Lamb wastage and its association with ewe body condition

Abstract

The effect of ewe body condition and live weight, and change in these factors, on lamb wastage was examined in a composite (principally Coopworth) flock of small- or large-framed ewes. Approximately equal sized flocks of small- and large-framed ewes, balanced for age and condition were selected from 9 commercial farms, and transported to Lincoln University’s Ashley Dene Pastoral Systems Research Farm where they were managed as one mob until pre-lambing in 2002, where the multiple bearing ewes remained for the following year’s trial (2003). Small-framed ewes in 2002 (n = 310) had significantly (P < 0.001) lower LW and GR (LW = 61.6 ± 0.73 kg; GR = 8.23 ± 0.26) compared with large-framed (n = 296) ewes (LW = 71.2 ± 0.75 kg; GR = 8.74 ± 0.26). Small-framed ewes in 2003 (n = 323) also had significantly lower (P < 0.001) average mating LW (52.4 ± 0.62 kg) compared with large-framed ewes (60.8 ± 0.63 kg). Crayon-harnessed Coopworth rams (n = 14) were introduced to all ewes on 11 April 2002 (n = 614) and 9 April 2003 (n = 643) and crayon-marked ewes were drafted once weekly for the first three weeks for determination of ovulation rate (0, 1, 2, 3, and 4) by laparoscopy. Pregnancy rates (0, 1, 2 or more fetuses) were recorded by use of transabdominal ultrasonic scanning on 4 July (2002) and 22 July (2003). Non-pregnant ewes and ewes with 1 fetus were sent back to original owners. Ewes were fully recorded at lambing. There was significantly (P < 0.001) higher embryonic mortality up to scanning (23 & 19%, in 2002 & 2003 respectively) compared with losses from scanning to lambing (1 & 3%, in 2002 & 2003 respectively). There were highly significant (P < 0.001) differences between GR and LW of ewes at mating in 2002 (GR = 8.48 ± 0.22; LW = 66.29 ± 0.60 kg) compared with 2003 (GR = 4.81 ± 0.21; LW = 56.56 ± 0.59 kg) that were attributable to climatic effects. As a result ewes in 2002 had a significantly higher OR (1.89 vs 1.74), scanning (165 vs 155%), and lambing percentage (207 vs 150%), than in 2003, respectively. There was a significant (P < 0.001) positive linear correlation between mating live weight and OR up to 3 CL in 2002 and up to 4 CL in 2003. Exclusion of non-pregnant ewes from the scanning data resulted in a positive relationship between number of lambs scanned and LW and GR. In 2002 ewes with embryo losses had a significantly (P < 0.05) lower reduction in GR (2.52 ± 0.35) compared with ewes that maintained their pregnancy (3.05 ± 0.19); however there was no effect of change in GR in 2003. Change in LW had a larger effect on embryo wastage than static weight did. It was concluded that ewes have higher embryonic losses up to scanning compared with after scanning, and that change in ewe body condition and LW have a greater effect on these embryo losses compared with static ewe body condition and live weight at mating.