The effect of mid-pregnancy shearing on lamb birthweight and survival to weaning

Mid-pregnancy shearing has been promoted in New Zealand as a technique to improve both lamb birthweights and survival. In the literature mid-pregnancy shearing has resulted in an increase lamb birthweight. However, the results have been inconsistent in magnitude and birthrank specificity. Additionally the mechanism responsible for the birthweight response has not been identified. The objectives of this study were to: ascertain the causes of the variation in the birthweight response to mid-pregnancy shearing, identify a possible metabolic mechanism for the birthweight response and lastly, to examine the effect of mid-pregnancy shearing on the newborn lamb's thermoregulatory capability and survival rate to weaning. The ftrst study was designed to determine whether dam nutrition during the midto latepregnancy period influenced the birthweight response to mid-pregnancy shearing. Mid-pregnancy shearing was found to significantly (P<O.05) increase lamb birthweight without differently affecting the birthweights singleor twin-born lambs. Dam feeding level post-shearing had no effect on the birthweight response. The newborn lamb's thermoregulatory capability was not affected by dam shearing treatment. When the results of this study were compiled with those of previous pregnancy shearing studies it was concluded that the birthweight response is greatest under conditions in which the unshorn ewe gives birth to a lamb(s) of low birthweight. The second study was designed to examine the birthweight response to mid-pregnancy shearing under two differing maternal treatments (one designed to restrict foetal growth (low group), the other designed not to limit foetal growth (maintenance group» allowing the conclusion made in Chapter 2 to be tested. Mid-pregnancy shearing was found to increase the birthweights of singletons (P<O.05) but not twins, and of lambs born to maintenance (P<O.05) but not low group ewes. Additionally, mid-pregnancy shearing had no effect on the thermoregulatory capacity of twin-born lambs. When the results of this study were considered with previous studies it became apparent that there are two criteria (not one) that must be met to achieve a birthweight response to mid­ pregnancy shearing. First, the dam must have the potential to respond (i.e. give birth to an otherwise lightweight lamb(s» and, second, she must have the means to respond (i.e. an adequate level of maternal reserves and/or level of nutrition to partition towards additional foetal growth).