Influence of diet and ambient temperature on bovine serum insulin-like growth factor-I and thyroxine: relationships with non-esterified fatty acids, glucose, insulin, luteinizing hormone and progesterone

Multiparous, nonlactating, Hereford cows were fed maintenance (M; n = 11) or restricted (R; n = 11) diets for 41 weeks to evaluate changes in insulin-like growth factor-I (IGF-I) and thyroxine during the onset of nutritionally induced anoestrus and during realimentation. Cows had a moderate body condition score (BCS = 5.5 ± 0.1; 1 indicating emaciated and 9 indicating obese) at the start of treatment. Cows on M diets maintained body weight (BW) and BCS, and R cows lost 1% of their initial BW weekly until they were anoestrus. When R cows were anoestrus their diets were increased to allow BW gain and resumption of ovarian activity. Relationships among concentrations of IGF-I, thyroxine, non-esterified fatty acids (NEFA), glucose, insulin, luteinizing hormone (LH), and progesterone in blood samples were determined. Anoestrus occurred when restricted cows had lost 24 ± 0.9% of their BW, had a BCS of 3.5 ± 0.1, and had nadir concentrations of IGF-I and thyroxine. Serum concentrations of IGF-I and thyroxine were reduced (P < 0.01) in R compared with M cows, except during realimentation when R cows had greater concentrations of thyroxine. Concentrations of IGF-I in M cows were positively correlated with concentrations of insulin (r = 0.31), glucose (r = 0.21) and progesterone (r = 0.24). In R cows, concentrations of IGF-I were positively correlated with insulin (r = 0.39), glucose (r = 0.23) and LH (r = 0.37). Mean weekly ambient temperatures were negatively correlated with IGF-I (r = −0.16), glucose (r = −0.37) and thyroxine (r = −0.40) in M cows. Our data indicate that ambient temperature and nutrient intake may both work to regulate concentrations of hormones and reproductive function in cattle.