Effect of Nutritional Status on Concentrations of Insulin- like Growth Factor-i in Milk from Holstein Cows

The effect of diet on concentrations of milk insulin-like growth factor-I (IGF-I), and their relationships with plasma IGF-I, and with somatic cell count (SCC) were evaluated in two trials. In Trial 1, 32 multi-parous Holstein cows at 4 to 5 wk of lactation received 4 different diets formulated to provide high (H) or low (L) dry matter intake (DMI) with H or L metabolisable energy (ME) density for 5 wk. Dietary treatment did not affect milk IGF-I concentrations, but concentrations were repeatable in individual cows. The intraclass correlation coefficients were 0.78 ± 0.05 and 0.73 ± 0.06 in a.m. and p.m. samples, respectively, when concentrations were measured for 7 consecutive days. The association between milk IGF-I and plasma IGF-I concentration was weaker (R2 = 0.140; P = 0.035) than the association between milk IGF-I concentrations and the natural logarithmic value (Ln) of SCC (R2 = 0.249; P = 0.004). The effects of body condition score (BCS) at calving and level of grain supplementation in early lactation on IGF-I concentrations in milk were evaluated in Trial 2 with 21 Holstein cows that grazed pasture. The BCS at calving did not affect milk IGF-I concentrations; neither did supplementation with 1 or 6 kg of grain. Milk and plasma concentrations of IGF-I were not associated (R2 = 0.002; P = 0.830), whereas the association between milk concentrations of IGF-I and Ln SCC was significant (R2= 0.342; P=0.005). Concentrations of IGF-I in milk were not a sensitive measure of dietary changes in lactating pasture-fed Holstein cows.