INFLUENCE OF YEAR, AGE, AND GEOGRAPHICAL LOCATION ON INDUCED ESTRUS IN EWES EARLY IN THE BREEDING-SEASON

Oestrous activity was monitored at two locations (Invermay in Otago an d Woodlands in Southland) in ewe flocks of similar age structure, sync hronised at the beginning of the breeding season each year for 5 years (1991-95). After an Ii-day CIDR treatment, interval to oestrus, durat ion of oestrus, and ovulation rate were measured. Age of ewe affected the mean interval to oestrus (32.4-41.0 h), with 2-tooths commencing u p to 10 h later than older ewes (P < 0.05). There was a significant lo cation effect in 1992 when oestrus commenced 8.6 h later at Woodlands than at Invermay (41.0 versus 32.4 h; P < 0.05). Duration of oestrus d iffered significantly between some years (P < 0.05) at Woodlands but n ot at Invermay. There was also a difference between locations whereby Woodlands animals had a longer oestrus (23.4-34.2 h), by 7.4-17.5 h (P < 0.001), than those at Invermay (15.0-18.1 h). This difference did n ot occur in 1995, when both flocks were located at Woodlands. Mean ovu lation rate (1.9-2.2) was not significantly different either within or between locations. These data suggest that even in the same flock, ag e of ewe, year, and location may influence oestrous activity in ewes t reated to manipulate the oestrous cycle at the beginning of the breedi ng season.