Shearing ewes at mid-pregnancy is associated with changes in fetal growth and development

Abstract

This study investigated the effect of mid-pregnancy shearing (at Day 70 of pregnancy, P70) on herbage intake of grazing single- and twin-bearing ewes, lamb birth weight, and cold resistance of new-born lambs. At pregnancy diagnosis on P50, 30 single-bearing and 30 twin-bearing ewes were allocated either to be shorn at P70 (n = 15 for each pregnancy rank) or to remain unshorn to serve as controls (n = 15 for each pregnancy rank). All ewes were mated over a 3-day period with synchronisation of their oestrus. Herbage intake was measured indirectly from in vitro pasture digestibility and faecal output of grazing ewes, with the use of intra-ruminal chromium slow-release capsules, over six 5-day periods from P64 to P105. The weights of placental and fetal tissues were assessed in a subgroup of 16 ewes at P140 and P141. In the remaining sheep, lamb liveweight at birth and during lactation until weaning at 103 days of age was measured, and cold-resistance of new-born lambs was assessed by measuring summit metabolic rate (SMR) by indirect calorimetry. Ewe liveweight (corrected for fleece weight), condition score, and herbage intake during pregnancy were not affected by shearing treatment. Mid-pregnancy shearing did not affect placental weight, but increased the relative weights (i.e. g/kg liveweight) of fetal thyroid gland and lungs and reduced the relative weight of adrenal glands and heart. The ratio of secondary to primary wool follicles in near-term fetal skin was about 10% higher in offspring of shorn than of unshorn ewes. The metabolic rate of fetal hepatic tissue was increased by mid-pregnancy shearing, particularly in twin fetuses, possibly indicative of an increase in placental transport of nutrients to the fetuses. This conclusion is supported by the greater birth weight (average response 0.5 kg) of lambs born to ewes shorn at mid-pregnancy. Mid-pregnancy shearing also increased the SMR of new-born twin lambs by 16%, but decreased the SMR of singleton lambs by 26%. These results indicate that mid-pregnancy shearing can increase lamb birth weight without increasing ewe herbage intake or placental weight. An increase in the efficiency of nutrient uptake by the placenta is implied, and possible effects on the activity of thermogenic tissues are discussed.

The body composition and metabolic status of twin- and triplet-bearing ewes and their fetuses in late pregnancy

“…In previous studies that have reported the effect of birthsize on the proportionality of organs in singles and twins, it has been observed that the smaller fetuses tend to display heavier not lighter organ weights especially the heart and liver (Clarke et al, 1997;Revell et al, 2002;Harding, 2001). Therefore in the present study, it might have been expected that the triplets, which were lighter on average than their twin born counterparts, would have displayed greater organ weights after correction for body weight.…”

“…Robinson (1977) stated that there was no evidence to suggest there is differential growth in fetal organs or tissues in comparison to fetal weight under conditions of under nutrition. More recently Clarke et al (1997), Harding (2001) and Revell et al (2002) reported that at least in singles and twins, treatments which resulted in smaller lambs also resulted in changes in the relative sizes of many organs. It appears that a detailed comparison of twin-and triplet-fetuses in late pregnancy has not been undertaken.…”

The effect of mid-pregnancy stressors on twin-lamb live weight and body dimensions at birth

“…However, increases in birth weight resulting from midpregnancy shearing have been observed without a corresponding increase in gestation length (Kenyon et al, 2002c;Corner et al, 2006). In studies where there was an increase in gestation length the duration was insufficient to account for the entire difference in lamb birth weight (Kenyon et al, 2002b;Revell et al, 2002).…”

“…An increase in the birth weight of light-weight lambs has the potential to increase survival rates to weaning (Morris et al, 1999). Mid-pregnancy shearing has consistently increased lamb birth weights (Morris and McCutcheon, 1997;Morris et al, 2000;Smeaton et al, 2000;Kenyon et al, 2002a,b,c;Revell et al, 2002;Kenyon et al, 2004;Corner et al, 2006). However, in New Zealand, mid-pregnancy shearing takes place in the winter when conditions can be cold, wet and windy and may result in hypothermia (Morris et al, 1962) with ewe mortality rates of 3-14% (Dabiri et al, 1995).…”

“…The lamb birth weight increase caused by midpregnancy shearing was not due to an increase in ewe feed intake (Kenyon et al, 2002b;Revell et al, 2002), longer gestation length (Kenyon et al, 2002c), elevated maternal thyroid hormone concentrations (Kenyon et al, 2005) or events involved with being yarded (Corner et al, 2006). Stressors during pregnancy caused by transport (cattle) and isolation (sheep) have resulted in increased fetal (Lay et al, 1997) and birth weights (Roussel et al, 2004), respectively.…”