In newborn calves, giving growth‑hormone‑releasing factor (GRF‑1‑29) under the skin or into a vein reliably raised growth hormone levels, no matter what they ate. Feeding colostrum (the first milk) boosted the calf’s own IGF‑I, while a synthetic IGF‑I version (Long‑R3‑IGF‑I) only worked when injected, not when taken by mouth. Giving extra bovine growth hormone also raised IGF‑I, but injecting Long‑R3‑IGF‑I actually lowered it.

Effects on the somatotropic axis [plasma levels of insulin-like growth factors (IGFs) I and II, IGF-binding proteins (IGFBPs), and growth hormone (GH)] of feeding different amounts of colostrum or milk replacer, of Long-R3-IGF-I (administered subcutaneously or orally; 50 micrograms.kg body wt-1.day-1 for 7 days), and of subcutaneously injected recombinant bovine GH (rbGH; 1 mg.kg body wt-1.day-1 for 7 days) were evaluated in calves during the 1st wk of life. Plasma Long-R3-IGF-I increased after subcutaneous application but not with the oral dose. Endogenous IGF-I was higher in calves fed colostrum six times compared with those fed only milk replacer. Native IGF-I was highest in rbGH-injected calves but was lowered by the subcutaneous injection of Long-R3-IGF-I. IGF-II concentrations were not modified by any of the treatments. IGFBP-2 increased in calves fed only milk replacer and those receiving subcutaneous Long-R3-IGF-I. GH was not modulated by differences in nutrition but increased after rbGH administration and similarly in all groups after intravenous injection of GH-releasing factor analog GRF-(1-29). Parenteral administration of Long-R3-IGF-I decreased GH concentration but did not affect the secretory pattern. The data demonstrate that the somatotrophic axis is basically functioning in neonatal calves and is influenced by nutrition, GH, and Long-R3-IGF-I.