Effects of luteectomy on the maintenance of pregnancy, circulating progesterone concentrations and lambing performance in sheep

Abstract

In sheep, there have been few and conflictingdata regarding the necessity of the corpus luteum (CL) for the maintenance ofpregnancy. The aims of the present study were to examine the effect ofluteectomy on and after Day 50 of pregnancy on maternal plasma progesteroneconcentrations and the progression of pregnancy, to determine the minimumplacental progesterone support required for the maintenance of pregnancy, andto evaluate the effect of luteectomy on lambing performance. In Experiment 1,four ewes luteectomized on Day 50 of pregnancy aborted 2-7 days aftersurgery, whereas pregnancy progressed and parturition occurred between Days143 and 149, with live lambs, in three of four ewes and in four ewesluteectomized on Days 60 and 70 of pregnancy respectively. The mean ( SEM)progesterone concentrations on the day before and one day after luteectomydecreased from 4.87 0.85 to 0.42 0.06 ng mL <emph type="7">-1(P&lt;0.01), from 4.57 0.51 to 0.80 0.12 ng mL<emph type="7">-1 (<emph type="7">P&lt;0.02) and from 6.050.52 to 1.67 0.11 ng mL <emph type="7">-1(P&lt;0.01), respectively, for the ewes luteectomized onDays 50, 60 and 70 of pregnancy. The fall in progesterone concentrations was90%, 80% and 71%, respectively, for the ewesluteectomized on Days 50, 60 and 70 of pregnancy. In Experiment 2, pregnancyprogressed in four ewes luteectomized on Day 70 and parturition occurredbetween Days 146 and 149, with live lambs. The mean progesteroneconcentrations declined (P&lt;0.01) from 6.9 0.7 ng mL<emph type="7">-1 on the day before luteectomy to 2.1 0.3 ng mL<emph type="7">-1 the day after surgery. The concentrations ofprogesterone in blood collected every 3 h during a 24-h period were stable onDays 60 and 80 of pregnancy, but they were lower(P&lt;0.03) on Day 80 than on Day 60 of pregnancy, foreach time period examined. In Experiment 3, the gestation length andbirthweights of single, twin and triplet lambs were not different between thecontrol intact ewes (n = 111) and the ewesluteectomized on Days 70-80 of pregnancy (n= 71). Lamb mortality was not different between the two groups(7.2% v. 8.4%, control v. luteectomized).In conclusion, these results showed that (1) the sheepCL is necessary to maintain pregnancy until at least Day 60,(2) progesterone withdrawal induced by luteectomy on andafter Day 50 of pregnancy must be of a critical magnitude to provoke abortion,(3) after Day 60 of pregnancy, the CL and the placentatogether secrete more progesterone than required for pregnancy maintenance,(4) there is no apparent 24- hour rhythm in maternalplasma progesterone concentrations before and after luteectomy, and(5) luteectomy at mid pregnancy has no apparent effecton gestation length, lamb birthweight or lamb mortality.