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

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 cro ssbred lambs allocated to three groups were fed ad libitum with concentrate s containing 0% (Control) or 25% of rapeseed meal obtained with either Samo urai (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 transamin ases (GOT and GPT). To evaluate the capacity of hormone production, the thy roid 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) dec reased (P <0.05) with the age of lambs and were lower (P <0.05) in the Samo urai 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 accu mulation 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 h ad no effect on the plasma concentrations in GH, cortisol and insulin nor o n other substances (glucose, GOT and GPT). These results indicate that the ingestion of rapeseed meal induces a low fu nctional disorder of the thyroid without affecting the animal performance o f 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 an d insulin. (C) 1999 Elsevier Science B.V. All rights reserved.