Plasma protein-bound iodine and growth rates of beef cattle

Plasma protein-bound iodine (P.B.I.) levels determined in February and August at Rockhampton, Qld., in 177 grazing heifers and steers were compared with post-weaning gains during summer (October to March) and winter (March to October). In contrast to a large difference in average gain between summer (96 lb) and winter (41 lb), reflecting changes in nutrition, there was no seasonal difference in P.B.I. Significantly lower mean gains in British (Hereford x Shorthorn) breed groups (40 lb) than in F1 (96 lb) or F2 (71 lb) Zebu (Brahman and Africander) x British crossbreds, and in F2 than in F1 crossbreds, were accompanied by similar breed differences in P.B.I. (British 2.91 µg %, F1 crossbreds 3.94 µg %, F2 crossbreds 3.55 µg %). P.B.I.'s were repeatable between seasons (r = 0.4) but gains were not (r = –0.07). Coefficients of correlation between summer gain and both summer and winter P.B.I. were significant in the British and F2 Zebu x British crosses, averaging about 0.5, but were near zero in the F1 crosses. Poor nutrition may have been responsible for the failure to observe significant correlations between winter gains and P.B.I. In summer, differences in P.B.I. accounted for about 25% of differences in gain between British and F2 Zebu x British crossbreds, and among animals within these breeds. The relationship between P.B.I. and gain was linear over the whole range of P.B.I. observed, which indicated that optimal levels of P.B.I. had not been exceeded. These data suggest that summer heat in the tropics may depress thyroid activity to an extent which imposes limitations on growth rates. Differences in performance among animals and breeds of beef cattle may be due in part to differences in ability to maintain adequate thyroid function under heat stress.