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Small Ruminant Research 56 (2005) 265–270
Technical note
The effect of melatonin treatment on wool growth and
thyroxine secretion in sheep
J.A. Abecia∗ , J.A. Valares, F. Forcada
Department of Animal Production and Food Sciences, Veterinary Faculty, Miguel Servet, 177, 50013 Zaragoza, Spain
Received 15 May 2003; received in revised form 5 March 2004; accepted 5 March 2004
Abstract
Melatonin implants are commonly used to control sheep reproduction during the winter solstice, but their effect on wool
growth is unclear. In this study, wool growth was measured in 17 Assaf (carpet wool), 21 Rasa Aragonesa (medium wool)
and 20 Merino (fine wool) ewes after inserting melatonin implants in January, February and April, and 13 ewes were used
as control. All ewes were previously ovariectomised and treated with oestradiol implants. The mean wool fibre diameter was
measured at the beginning of the study (1 January), after shearing (1 June), and at the end of the study (31 August). Wool
growth was lowest in April for ewes implanted on 1 January. Plasma thyroxine (T4) concentrations differed significantly
among breeds, except when peaked in June. In controls ewes, T4 levels were significantly lower in July. Fibre diameter was
affected by breed but not melatonin treatment. Inserting the implants in spring appeared to influence wool production. At the
same time, early treatment during the winter solstice could have a negative effect on wool growth in spring, except for fibre
diameter. The differences in wool growth and fibre diameter among breeds remained constant throughout the year.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Sheep; Melatonin; Wool; Thyroxine
1. Introduction
In ruminants, photoperiod can influence reproduction cycles (Yeates, 1949), pelage growth (Ryder,
1964), spring moult (Zeuner, 1963), appetite and
weight changes (Lincoln and Ebling, 1985) and horn
growth (Lincoln, 1990). However, it appears to have
little direct effect on wool growth rate in sheep, which
appears to be more breed specific (Sumner et al.,
1994).
Melatonin implants are commonly used to advance
the breeding season in sheep by mimicking an increas∗ Corresponding author. Tel.: +34-976761000;
fax: +34-976761612.
E-mail address: [email protected] (J.A. Abecia).
ing photoperiod (Haresign et al., 1990), but their effect
on wool growth is unclear. In the Mediterranean countries, the combination of spring shearing and melatonin treatment may have a negative effect on wool
production, but there is no such evidence for animals implanted with melatonin in the spring equinox
(Abecia et al., 2001) or early summer (Harris et al.,
1989). It has been reported that advancing the breeding season following the insertion of melatonin implants during the winter solstice (Forcada et al., 2002)
may have a detrimental influence on wool production.
The Assaf, Rasa Aragonesa and Merino ewes were
evaluated for wool growth in January, February and
April. Assaf is a dairy breed developed in Israel by
crossbreeding East Friesian (5/8) with the Awassi
(3/8) breed (Goot, 1986). Ewes yield 2.5–3.0 kg
0921-4488/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.smallrumres.2004.03.001
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J.A. Abecia et al. / Small Ruminant Research 56 (2005) 265–270
coarse fleece, long and mostly white of carpet quality and tail length is average with fat at the base.
Rasa Aragonesa is the second most common breed in
Spain (2.5 million heads) with medium wool, which
produces a medium-quality fleece (white with square
strands). The Spanish Merino is the most common
sheep in Spain (3 million heads) with very fine wool.
The objective of the present study was to determine
whether melatonin treatment could explain changes
in wool production under an increasing photoperiod
(from the winter solstice to the spring equinox). Since
exogenous T4 (Donald et al., 1994) or thyroidectomy
(Hynd, 1994) can have a direct effect on wool growth,
the role of thyroxine was also examined.
2. Material and methods
The study was carried out at the experimental research farm at the University of Zaragoza, Spain (latitude 41◦ 41 N), which complies with the European
Union Standards for Scientific Procedure Establishments. All protocols were approved by the Animal
Experimentation Ethics Committee of University of
Zaragoza.
The study included 17 Assaf (A), 21 Rasa
Aragonesa (RA) and 20 Merino (M) ewes. The body
weights and condition scores at the beginning of the
experiment were 73.6 ± 2.9 and 3.2 ± 0.1 for Assaf,
50.5 ± 0.5 and 2.7 ± 0.1 for Rasa Aragonesa, and
40.0 ± 2.5 and 2.4 ± 0.1 for Merino ewes. Treatment
included 17 animals (5 A, 6 RA, 6 M) receiving a
subcutaneous implant (18 mg melatonin, CEVA Sante
Animal, Libourne, France) at the base of the ear on
1 January, 14 ewes (4 A, 5 RA, 5 M) on 15 February and 14 ewes (4 A, 6 RA, 4 M) on 1 April. The
remaining animals, 13 ewes (4 A, 4 RA, 5 M), were
used as controls.
All ewes were ovariectomised and oestradiolimplanted 4 months before the beginning of the experiment to avoid any intromission due to changes
in reproductive activity and associated changes in
steroid hormone profiles. A 1.5 cm silastic subcutaneous implant (3.3 mm internal diameter, 4.6 mm external diameter; Karsch et al., 1973) with crystalline
oestradiol (Sigma Chemical Co., St Louis, USA),
previously soaked in water to prevent an initial peak
of steroid release was used.
Areas of wool on the flanks were dyed on 31 December using black commercial hair dye (Turcolor,
Egalle S.L., Zaragoza, Spain) to determine fibre
length. To calculate wool growth, a calliper was used
to measure the relaxed length (to the nearest millimetre) of the un-dyed section of the fibres each month,
starting 31 January. Blood samples were also taken to
measure T4 levels. Ewes were sheared on 1 June and
the fleeces weighed.
Mean fibre diameter was measured at the beginning
of the experiment (1 January), at shearing (1 June), and
at the end of the experiment (31 August), by an adaptation of the projection microscope method, which
is compatible with the official ISO 9000 137/1975
method. A bundle of wool fibres was mounted on a
microtome and sections were cut off on a microscope
slide and magnified 400×. Fibre width was calculated
using a calibrated ruler and the actual width (␮m) was
obtained by multiplying by 2.5.
The concentration of thyroid hormone was determined in single assay using the Coat-A-Count®
solid-phase 125 I radioimmunoassay for total T4 (Diagnostic Products, Los Angeles, CA). The intra-assay
coefficient of variation was 9% (detection limit:
2.5 ng/ml).
The mathematical model for the analysis of fibre
length, fibre growth, fibre diameter, fleece weight,
fleece weight/live weight0.75 and plasma T4 concentrations included fixed effects due to breed (A, RA
and M) and treatment month (melatonin in January,
February or April, or control) and residual error.
Interactions were tested and were found to be not
significant, therefore were not included in the model.
An LSD test was used to compare differences among
breeds and treatments. A paired t-test was used to
compare differences in fibre diameter.
3. Results and discussion
The wool growth rate was highest in winter and
peaked in June in all breeds (Fig. 1). Growth rates
were significantly different among breeds in January, March, April and June with the highest growth
in Assaf (P < 0.01). Mean plasma T4 concentrations (Fig. 1) remained constant (except for a peak
in June) and were always higher in Assaf ewes
(P < 0.01).
J.A. Abecia et al. / Small Ruminant Research 56 (2005) 265–270
267
Fig. 1. Least-squares mean (±S.E.) fibre growth (histogram, mm) and plasma T4 concentrations (line, pg/ml) throughout the experiment
of ewes implanted with melatonin on 1 January, 15 February and 1 April, or non-implanted (control), of the breeds Assaf (A), Rasa
Aragonesa (RA) and Merino (M).
All ewes implanted on 1 January had the lowest rate
of wool growth in April (Fig. 1). The T4 concentrations were significantly lower in control ewes in July
(P < 0.05). The three measurements of fibre diame-
ter varied significantly with breed but not melatonin
treatment. Fibre diameter remained constant throughout the experiment among groups and breeds (Fig. 2).
Finally, fleece weight and fleece weight/live weight0.75
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J.A. Abecia et al. / Small Ruminant Research 56 (2005) 265–270
Fig. 2. Least-squares mean (±S.E.) fibre diameter (␮m) of ewes implanted with melatonin on 1 January (1), 15 February (2) and 1 April
(3), or non-implanted (C), of the breeds Assaf (A), Rasa Aragonesa (RA) and Merino (M), at the beginning of the experiment (1 January),
at shearing (1 June) and at the end of the experiment (31 August).
varied significantly with breed (Table 1) while melatonin treatment had no effect. Wool growth rate was
highest in the winter season (January) for all breeds, in
agreement with Sumner et al. (1998), and fibre growth
peaked in June. Perhaps the shearing on 1 June affected the normal pattern of growth since it has a positive effect on growth rate (Hawker et al., 1985). Fibre
diameter was also significantly affected by breed.
There is little data on wool growth parameters in
Rasa Aragonesa sheep. According to the National
Association of Rasa Aragonesa Breeders, the mean
diameter is 25–28 ␮m and fleece weight is 1.8–3 kg
(MAPA, 1985), which are similar to the present study.
For Spanish Merino, the Spanish Association of
Merino Sheep Breeders reported that the average fibre
diameter is 21–23 ␮m and the average fleece is 3.73 kg
(Valera et al., 2002). In the present study, Merino
ewes had higher diameters and lower fleece weight
than that reported by the Association, reflecting the
specialisation for meat versus wool production on the
farms of origin. No data on wool quality of the Assaf
breed have been reported, except for fleece weight
(about 3 kg, Sheep & Goats Breeders Association
from Israel, web page: http://www.sheep-goats.org.il).
In general, melatonin treatment during the winter solstice until spring had no effect on mean wool
Table 1
Least-squares mean (±S.E.) fleece weight (kg) and ratio fleece weight/live weight0.75 of ewes implanted with melatonin on 1 January, 15
February and 1 April, or non-implanted (control), for the Assaf (A), Rasa Aragonesa (RA) and Merino (M) breeds
Ewes implanted
1 January
Fleece weight
A
RA
M
2.67 ± 1.10 (5) a
1.66 ± 0.13 (6) b
2.51 ± 0.34 (6) a
Fleece weight/live weight0.75
A
0.088 ± 0.034 a
RA
0.081 ± 0.007 a
M
0.133 ± 0.006 b
Control
15 February
2.97 ± 0.53 (4) a
1.54 ± 0.20 (5) b
2.27 ± 0.26 (5) a
0.116 ± 0.015 a
0.079 ± 0.010 a
0.150 ± 0.011 b
1 April
2.36 ± 0.17 (4) a
1.59 ± 0.23 (6) b
2.42 ± 0.22 (4) a
0.091 ± 0.032 a
0.079 ± 0.011 a
0.151 ± 0.024 b
2.43 ± 0.47 (4) a
1.91 ± 0.39 (4) b
2.26 ± 0.29 (5) b
0.098 ± 0.017 a
0.086 ± 0.014 b
0.123 ± 0.019 a
Means within a column within a trait not followed by the same latter differ (P < 0.05). Number of animals is given in parenthesis.
J.A. Abecia et al. / Small Ruminant Research 56 (2005) 265–270
growth rate or diameter. The only exception was
in April, when ewes implanted during the winter
solstice (1 January) had the lowest growth rates. It
has been suggested that natural and experimental
increases in daylength have a short-term inhibitory
effect on growing wool follicles (Pearson et al.,
1996). Thus, if animals implanted in January become
sensitive to increasing days when implants become
exhausted (90–100 days after implantation; Forcada
et al., 2003), the sudden change in photoperiodic
status from short-day perception (due to the implants) to long-day perception (natural photoperiod
of April), could decrease wool growth. Regarding the
other dates of implantation, the present findings support previous work based on Rasa Aragonesa ewes
implanted in March in the same latitude (Abecia
et al., 2001), where melatonin had no effect on wool
growth. Other reports suggest in Merino sheep that
melatonin is not directly involved in the seasonality effect of wool growth following implantation in
spring (McCloghry et al., 1992). Harris et al. (1989)
concluded that, although exogenous melatonin alters
the reproductive characteristics of Romney rams, it
does not completely explain a measurable change in
the pattern of wool production.
The secretion pattern of T4 was affected by breed,
in accordance with both wool parameters. The highest T4 concentrations and mean fibre growth rate were
in Assaf ewes, the lowest in Merino and intermediate for Rasa Aragonesa, which supports the idea that
T4 has an important influence on wool growth pattern
(Donald et al., 1994; Hynd, 1994). A significant proportion of the variation in wool growth rate and mean
fibre diameter in the present study may have been associated with permanent phenotypic differences among
ewes according to the rhythm based on seasonal wool
growth cycle (Sumner et al., 1994).
The T4 concentrations peaked in June in all three
breeds. Souza et al. (2002) reported the same peak in
December in the southern hemisphere. Differences in
the seasonal pattern of thyroid hormones reflect the
response of the thyroid to changes in the photoperiod (Hafez, 1959). The only effect of melatonin on
plasma T4 concentrations was observed in July, when
the control ewes had significantly lower T4 values,
after the June peak. Since these animals did not receive any implants, one can assume that the reduction
of plasma T4 levels after the summer peak reflects
269
circannual rhythm of secretion, as described by Souza
et al. (2002). Breed differences in plasma T4 levels
were observed throughout the experiment, which may
be associated with breed differences in wool growth.
Melatonin treatment did not affect fleece weight.
These findings are in agreement with Harris et al.
(1989) that no effect was observed for greasy fleece
weight or wool production.
In conclusion, treating ewes in spring with exogenous melatonin to improve reproduction has no influence on wool production in breeds with different wool
types. However, earlier treatments during the winter
solstice could have a negative effect on wool growth
in the medium term in spring, without affecting fibre
diameter. Breeds varied in wool growth rate and fibre diameter which remained constant throughout the
study.
Acknowledgements
This study was supported by the grant AGL20011817 and PETRI 1995-0520-OP from C.I.C.Y.T.,
Spain. The authors would like to thank Dr. Alex
Martino (CEVA Salud Animal, Spain) for generously
donating the melatonin implants.
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