Thyroid secretion rates in grazing British and Zebu cross bred steers: Seasonal effects and relationships with growth

Abstract

Marked monthly fluctuations in thyroid secretion rates and component parameters in 16 Hereford x Shorthorn, Africander x Hereford, and Brahman x Hereford steers did not follow seasonal changes in ambient temperature or day length in subtropical Queensland, but were more consistently related to changes in pasture quality. Secretion rates when the pasture was at its best were more than double those recorded when the pasture was poorest. British steers as compared with Zebu crossbreds were similar in thyroxine turnover rate, lower (P < 0.01) in thyroid secretion rate, plasma protein-bound iodine (P.B.I.), and extrathyroidal thyroxine, and higher (P < 0.01) in thyroxine volume of distribution.

There were no breed x month interactions. Determinations of blood volumes and extracellular water volumes did not provide any explanation of differences in P.B.I. and thyroxine volume of distribution.

Correlations betweefl thyroid secretion rates and body weight gains were positive (r = 0.85, P < 0.01, among months; r = 0.70, P < 0.01, among animals) in summer when pasture was good and gains averaged 2.06 lb per day. Correlations were negative (r = –0.78, P < 0.01, among months; r = –0.43 among animals) in winter when pastures were poor and gains averaged –0.28 lb per day.

Of the different components of thyroid secretion rate, P.B.I. and extrathyroidal thyroxine were superior to thyroxine turnover rate and thyroxine volume of distribution in reflecting seasonal changes in thyroid activity and its relation with growth rates. P.B.I. and body weight contributed most and thyroxine turnover rate and thyroxine volume of distribution least to differences in thyroid secretion rate between breeds and between animals within breeds.