Genetic parameters of spontaneous spring ovulatory activity in Merinos d'Arles sheep

The genetic parameters of spontaneous spring ovulatory activity were invest igated in the Merinos d'Arles breed under the usual pastoral and transhuman t management conditions of this breed in southeastern France. Ovulatory act ivity was determined by assaying the plasma progesterone concentration in t wo blood samples taken 8-10 days apart. The data set consisted of 1887 ovul atory activity performance measurements in 1995, 1996 and 1997 on 933 ewes, daughters of 176 rams. The effects of the 'physiological status' (hoggets, adult ewes with or without lambing in the previous autumn), age and live w eight just before the mating period were found to be highly significant. Th ey were included in the linear animal model and the threshold sire model us ed to estimate genetic parameters. On average, 27.9 % of ewes exhibited ovu latory activity in April. Age and live weight just before the mating period had a marked positive effect on ovulatory activity. A difference of about 8-9 % was observed between extreme classes for these factors. The heritabil ity and repeatability estimated through the linear model were 0.20 (standar d error: 0.04) and 0.30 (0.07), respectively. When using the threshold mode l, the heritability was 0.37. These values led us to conclude that a geneti c approach for improving spontaneous spring ovulatory activity should be fu rther developed. Nevertheless, further studies are necessary to determine a ll the implications of such selection. (C) Inra/Elsevier, Paris.