The incidence of luteal activity, as determined by peripheral plasma progesterone concentration, before the onset of the breeding season in the Rasa Aragonesa breed of sheep

Abstract

It has been reported in several sheep breeds that the first oestrus of the breeding season is preceded by at least one ovulation unaccompanied by oestrous behaviour. This is associated with seasonal variations in sensitivit)' to oestrogen (Karsch el al., 1978), and progesterone could be involved in the initiation of the fldl-length oestrous cycle (I'Anson & Legan, 1988). Since the occurrence of silent ovulations has been previously demonstrated, the aim of this experiment was to study the pattern of progesterone secretion at the end of seasonal anoestrus, and its capacin' to establish the first oestrous bebaviour of the breeding season in the Rasa Aragonesa breed of sheep. A threshold of 0.5 ng ml -~ of progesterone was selected as an indication of luteal activity (Schirar el al., 1989). From approximately 60 days before the expected onset of the breeding season at 41 ° 40'N (5-21 August; Forcada el al., 1992), blood samples were collected evm T 6 days fi'om 144 anoestrous ewes. Oestrus was detected daily using aproned rams, and the blood salnpling continued in an individual ewe until first detected in oestrus. Only those samples collected fi'om 34 days (day-34) to 1 day before the first detected oestrus (day-1) (nvo complete oestrous cycles) were nsed. Animals were fed as a group to support their maintenance requirements. Plasma progesterone concentrations were determined by radioimmunoassay (Forcada et aL, 1992). For the purpose of statistical analysis, samples (n=744) were grouped every two consecutive days (-34+ -33; ...; -2+ -1 days beiore first oestrus). Mean (_+sE.x0 plasma progesterone concentl-ations were compared by analysis of variance, with the time of sampling as treatment effect and individual treated as a random effect. Data were recoded on the basis of progesterone concentration ( 0.5 ng ml <, 1). The hypothesis that the probability of plasma samples with progesterone concentrations lower than 0.5 ng ml -~ was equal to 1 was checked using a t)inomial test, with a test proportion of 0.999 (as the maximum value accepted by