The reproductive efficiency of big and small ewes

Abstract

The reproductive performance of big and small ewes was investigated in a study carried out on the Lincoln University Research Farm, Ashley Dene, between January and August 2002. 635 mixed age ewes (mainly Coopworth) were sourced from 8 commercial farms as either big (330) or small (305) depending on their frame size.

The big ewes weighed 68kg on average, 16% heavier than the small ewes throughout the study, but had similar levels of body condition.

The mean onset of the breeding season was 4 days earlier for the big ewes. The mean date for onset of oestrous activity for both big and small ewes was 19 March and 23 March 2002 respectively.

The mean ovulation rate (from laparoscopy over the first cycle of joining (18 April, 19 April, 26 April and 2 May)), for the big and small ewes was 1.92 and 1.78 respectively, and the number of foetuses (from ultrasonic scanning) was 1.69 for big ewes and 1.62 for small ewes. The OR and number of foetuses of small ewes increased with increasing liveweight (2%/kg and 2.3%/kg respectively) and body condition and joining. There were no similar relationships for big ewes. Liveweight changes from joining to scanning has no significant affect on the number of foetuses for either big or small ewes.

Based on the difference in liveweight, 16% more small ewes could be stocked per hectare, and even with lower ovulation rates, would be capable of producing more lambs per hectare (32) than the smaller number of large ewes (29), thus increasing New Zealand’s lamb output.