Lambing behaviour of Merino ewes from lines subjected to divergent selection for multiple rearing ability from the same base population

Abstract

Timing of birth and birth site selection were investigated in two lines of grazing Merino sheep that had been divergently selected for multiple rearing ability. Time budgets in the neonatal phase were also compared between the two lines. Parturition was slightly more likely to commence during daylight hours (06:00-18:00) than at night (18:00-06:00) (0.548 vs. 0.452). Intervals between consecutive onsets of parturition in 685 ewes conformed to an exponential distribution, expected for intervals between consecutive, random occurring events. A significant excess of observed short intervals above those expected was found. This observation could either be coincidental, or indicative of an underlying mechanism that may operate as a trigger to synchronize the commencement of parturition. Recorded birth sites of 606 ewes not assisted at birth were found to differ from a random distribution in some of the 10 lambing paddocks used during the study. In four night paddocks, the frequency of birth sites in the 50% of the area situated on the boundaries was higher than in the middle portion. In the paddocks used during daytime an increased frequency of birth sites appeared to be associated with a tree on the boundary of two adjacent paddocks. In the five paddocks used at night, birth site frequencies differed when the paddocks were divided along the length in three blocks of equal size. In the paddock nearest to the base building, birth sites appeared to be concentrated along the boundary furthest from the building that served as base for the observers and the nearby floodlights. Selection line did not appear to influence the choice of a birth site. The period that ewes were observed to groom their lambs was lowly repeatable (0.13?0.06). No line difference was observed in the period that ewes spent grooming. Ewes caring for viable multiples groomed their offspring for a longer period than those caring for singles. Mature ewes tended to groom their lambs for a longer period than primi-parous maidens. A higher proportion of High (H) line ewes groomed their progeny shortly after birth than Low (L) line ewes. Later on (90-120 min. postpartum), L line ewes were more likely than those in the H line to graze. The latter group of ewes was more likely to stand with their offspring at this stage.


South African Journal of Animal Science Vol.32(1) 2002: 57-65