EFFECT OF A SUSTAINED-RELEASE FORM OF SOMATOTROPIN ON THE PROFILE OF MILK PROTEINS AND FATTY ACIDS DURING A FULL LACTATION

Twenty-three Holstein cows received one injection of recombinant bovine somatotropin (bST) every 28 d at one of four doses: 0, 320, 640, or 960 mg 28 d−1. All injections started in early lactation and continued until the cows were dried-off. Composites of AM and PM milkings were analyzed for fatty acids by gas-liquid chromatography, for percent milk protein by infrared analysis and for casein by electrophoresis. For the 0, 320, 640, 960 mg cow−1 28 d−1 bST treatment groups, average ± SE milk protein percents for the entire lactation were 3.24 ± 0.08%, 3.25 ± 0.07%, 3.31 ± 0.08% and 3.24 ± 0.07%, respectively (P > 0.05); average milk caseins were 2.59 ± 0.07%, 2.61 ± 0.06%, 2.60 ± 0.07% and 2.57 ± 0.06%, respectively (P > 0.05), and average whey proteins were 0.65 ± 0.03%, 0.64 ± 0.03%, 0.71 ± 0.03% and 0.67 ± 0.03%, respectively (P > 0.05). No effect of bST was observed on milk fatty acids and protein at any dose or stage of lactation. Concentrations of αs-, β- and K-casein were not affected by bST treatment (P > 0.05). Fat percentages were 3.78 ± 0.21%, 3.91 ± 0.19%, 3.76 ± 0.19% and 4.02 ± 0.16% for the 0, 320, 640 and 960 mg 28 d−1 treatment (P > 0.05). Milk fatty acids were not affected by bST treatment (P > 0.05). Short chain fatty acids and medium chain fatty acids were decreased (P < 0.05) with progressing lactation while long chain fatty acids were increased in late lactation (P < 0.05). Key words: Somatotropin, composition of milk, caseins, milk fatty acids