The effect of maternal shearing and thyroid hormone treatments in mid pregnancy on the birth weight, follicle, and wool characteristics of lambs

Abstract

Mid pregnancy shearing of ewes has been associated with a short-to medium-term increase in maternal thyroid hormone concentrations. To determine the role of this elevation in the observed changes in lamb birth weight and fibre characteristics, ewes in mid pregnancy were either: shorn at day 70 of pregnancy with thyroid glands left intact (Shorn), unshorn with intact thyroid glands (Unshorn), thyroidectomised but left unshorn with a short-term elevation in thyroxine (T4) concentrations in mid pregnancy (Tx+T4), or unshorn with intact thyroid gland and prolonged elevation in T4 concentrations (+T4). The Tx+T4 regime successfully increased maternal thyroid hormone for a short period post-treatment, while the +T4 regime increased maternal thyroid hormone levels for a prolonged period during mid to late pregnancy. Lambs born to Shorn ewes were significantly (P < 0.05) heavier than those born to Tx+T4 and +T4 ewes and tended to be heavier than those born to Unshorn ewes (P = 0.10). Birth weights of lambs born to Tx+T4 and +T4 ewes did not differ from those born to Unshorn ewes. Ewe treatment had no effect on lamb wool follicle characteristics or fibre diameter, coefficient of variation of mean fibre diameter, yield, or loose wool bulk; but treatment tended to affect colour (P = 0.05) and staple length (P = 0.09). These results suggest that an elevation in A04064; maternal thyroid hormones is not the sole endocrine mechanism responsible for either the birth weight effect or changes in fibre characteristics observed in lambs born to mid pregnancy shorn ewes. However, it is still possible that an elevation of T4 and T3 concentrations in conjunction with other factors are required to achieve the observed effects.

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

“…Chronic cold stress resulting from a short fleece during winter was likely to have long term consequences on the ewe and the fetus. Thyroid hormone concentrations became elevated after mid-pregnancy shearing as a result of cold stress (Symonds et al, 1989), however, elevated thyroid hormones alone did not result in increased lamb birth weight (Kenyon et al, 2005). Therefore, the mechanism of mid-pregnancy shearing that produces the increase in lamb birth weight may be chronic stress.…”

“…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.…”

On-farm factors affecting physical quality of Merino wool. 1. Nutrition, reproduction, health and management

“…Such changes could potentially increase foetal thyroid hormone and effect follicle development at a critical time. Kenyon et al (2005) investigated this hypothesis through use of thyroidectomies and exogenous administration of triiodothyronine and thyroxine, but found that any effects on fibre attributes could not be solely attributed to elevated maternal thyroid hormones. Another study showed thyroxine definitely involved in the development of follicle morphology, but that any alterations which occurred from supplementation with thyroxine or inhibition of thyroxine (with propylthiouracil) did not alter subsequent fleece attributes (McDowall et al, 2011).…”

The effect of mid-pregnancy shearing or yarding stress on ewe post-natal behaviour and the birth weight and post-natal behaviour of their lambs

“…The increase in lamb birth weight observed is not due to increased gestation lengths (Kenyon et al, 2002c), increased ewe feed intakes (Kenyon et al, 2002b;Revell et al, 2002), changes in the ewe's metabolism (Kenyon et al, 2002b) or increased maternal thyroid hormone (T3 and T4) concentrations (Kenyon et al, 2005). Other stressors such as transport and isolation increase fetal weight in late gestation.…”