Impacts of pregnancy shearing ewes on lamb performance : a systematic review, meta-analysis and narrative synthesis of birth-, weaning- and survival-related outcomes : submitted in partial fulfilment of a Master of Science in Animal Science, Massey University, Manawatu, New Zealand

Abstract

The sale of lamb is now the largest income driver for New Zealand sheep farmers. Therefore, improving lamb pre-weaning survival and weaning weight are important for driving profitability. A management option that has been shown in many studies to result in increased lamb birth weights is shearing ewes during pregnancy. Many studies have also investigated the effects of mid-pregnancy shearing on lamb performance to weaning. This wealth of data can, therefore, provide data to conduct a meta-analysis to quantitatively summarize the effects of pregnancy shearing on both lamb birth and weaning weight. A meta-analysis is a systematic and quantitative literature review method that uses statistical methods to bring together data from existing research to provide a more objective overall conclusion than a purely narrative review. The meta-analysis showed that overall, pregnancy shearing significantly (p < 0.05) increased lamb birth weight by an average of 0.50 standard deviations (Hedge’s d = 0.52), with greater increases observed when shearing occurred between days 42 and 100 of gestation (β = 0.63–0.70). The positive effect of shearing was not present when shearing was conducted after 100 days of pregnancy. The single predictor of “timing of shearing” explained 65% of the variation across trials (I²total = 77.5%). In contrast, the pregnancy shearing effect on weaning weight was non-significant (d = 0.09, p >0.05). A qualitative review of the literature indicated that in addition to the timing of shearing, parameters potentially related to the lamb birthweight response included: the type of comb used increased gestation length, level of ewe nutrition offered in pregnancy, increased ewe intake, increased maternal glucose and elevated thyroid hormones (T3 and T4), enhanced placental development and whether the dam had the potential to deliver otherwise lightweight lambs (i.e. giving birth to multiples) and had the capacity and/or adequate body reserves at time of shearing. Pregnancy shearing under either housed or pasture grazing conditions has on some occasions resulted in higher perinatal lamb survival rates. In addition, some reports on lamb behaviour have shown that pregnancy shearing can improve behaviours relating to survival and the establish lamb-ewe relationships. Pregnancy shearing, however, had little to no effect on lamb wool characteristics. Pregnancy shearing has an effect on ewe behaviour, increasing their comfort behaviour and maternal behaviour, and resulted in increased ewe milk yield, higher milk protein and fat percentages, but had little or no effect on ewe wool traits. In summary for farmers to get a consistent birth weight response to pregnancy shearing there are clear management plans they should follow. Ewes should be shorn between 42 and 100 days of gestation, have a minimum BCS of 2.5, be shorn with cover/winter comb, be offered pasture covers of a minimum of 1300 kg DM/ha and be provided with shelter post shearing.