03829naa a2200289 a 450000100080000000500110000800800410001902400310006010000200009124501850011126000090029652029480030565000180325365000090327165000140328065000190329465000170331365000170333065000100334765300190335765300120337670000160338870000180340470000180342270000190344077300800345915303102023-08-28       1999    bl uuuu     u00u1 u    #d7 a10.1530/jrf.0.11501332DOI1 aMITCHELL, L. M.  aOvulation, fertilization and lambing rates, and peripheral progesterone concentrations, in ewes inseminated at a natural oestrus during november or february.h[electronic resource]  c1999  aThe objective of fuis study was to determine the relative importance of seasonal changes in ovulation rate, fertilization rate and embryo survival as the cause of reduced lambing rates in ewes mated in February compared with those mated in November. The study was conducted at 57°N using mature Mule ewes and Suffolk rams. Sixty ewes were allocated equally to five groups: unbred (OB) or mated at a natural ?strus during November (N) or February (F) by natural (N) or cervical artificial (A) insemination. Groups were maintained separately at pasture supplemented with haro A raddled vasectomized or non-vasectomized rarn was present with OB, NN and NA groups from 26 October 1995 to 1 January 1996 and with UB, FN and FA groups from 25 January 1996 to 31 March 1996. Ewes marked by the ram were recorded twice a clay, and those in groups NN, NA, FN and FA were inseminated at their second behavioural oestrus. For all ewes, blood samples were obtained once a clay from introduction of the vasectomized rams until30 days after mating (groups NN, NA, FN and FA) or 20 days after the first oestrus (group UB), and ovulation rate was measured by laparoscopy 7 days after the first oestrus. For ewes in groups NN, NA, FN and FA, ovulation rate was measured again after the second oestrus and ova were recovered from six ewes Fer group for assessment of fertilization before autotransfer. Pregnancy and lambing rates were recorded at term. Mean (+- SE) dates of the first recorded oestrus for ewes in groups NN, NA and OB, and FN, FA and UB were 4 +- 1.1 November and 4 +- 0.9 February, respectively, and intervals between the first and second ?strus were 16 +- 0.2 and 17 +- 0.3 days (P &lt 0.01), respectively. Ovulation rates were 2.6 +- 0.08 and 2.0 +- 0.05 (P &lt 0.001), and peripheral progesterone concentrations during the luteal phase were 8.5 +- 0.25 and 7.6 +- 0.31 ng ml-1 (P &lt 0.05), for November and February, respectively. The difference in peripheral progesterone concentration was not solely attribulable to the difference in ovulation rate. There was no significant effect of month or method of insemination, or of embryo recovery and autotransfer procedures on pregnancy rates and the proportion of ewes that became pregnant were NN 0.92, NA 0.83, FN 0.67 and FA 0.75. For ewes undergoing embryo recovery and autotransfer, ova reoovered Fer corpus luteum were 1.00, 0.93, 1.00 and 0.92, fertilized ova Fer ovum recovered were 0.69,0.92,1.00 and 0.83, and lambs born Fer corpus luteum were 0.62, 0.79, 0.78 and 0.58 for NN, NA, FN and FA groups, respectively. There were no significant seasonal effects on fertilization rate or embryo survival. It is concluded that a seasonal decline in ovulation rate is the primary cause of reduced lambing rates in ewes mated in February compared with those mated in November. Pregnancy rates were high after mating in both periods and were not enhanced by the use of cervical insemination.  aEndocrinology  aEwes  aOvulation  aPregnancy rate  aProgesterone  aReproduction  aSheep  aOestrous cycle  aOestrus1 aKING, M. E.1 aAITKEN, R. P.1 aGEBBIE, F. E.1 aWALLACE, J. M.  tJournal of Reproduction and Fertilitygv. 115, n. 1, p. 133-140, Jan. 1999.