SOMATOTROPIN AND INSULIN-LIKE GROWTH-FACTOR-I CONCENTRATIONS IN PLASMA AND MILK AFTER DAILY OR SUSTAINED-RELEASE EXOGENOUS SOMATOTROPIN ADMINISTRATIONS

Effects of daily injectable or sustained-release bovine somatotropin ( bST) administrations on plasma and milk bST and insulin-like growth fa ctor-I (IGF-I) concentrations were monitored in 74 lactating cows thro ugh early, mid- and late lactation. Treatments beginning at wk 4 of la ctation were excipient (CO, 24 cows) at 2 wk intervals, daily injectio ns of 10.3 mg bST (DI, 25 cows) and 350 mg sustained-release bST at 2 wk intervals (SR, 25 cows). The duration of treatments was 40 wk. Data were first analyzed for the overall mean concentrations covering the 40 wk treatment period. Overall mean plasma bST, milk bST and plasma I GF-I concentrations were significantly increased by both bST treatment s (p<0.05). On the other hand, milk IGF-I concentrations were signific antly increased (p<0.05) only in the DI group. Next, data were analyze d according to stage of lactation. The bST treatments resulted in sign ificant increases (p<0.05) in plasma and milk bST concentrations for a ll early, mid- and late lactation periods. Even though plasma IGF-I co ncentrations were higher (p<0.05) in al lactation periods for bST trea tment groups, higher milk IGF-I concentrations (p<0.05) occurred only in mid- and late lactation periods for the DI group. The patterns of b ST and IGF-I concentrations in milk follows those of the plasma after bST treatments.