The relationship between joining live weight and ovulation rate and number of lambs born was examined in a composite flock of small- and large-framed ewes. Approximately equal numbers of small- (n = 315) and large (n = 305) framed, mixed-age ewes of equal average condition score, selected in the same proportion from 9 commercial flocks (mainly Coopworth), were managed together from early March to pre-lambing. The average pre-joining live weight (±SD) of small-framed ewes (61.3 ±6.35 kg). was significantly (P<0.001) lower than that of large-framed ewes (71.1±6.58 kg). Ewes were joined with 14 Coopworth ram hoggets fitted with ram harnesses on 8 April. Ewes marked by rams were temporarily removed from the flock every 7 days for 21 days and ovulation rate (0, 1, 3, 4) recorded by laparoscopy the following day after fasting. Pregnancy status (0, 1, 2 or more) was recorded by ultrasonography on 4 July. Ewes were fully recorded at lambing. The mean ovulation rate (OR) of small-framed ewes was significantly lower (1.76, P<0.05) than that of large-framed ewes (1.92). The regression of OR on joining live weight (jLW) was significant (P<0.001) for small-ewes (OR = 0.0216 x jLW + 0.463) but there was no relationship between OR and jLW in large-framed ewes. Maximum OR (1.92) was predicted to occur at 67.5 kg jLW. The results of this study indicate an upper limit to the live weight ovulation rate relationship. Compared on the basis of equal ewe live weight per ha, OR per ha was 12% higher for small- than large-framed ewes.
Proceedings of the New Zealand Society of Animal Production, Volume 63, Queenstown, 140-143, 2003
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