VARIATION IN OVULATION RATE AND LITTER SIZE OF CAMBRIDGE SHEEP

Ovulation rate (OX) and litter size (LS) were recorded in a flock of C ambridge sheep from 1984 to 1994 and from 1976 to 1994 respectively. T here were 1516 and 2480 OR and LS records with means of 3.2 and 2.5 re spectively. There was considerable variation in results between years ranging from 2.2 to 3.2 for LS and from 2.8 to 4.2 for OR. Ewe lamb OR was affected by year of recording, some years having higher OX than o thers. Rearing type (single, twin or triplet) also affected ewe lamb O R, twins having lower OX than both singles and triplets. Ewe lamb OR d ecreased with later date of birth reflecting a lower age at OR countin g. Ewe lamb LS was affected by year of recording and there was also an interaction between date of birth and year indicating that OR decreas ed with later date of birth in some years but not in others. OR of adu lt ewes was affected by year of recording which also interacted with a ge (2, 3 and 4). Generally, OR increased with age but this trend was n ot observed in all years. LS of adult ewes was significantly affected by year and increased with ewe age (2, 3 and 4). Heritability and repe atability were determined for ewe lamb OR and LS and adult ewe OX and LS in univariate and two-trait analyses using DFREML. Heritabilities o f ewe lamb OR (0.18 to 0.21) and LS (0.05 to 0.08) were lower than cor responding adult ewe values for OX (0.19 to 0.34) and LS (0.19 to 0.22 ). Ewe OR had a lower repeatability (0.43 to 0.55) than LS (0.30 to 0. 80). High genetic correlations (>0.9) were obtained for combinations o f ewe lamb OX, ewe lamb LS, adult ewe OR and adult ewe LS. The importa nce of analysing ewe lamb and ewe records separately is discussed as i s the value of ewe lamb OX as a selection criterion.