Abstract
Experiments were conducted to investigate the counteracting effect of the irradiation treatment with gamma rays on the goitrogenic effect of rapeseed meal. Rapeseed meals were exposed to gamma rays of 20-70 Mrad and the diets containing 10 or 20% of these irradiated meals were fed to day-old White Leghorn male chicks for 3 or 4 weeks. These diets were formulated to be isonitrogenous and isocaloric, respectively. In addition, the influences of irradiation treatment on the contents of oxazolidinethione (OZT), which plays a significant role in the goitrogenic effect of meal, and several other constituents of meal were also examined.
When rapeseed meals were irradiated at a level of 60Mrad, their OZT, isothiocyanates and crude fiber contents decreased markedly and such a irradiation dosage also resulted in a little loss of the available lysine of meals. The extent of the above changes in the content of OZT was similar to that in rapeseed meal which was autoclaved under the reasonable condition (moisture content 20%, 120°C, 30min).
The irradiation treatment at any dosage had no adverse effects on growth rate, feed consumption and feed efficiency in chicks fed diets containing the treated meals. When the level of dosage was increased from 20 to 70 Mrad, thyroid weight of chicks tended to be lighter and an increase in thyroid weight of those receiving meals, which were irradiated at a level of about 60 Mrad, was very slight as was to be expected accordin to the chemical assay of OZT in the these meals. The rate of counteracting effect of treatment with above dosage was comparable to that of the treatment by autoclaving under the reasonable condition.
From these results, it may be concluded that the gamma irradiation at dosage of about 60 Mrad is effective in counteracting the goitrogenic effect of rapeseed meal and does not cause any adverse effect on growth performance in birds.
