Effects of the presence of rams during pregnancy on lambing performance in ewes

Ewes of the Prealpes-du-Sud breed (n = 112) were mated with fertile rams and were used to investigate the effect of the presence of vasectomized rams during pregnancy on reproductive outcomes. Ewes in the control group (n = 56) were isolated from rams during the whole period of pregnancy, whereas those in the experimental group (n = 56) were kept with vasectomized rams from day 10 post-mating until lambing. At day 10 post-mating, a series of blood samples was collected every 15 min for 8 h from five control ewes and five experimental ewes to determine their patterns of LH secretion. The introduction of the ram was associated with a rapid increase of pulsatile LH release. The lag between the introduction of the ram and the onset of the first episodic LH release was less than 15 min. The mean(+/- sem) number of LH pulses/4 h after the introduction of the ram (2.8 +/- 0.4) was significantly higher (P < 0.01) than that observed/4 h before the introduction of the ram (1.4 +/- 0.2). Although more ewes were pregnant in the control group (87.5%) than in the ram-exposed group (82.1%), the difference was not significant. The presence of rams did not affect gestation length (145.8 days), overall lamb mortality (3.5%) or birth weights of single (3.95 kg), twin (3.24 kg) or triplet (2.59 kg) lambs. The proportion of ewes with multiple births in the control group (69.4%) was significantly greater(P < 0.05) than that in the ram-exposed ewes (47.8%). The ewes in the control group had significantly more (P < 0.01) twin lambs born alive (72.3%) than the ewes in the ram-exposed group (50.0%). In conclusion, the presence of vasectomized rams during early pregnancy affected the incidence of multiple births but did not affect pregnancy rate or gestation length. The altered fertility of ewes exposed to vasectomized rams may reflect changes in embryonic loss during early pregnancy. (C) 1998 Elsevier Science B.V. All rights reserved.