Insulin-like growth factor I in follicular development and function in postpartum beef cows

The objective of this study was to determine whether nutrition affects follicular growth and(or) steroid and insulin-like growth factor I (IGF-I) concentrations in follicular fluid. Beginning 6 d after calving, Hereford-cross cows (n = 28) were fed either 14 (ad libitum) or 7 (restricted) kg.animal1.d-1 of chopped alfalfa-brome hay. Half the cows in each treatment were ovariectomized on d 20 (OVX-20) and the remaining half on d 35 (OVX-35 postpartum) Cow weight and condition score were recorded weekly, and blood was collected thrice weekly for determination of insulin, IGF-I, glucose, and free fatty acid (FFA) concentrations. At ovariectomy, follicular fluid from each follicle greater than or equal to 4 mm in diameter was aspirated for determination of IGF-I, progesterone (P4), and estradiol-17 beta (E2) concentrations. Restricted cows lost more weight after calving than did ad libitum cows (P < .0001), although all cows lost similar amounts of body condition (time postpartum, P = .008). Concentrations of FFA were elevated (P < .0001) in restricted cows from wk 2 through 5 after calving but did not change with time in ad libitum cows. Plasma concentrations of glucose were lower in restricted than in ad libitum cows (59.6 +/- .4 vs 61.8 +/- .4 mg/dl; P = .05), but insulin and IGF-I were similar (P > .10) between dietary treatments. Restricted cows had less (P < .05) follicular fluid volume in both medium-sized (7 to 9.9 mm) and large (greater than or equal to 10 mm) follicles and had smaller (P < .05) medium-sized follicles than did ad libitum cows. Restricted cows had greater (P < .05) follicular fluid concentrations of E2 in small (4 to 6.9 mm) and large follicles than did ad libitum cows. Follicular fluid concentrations of P4 and IGF-I were not affected (P > .10) by dietary treatment, day of ovariectomy, or follicle size class. We concluded that nutrient restriction altered follicular growth during the postpartum period, which was not reflected by changes in the follicular fluid concentrations of IGF-I.