SummarySix lactating, non-pregnant Jersey cows were given subcutaneous injections of recombinantly derived bovine growth hormone for 7 d. Milk yield was increased by 4·5 kg/d on d 7, compared with the average yield of 10·7 ± 0·4 kg/d (mean ± s. e. m.) for the 7d preceding treatment. Concentrations of insulin-like growth factor I (IGF-I) in the milk increased from 0·44 ± 0·04 nmol/1 (mean ± s. e. m.) during the 7 d preceding treatment to 1·6 ± 0·2 nmol/1 on d 7 of treatment. Taking the increase in milk yield into account the total increase in the secretion of IGF-I into milk of one udder half was 6-fold. Plasma concentrations of total IGF-I rose from 15·5 + 1·3 nmol/1 (mean ± s. e. m.) on the day preceding treatment to 56·9 ± 3·6 nmol/1 (mean ± s. e. m.) on d 7 of treatment. Mammary plasma flow increased from 1·6 ± 0·09 to 2·2 ± 0·06 1/min udder half over the same time. Estimates of the amount of IGF-I that reached the mammary gland gave values of 24 and 116 nmol/min udder half before and during treatment respectively. IGF-I in milk of treated cows was associated predominantly with proteins ranging from 40000 to 150000 mol. wt, but a significant proportion (19%) of the total IGF-I was present in the free unbound form. IGF-I crosslinking studies revealed the presence in milk of one specifically labelled band at 31000 mol. wt.
21. Plasma concentrations of insulin-like growth factor-I (IGF-I) and resumption of cyclicity in pasture-fed dairy cows