Effect of previous lambing date and subsequent month of mating on reproductive performance in Mule ewes

The objective was to determine the effect of previous lambing date and subs equent month of mating on reproductive performance in Mule (Bluefaced Leice ster X Scottish Blackface) ewes. Sixty-four ewes which had previously lambe d in January (13 January (s.e. = 1 day)) and 80 ewes which had previously l ambed in May (15 May (s.e. = 1 day)) were allocated equally to four mating periods (30 August to 17 September, 1 November to 19 November, 3 January to 21 January and 14 February to 4 March) in a 2 X 4 factorial design. From 2 0 days before and during their designated mating period, January- and May-l ambing ewes were separately housed in straw-bedded pens under natural photo period and were given 1 kg per head per day dried grass pellets. A-vasectom ized ram was continuously present with each group for 17 days and was repla ced by a raddled, fertility tested entire Suffolk ram at the start of the m ating period. Ewes were mated at a single natural oestrus and those marked by the ram were recorded daily. Ovulation rate was measured by laparoscopy on day 6 after mating. For ewes which had previously lambed in January (16 per group), numbers by month of mating that showed oestrous behaviour, ovul ated and were pregnant, respectively were: September, 16, 15 and 12; Novemb er, 16, 15 and 24; January, 15, 15 and 10 and February, 15, 16 and 7. Mean (s.e.) ovulation rates by month of mating were 2 1 (0.16), 25 (0.19), 2.1 ( 0.09) and 2.2 (0.19) corpora lutea per ewe ovulating, and lambing rates by month of mating were 2.3 (0.25), 1.9 (0.25), 2.2 (0.24) and 0.8 (0.23) lamb s per ewe to the ram. For ewes which had previously lambed in May (20 per g roup), numbers by month of mating that showed oestrous behaviour, ovulated and were pregnant, respectively, were: September, 13, 20 and 12; November, 20, 20 and 19; January, 20, 20 and 17 and February, 20, 20 and 13. Mean (s. e.) ovulation rates by month of mating were 2.0 (0.13), 2.3 (0.12), 2.1 (0. 05) and 2.2 (0.11) corpora lutea per ewe ovulating and lambing rates by mon th of mating were 1.0 (0.21), 2.1 (0.15), 1.5 (0.17) and 1.2 (0.21) lambs p er ewe to the ram. Ovulation, pregnancy and lambing rates were not influenc ed by previous lambing date, but lambing rates were significantly (P < 0.01 ) reduced for ewes mated in September and February compared with November. Results demonstrate that in Mule ewes acceptable ovulation rates can be ach ieved throughout the period September to February but lambing rates are red uced when ewes are mated at the extremes of their natural breeding season. The main factor contributing to the reduction in lambing rates was an incre ase in the number of ewes failing to establish pregnancy as a consequence o f ovulation without oestrous behaviour, fertilization failure and (or) tota l embryo loss.