Organising the breeding plan of a seasonally breeding species, such as
sheep, presents a challenge to farmers and the industry as a whole,
since both economical and biological considerations need to be
carefully balanced. Understanding the breeding activity of individual
animals becomes a prerequisite for a successful breeding program. This
study set out to investigate the sources of variation and the genetic
profile of the spontaneous, out-of-season ovulatory activity of ewes
of the Chios dairy sheep breed in Greece. The definition of the trait
was based on blood progesterone levels, measured before exposing the
ewes to rams, which marks the onset of the usual breeding season. Data
were 707 records, taken over two consecutive years, of 435 ewes kept
at the Agricultural Research Station of Chalkidiki in northern
Greece. When all available pedigree was included, the total number of
animals involved was 1068. On average, 29% of all ewes exhibited
spontaneous, out-of-season ovulatory activity, with no substantial
variation between the years. Significant sources of systematic
variation were the ewe age and live weight, and the month of previous
lambing. Older, heavier ewes, that had lambed early the previous
autumn, exhibited more frequent activity. Heritability estimates were
0.216 (±0.084) with a linear and 0.291 with a threshold
model. The latter better accounts for the categorical nature of the
trait. The linear model repeatability was 0.230 (±0.095). The
results obtained in this study support the notion that spontaneous
out-of-season ovulatory activity can be considered in the development
of a breeding plan for the Chios sheep breed