New Zealand Journal of Agricultural Research abstracts
The effect of maternal shearing and thyroid hormone treatments in
mid pregnancy on the birth weight, follicle, and wool characteristics
of lambs
P. R. Kenyon
R. G. Sherlock
T. J. Parkinson
S. T. Morris
Institute of Veterinary, Animal and Biomedical Sciences
Massey University
Palmerston North, New Zealand
Abstract Mid pregnancy shearing of ewes has been
associated with a short- to medium-term increase in maternal thyroid
hormone concentrations. To determine the role of this elevation in the
observed changes in lamb birth weight and fibre characteristics, ewes
in mid pregnancy were either: shorn at day 70 of pregnancy with thyroid
glands left intact (Shorn), unshorn with intact thyroid glands
(Unshorn), thyroidectomised but left unshorn with a short-term
elevation in thyroxine (T4) concentrations in mid pregnancy (Tx+T4), or
unshorn with intact thyroid gland and prolonged elevation in T4
concentrations (+T4). The Tx+T4 regime successfully increased maternal
thyroid hormone for a short period post-treatment, while the +T4 regime
increased maternal thyroid hormone levels for a prolonged period during
mid to late pregnancy. Lambs born to Shorn ewes were significantly (P < 0.05)
heavier than those born to Tx+T4 and +T4 ewes and tended to be heavier
than those born to Unshorn ewes (P = 0.10).
Birth weights of lambs born to Tx+T4 and +T4 ewes did not differ from
those born to Unshorn ewes. Ewe treatment had no effect on lamb wool
follicle characteristics or fibre diameter, coefficient of variation of
mean fibre diameter, yield, or loose wool bulk; but treatment tended to
affect colour (P = 0.05) and staple length (P = 0.09).
These results suggest that an elevation in maternal thyroid hormones is
not the sole endocrine mechanism responsible for either the birth
weight effect or changes in fibre characteristics observed in lambs
born to mid pregnancy shorn ewes. However, it is still possible that an
elevation of T4 and T3 concentrations in conjunction with other factors
are required to achieve the observed effects.
Keywords mid pregnancy shearing; lamb birth
weight; wool follicle development; thyroxine
A04064; Received 25 June 2004; accepted 28 April 2005; Online
publication date 4 August 2005
New Zealand Journal of Agricultural Research, 2005, Vol. 48:
293–300
0028–8233/05/4803–0293 © The Royal Society of New Zealand 2005
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