Summary Effects of subcutaneous (s.c.) administration of 50 μg/kg body weight of recombinant bovine growth hormone (rbGH) or saline were studied for 11 weeks in 40 intact male veal calves supplied 50 mg or 10 mg of iron (Fe)/kg of milk replacer (MR). Feed intake, average daily gain and growth: feed ratio were reduced in Fe‐deficient calves, but not significantly influenced by rbGH. Plasma Fe and haemoglobin concentration, red‐cell number and packed cell volume were decreased in Fe‐deficient calves (P < 0.05) and rbGH further reduced red‐cell number in Fedeficient calves (P < 0.05). The age‐dependent increase of total Fe binding capacity was greater in Fe‐deficient calves and enhanced by rbGH (P < 0.05). Plasma urea concentrations increased, whereas glucose (G) and triiodothyronine (T3) levels decreased in Fe‐deficient calves. rbGH significantly increased G in calves fed MR containing 50 mg/kg (P < 0.05) and influenced urea concentrations (P < 0.05). Plasma insulin (I) and IGF‐I concentrations were lower in Fe‐deficient calves (P < 0.05). Plasma GH in the first hours after rbGH injections increased (P < 0.05) to higher levels in calves fed 10 than in those fed 50 mg Fe/kg MR, but incremental changes were comparable. In conclusion, low Fe intake caused haematologic, metabolic and endocrine changes. Plasma IGF‐I, I and T3 concentrations after rbGH administration and effects of rbGH on IGF‐I in Fe‐deficient calves were reduced, even though plasma GH levels were increased.

In calves with severe iron (Fe) deficiency, insulin-like growth factor (IGF)-I levels and IGF-I responses to exogenous growth hormone (GH) are reduced, while insulin-dependent glucose utilization is enhanced. Blood plasma concentrations of im-munoreactive insulin (IRI), IGF-I, IGF-II and GH, and the half-life of blood plasma GH [after an i.v. injection of recom-binant bovine GH (rbGH; 100 µg rbGH/kg body weight)] were measured in 20 calves at body weights between 160 and 190 kg. Calves were fed milk replacers containing 50 or 10 mg Fe/kg (groups Fe50 and Fe10, respectively). Daily weight gain and feed utilization were similar in both groups. Group Fe10 developed mild Fe deficiency anemia and blood plasma ureanitrogen concentrations were higher (p < 0.05) than in group Fe50. IGF-I and IGF-II concentrations did not vary consistently over a 10-hour period and were not significantly influenced by different Fe intakes. The IRI concentration increased transiently (p < 0.05) after feed intake, but the total response was (not significantly) smaller in Fe-deficient calves. Plasma GH concentration changed episodically and was similar in both groups. Loss of GH from the circulation after i.v. rbGH injection, estimated by biexponential analysis, during the distribution or α phase (first 16 min) was similar in both groups, but during the β phase was shorter (p < 0.05) in group FelO than in group Fe50 (29.9 and 34.2 min, respectively). The increased disappearance rate of GH, seen even in mild Fe deficiency, may contribute to reduced GH levels and IGF-I responses to GH in severe Fe deficiency.