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Responses to an insulin challenge in dairy cows classed as efficient or inefficient based on residual feed intake (RFI) during mid lactation and the dry period
Cows selected for milk yield are insulin resistant and readily mobilize lipid energy stores during lactation. Upon exposure to a stressor, insulin secretion from the pancreas is inhibited by epinephrine and consequently adds to the hyperglycemic responses to stress. Insulin-induced hypoglycemia is an indirect means of stimulating the hypothalamic-pituitary-adrenal (HPA) axis in order to release adrenocorticotropic hormone (ACTH) and subsequently cortisol. This experiment was designed to explore the nutrient partitioning and stress hormone responses to an insulin challenge in mid lactating and dry cows classed as efficient or inefficient based on residual feed intake (RFI). Sixteen multiparous Holstein-Friesian cows (589±37 kg) were selected based on RFI extremes (8 inefficient and 8 efficient). On two occasions, during mid lactation (122±23 DIM) and the dry period (~38 days dry), an i.v. insulin challenge was conducted. Animals were housed in metabolism stalls and fed lucerne hay cubes ad libitum and recieved 6 kg DM crushed wheat grain (and minerals) per day at milking (18% CP and 10.5 MJ ME/kg DM) and had food removed 12 hours prior to the challenge. The day prior to the challenge cows were fitted with interdwelling jugular catheters. Insulin (.12U/kg) was infused via the catheter and blood samples collected at regular intervals pre- and post-infusion. Isolated plasma was analysed for cortisol, growth hormone (GH), insulin like growth factor-1 (IGF-1), non-esterified fatty acids (NEFA) and glucose concentrations. Overall, responses to the insulin challenge were more pronounced during mid lactation compared to the dry period. Basal plasma IGF-1 concentrations were greater in inefficient compared to efficient cows during mid lactation (11.2 vs 15.9 ng/ml respectively, P=.006) but not the dry period (P=.78). Peak plasma cortisol concentrations were not influenced by efficiency group during either measurement periods. Plasma cortisol and NEFA concentrations were greater, and glucose concentrations lower, in mid lactation compared to the dry period. This experiment demonstrated a stress (cortisol) response to the insulin-induced hypoglycemia, although this response did not vary with efficiency. The diminished responses to insulin during the dry period are likely due to the varied metabolic states of the animals at these two varied stages of production. The greater basal IGF-1 concentrations in inefficient animals may in part explain their inefficiency as IGF-1 mimics the actions of insulin and reduces circulating glucose concentrations, reducing the concentration of glucose available for use in mammary tissues.
Keywords: Stress responses, efficiency, insulin