Chemical changes and influences of rapeseed antinutritional factors on lamb physiology and performance: 2. Plasma substances and activity of the thyroid

Abstract

In order to study the effects of a high level of rapeseed meal in the diet on endocrine function and thyroid activity, sixty six Texel, Suffolk or crossbred lambs allocated to three groups were fed ad libitum with concentrates containing 0% (Control) or 25% of rapeseed meal obtained with either Samourai (Samourai) or Honk (Honk) rapeseed varieties. Lambs were weaned at 88 ± 8 days of age and 24 ± 5 kg of live weight and were slaughtered when their fattening state was estimated to be satisfactory. Blood samples were taken fortnightly for the determinations of hormones, glucose and transaminases (GOT and GPT). To evaluate the capacity of hormone production, the thyroid glands were collected at slaughter and the in-vitro release of thyroid hormones was recorded. The plasma concentrations in triiodothyronine (T 3) and thyroxine (T 4) decreased ( P <0.05) with the age of lambs and were lower ( P <0.05) in the Samourai and Honk groups than in the Control. Moreover, the capacity of T 3 and T 4 production by the thyroid gland shown by the in-vitro T 3 and T 4 accumulation in the thyroid tissues was higher ( P <0.05) in the Control than in the two rapeseed groups. In contrast, the concentrate with rapeseed meal had no effect on the plasma concentrations in GH, cortisol and insulin nor on other substances (glucose, GOT and GPT). These results indicate that the ingestion of rapeseed meal induces a low functional disorder of the thyroid without affecting the animal performance of lambs, as we reported previously. This is because the thyroid hormones do not appear to play a major role in the processes of growth and fattening, these having been successively taken under the control on priority by GH and insulin.