Glucose clearance studies in lactating cows: influence of stage of lactation and ration composition

Glucose pool size (PS), pool space (PV) and clearance rate (CR) vere determined for eight first-lactation Holstein cows at three lactation stages [early (E) = 75-100 days, late (L) = 205-250 days, and dry (D) ]. A corn silage-based ration (15% CP, 22% ADP) was fed ad libitum during lactation and at maintenance during D, using 4-hr feeding intervals. Three intravenous glucose (.1 g/kg BW) injections were administered each period. Serial blood samples were collected for 40 min post-dosing. Parameters were calculated by the least squares method using natural logarithms of glucose concentration. Basal plasma glucose and insulin concentrations were 57.4, 60.1, and 62.3 mg/dl and .78, .97, and 1.24 ng/ml, respectively, for E, L, and D, respectively. PS and PV increased (P<.05) from E to D; however, when adjusted for body weight there was no difference across stages. CR decreased (P<.05) from E (2.43%/min) to D (1.71%/min) and was correlated with dry matter intake (r = .70, P<.01). Glucose-induced insulin response was greater (P<.05) during D than during lactation due to higher peak (P<.05) and lover insulin clearance (P<.05) values.

Glucose parameters were determined by single injection in lactating cows fed a high-concentrate ration (16.8% CP, 13% ADF) (n=5) and a control ration (16.3% CP, 24.4% ADF) (n=4) , at 90 and 120 days postpartum. No differences due to stage or diet were observed, due to insufficient observations. Plasma glucose and insulin tended to be higher with HC-feeding and at the later stage.