Development of insulin resistance in dairy cows by 150 days of lactation does not alter oocyte quality in smaller follicles

The objective of this study was to test the hypothesis that high-producing dairy cows become increasingly resistant to insulin throughout lactation and that, consequently, oocyte quality is compromised. We used Holstein cows at 50 (51.5 ± 3.7; n = 30), 100 (102.3 ± 9.4; n = 30), and 150 (154.5 ± 18.9; n = 30) days in milk (DIM). We measured circulating insulin and glucose and performed a glucose tolerance test (GTT) after 5 h of fasting. To evaluate oocyte quality, we performed ovum pickup on the day before the GTT (581 oocytes). We performed statistical analyses using the MIXED procedure of SAS. The model included the fixed effects of DIM, period, time, parity, and an interaction between DIM and time. We observed no difference in the GTT between groups for any variable related to circulating glucose (for example, glucose peak = 203.3 ± 7.2, 208.8 ± 6.3, and 194.3 ± 5.9 mg/dL). However, various measures of circulating insulin were different in cows at 150 DIM compared with 50 or 100 DIM: higher basal insulin (8.8 ± 0.9, 8.8 ± 0.8, and 11.9 ± 0.8 µIU/mL), peak insulin (61.9 ± 6.2, 69.1 ± 5.7, and 89.0 ± 6.1 µIU/mL), delta maximum insulin (51.1 ± 5.5, 59.4 ± 5.0, and 73.5 ± 5.4 µIU/mL), and area under the curve 5-60 (1,874.8 ± 171.0, 2,189.5 ± 157.8, and 2,610.5 ± 174.0 µIU/mL × min). Nevertheless, we observed no difference among groups in the number of viable oocytes (3.2 ± 0.7, 3.9 ± 0.7, and 3.6 ± 0.7 per cow per ovum pickup) or percentage of viable oocytes (49.3, 52.2, and 51.8%). Increased circulating insulin before and throughout the GTT in cows at 150 DIM indicates that cows develop increasing insulin resistance with increasing DIM; however, increased insulin resistance was not associated with a detectable alteration in the quality of oocytes aspirated from small and medium-sized follicles.