Effect of selection for early lambing performance on the seasonal
patterns of gonadotrophin levels; response to GnRH and semen charcteristics
in adult rams
J.C. Brewer, R.M. Briggs, J. Parr, R.J. Wilkins, J.F. Smith
Postgraduate School of Animal Biology, University of Waikato,
Hamilton
Proceedings of the New Zealand Society of Animal Production. 1995,
55: 228-231
Little research has been done on within breed differences in seasonality
of reproduction in rams. We took 16 Dorset x Romney rams (8 BV+ and 8
BV-, selected on the basis of their breeding value for date of lambing derived
for the lambing performance of their female relatives). These rams were
housed indoors under natural light conditions and fed a ration of pellets and
hay. Semen samples, testis diameter and body weight measurements were
taken weekly. Blood samples were taken monthly (every 20mins for 26hrs,
and then every 10mins for a further 4hrs following a 1 ml IV injection of
0.001 mg GnRH). Samples were assayed for oLH and oFSH. The trial ran
from September 1993 to August 1994. More BV- (early lambing group)
rams (70% vs 25%) produced ejaculates in the first 3 months of the trial.
The BV- rams had significantly (P<0.001) lower ejaculate volumes than the
BV+ rams with both showing significant (P<0.001) seasonal changes. There
were significant (P<0.01) seasonal effects on semen concentration with a
January maximum of 6300 x 10 6 sperm/ml and a minimum in July of 3350
x 10 6 sperm/ml. There was a seasonal effect on testis diameter (55mm in
Dec-Feb and 38mm in July). There was a significant (P<0.05) seasonal shift
in live weight between Feb-Apr (70kg) and July 58kg). There was
significant (P<0.01) seasonal effect on basal LH levels and LH pulse
frequency, but there were no detectable BV effects. The BV- rams had
greater pulse amplitude and longer durations of peaks than did the BV+
animals. LH response to GnRH was significantly different only in Nov (BV-
rams showed greater area under the curve 162±35 vs 122±28 units). The
BV- rams showed a seasonal effect with a rise in Nov and a decline Feb
(P<0.05) while the BV+ rams showed little seasonal change. There was a
significant (P0.01) seasonal shift in FSH levels from a low of 2.3 ng/ml in
Sept to 40.0 ng/ml in Feb then a decline to 4.3 ng/ml in April but no effect of
BV group.
These results suggest that animals selected for early lambing may be more
sensitive to GnRH prior to the summer solstice.
Keywords: NZSAPAB;
out-of-season lambing; ram seasonality; reproductive hormones
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Last Updated 25-01-1997
