Plasma progesterone concentrations during pregnancy and parturition of ewes grazing yarloop clover.

Abstract

Summary. Progesterone concentrations in the peripheral plasma of pregnant ewes grazing oestrogenic (yarloop clover) and non-oestrogenic (rye grass) pastures were compared. Whilst the overall pattern of plasma progesterone throughout pregnancy in the two groups was similar, the mean of values determined from the 90th day of gestation in ewes on yarloop was significantly lower than those on grass. No significant differences were recorded in the length of pregnancy or birth weight of lambs (P>0\m=.\05)although some evidence of maternal dystocia was obtained for the ewes on yarloop. In relation to parturition, mean plasma progesterone values in both groups began to fall 96 hr before birth, at first gradually then rapidly in the 16 hr preceding birth. Ewes on grass had a higher (P<0\m=.\01) plasma progesterone concentration 0 to 8 hr before birth but neither the level nor the pattern of progesterone disappearance appeared to be related to the lambing difficulties observed. Two classical symptoms of clover disease in ewes, infertility and dystocia were described by Bennetts, Underwood & Shier (1946). Determination of plasma progesterone concentrations throughout the oestrous cycle of young ewes grazing oestrogenic (yarloop clover) pastures indicated that corpus luteum function in the infertile animals was reduced (Obst & Seamark, 1970). To investigate the possibility of there being similar endocrinological disturbance underlying dystocia, the concentrations of plasma progesterone of pregnant ewes exposed to oestrogenic pastures have been determined throughout pregnancy and parturition. The observations made reveal that plasma pro¬ gesterone levels in ewes grazing yarloop pastures are significantly lower in late