Effect of body condition score on insulin sensitivity of precalving dairy cows

Insulin plays a central role in the transition period of dairy cows by influencing the glucose, lipid and protein metabolism. The effect of insulin is regulated by the secretory capacity of the pancreas and the insulin sensitivity of peripheral tissues. Overconditioning is known to induce insulin resistance in heifers and sheep. In the present study, we identified the effect of BCS on insulin sensitivity in 9 healthy, dry dairy cows at the end of pregnancy. To assess insulin resistance, we performed hyperinsulinemic euglycemic clamp (HEC) tests, generally accepted as the gold standard method, consisting of 4 consecutive insulin infusions 0.1; 0.5; 2 and 5mU/kg/min. At regular time intervals during the infusions, blood glucose concentration was determined using a glucometer and the speed of a concomitant glucose infusion was adapted to keep the blood glucose concentration constant. At the end of each infusion, a steady state was maintained for at least 30 minutes.During the steady state, minor changes of the glucose infusion were necessary to maintain a normal blood glucose level. The steady state glucose infusion rate (SSGIR) was calculated as the amount glucose infused per kg bodyweight and per minute. Statistical analysis was done using SPSS.Cows had been selected to assure a good spread in BCS at the end of lactation. Four cows had a normal BCS (3-3.5) and 5 cows were overconditioned (4-5). During the dry period, cows were fed according to the NRC requirements to avoid changes in BCS and hence confounding of the results by under- or overfeeding. The average SSGIR(±SD) was 1.44±0.44; 8.77±2.50; 13.61±2.91 and 17.92±4.16 μmol/kg/min for the insulin infusions of 0.1; 0.5; 2 and 5mU/kg/min respectively. The Pearson correlation coefficient was not significant for the insulin dose of 0.1mU/kg/min but significant negative correlations were found between the BCS and SSGIR at insulin doses of 0.5(R=-0.906; P<0.01), 2(R=-0.872; P<0.01) and 5mU/kg/min (R=-0.888; P<0.01). The negative correlations point to a lower SSGIR for fatter cows which is indicative for a higher insulin resistance of the glucose metabolism in fatter cows. It can be concluded that at the end of pregnancy, dairy cows with a higher BCS are more insulin resistant compared to leaner cows. This insulin resistance is characterized by decreased insulin sensitivity and decreased insulin responsiveness.