in the EU.
b
Synthetic HGPs
Trenbolone acetate mimics the activity of testos-
terone and is administered as a subcutaneous
Table 17.1.
Predicted total excretion of oestrogens from the human and farm animal populations in
the UK (2004).
Population
(million)
Oestrone
(kg year
−1
)
Oestradiol
(kg year
−1
)
Discharge
percentage
a
Type
Human
59
219
146
17
Dairy
2.2
693
365
49
Pig
5
367
19
18
Broiler chicken
112
15
34
2
Laying hens
b
29.2
NC
NC
12
Breeding ewes
7.6
19
6
1.9
Non-breeding sheep
1.5
1.6
0.4
0.1
Total farm animals
157.5
1096
424
83
Total
216.5
1315
570
100
a
Based on the total amount of oestrone and oestradiol.
b
The combined amount of oestrone and oestradiol is 260 kg year
−1
.
NC, not calculated because of insufficient data.
