Responses to metabolic challenges in dairy cows with high or low milk yield during an extended lactation

Responses of dairy cows with high or low milk yield (MY) beyond 450 d in milk (DIM) to 3 metabolic challenges were investigated. Twelve multiparous Holstein-Friesian cows that calved in late winter in a pasture-based system were managed for a 670-d lactation by delaying re-breeding. Cows were selected for either high MY (18.9 ± 1.69 L/cow per d; n = 6) or low MY (12.3 ± 3.85 L/cow per d; n = 6) at 450 DIM. Cows were housed indoors for 2 periods of 12 d at approximately 460 and 580 DIM. Each cow was fed freshly cut pasture (460 DIM) or pasture silage (580 DIM) plus 6.0 kg of DM barley grain daily (approximately 200 MJ of total metabolizable energy/cow per day). At all other times, cows were managed as a single herd and grazed pasture supplemented with cereal grain to an estimated intake of 180 MJ of metabolizable energy/cow per d. Cows were fitted with a jugular catheter during the final week of each experimental period. Over a period of 3 d, each cow underwent an intravenous glucose tolerance test (0.3 g/kg of body weight), an insulin tolerance test (0.12 IU of insulin/kg of body weight), and a 2-dose epinephrine challenge (0.1 and 1.6 µg/kg of body weight). Cows selected for high MY had greater milk and milk solids yields between 450 and 580 DIM than low MY cows (17.3 vs. 10.8 ± 1.49 kg of milk/d and 2.4 vs. 1.5 ± 0.23 kg of milk solids/d). The results indicated that whole body and peripheral tissue responsiveness to insulin may vary between cows of high and low MY. Following the glucose tolerance test, high MY cows had a lower plasma insulin response with a greater glucose area under the curve than low MY cows. Further, high MY cows had slower plasma glucose clearance compared with low MY cows during an insulin tolerance test. The plasma nonesterified fatty acid (NEFA) responses to the IVGTT and the ITT were similar between cows of high and low MY, but the clearance of NEFA from the plasma following both the IVGTT and ITT were slower at 580 compared with 460 DIM. The sensitivity to epinephrine was greater in high MY cows compared with low MY cows as the glucose and NEFA area under the curve and the percentage change in NEFA were greater in high MY after the low dose epinephrine challenge. However, the lipolytic but not the glucose appearance in response to epinephrine was greater in high MY cows than low MY cows. Following the high dose of epinephrine, the glucose response was lower, but the NEFA response was greater in high MY compared with low MY cows. Cows able to sustain greater MY to 580 DIM had a greater propensity for lipid mobilization, possibly enhancing nutrient partitioning to the mammary gland during the late stages of an extended lactation.