Abstract

Differences between breeds in seasonal breeding activity have been linked to seasonal shifts in both LH levels and the magnitude of oestradiol negative feedback. Studies were undertaken to determine the endocrine differences in ewes of the same breed with high or low breeding values (BVs) for date of lambing. Forty-eight ewes (24BV- early lambing and 24BV + later lambing) were selected from the flocks and 16 ewes (8 of each BV class) were ovariectomized (ovx) and implanted with silastic implants containing oestradiol- 17Beta. All ewes were bled 2 x week for a 12 month period commencing in November to provide data on basal LB. The ovx ewes were subjected to rapid sampling (every 20 min for 24 h) using automated sampling equipment, on 5 occasions approximately 1 month after each equinox and solstice in that year and samples were analysed for LH pulse frequency. Basal LH levels showed a seasonal shift with the ovx ewes having a greater amplitude of change than the entire ewes. LH levels in the ovx ewes were lower in November to January, and higher in March, April, May and June; and lower again in August, September, October than in entire ewes. There was no evidence of a major difference in pattern due to BV. In the ovx ewes the values for the BV- animals tended to be lower than those for the BV+ ewes during March through July. At the initial rapid bleed no animals showed pulse activity. In April the BV- group had more pulses per 24 h (6.0 v 2.8). In July there was no difference in proportion having pulses (8/8) nor in the pulse frequency (5.3 v 5.6/24 h). In October the BV- animals had a higher pulse frequency (2.8 v 1.0), while the proportion of animals (3/8) exhibiting pulses was the same. More BV- ewes had pulses in January (6 v 3) and of those with pulses the BV- group had higher frequency (7.7 v 4.7). These data confirm the seasonal shifts in basal LH levels and in LH pulse frequency. Selection for earlier lambing had no effect on the pattern of basal levels. In the ovariectomized ewe the differences in LH pulse frequency due to BV detected in October and final bleed in January were not as marked as expected for the differences in the breeding activity of the two flocks but they confirm that differences in sensitivity to oestradiol 17Beta negative feedback are associated with differences in the onset of breeding season of ewes selected for earlier lambing within one breed. It would appear that selection for earlier lambing has disturbed the association between day length and LH pulse activity and the sensitivity of the pituitary to GnRH.

P, Sharp, TW Knight, and J Hodgson

Proceedings of the New Zealand Society of Animal Production, Volume 55, , 183-185, 1995