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Plasma insulin and glucose concentrations of feedlot cattle during summer
Periods of heat stress are typically associated with an increase in basal insulin (INS) concentration and a decrease in basal glucose (GLU) concentration, thereby altering energy metabolism. The purpose of this study was to investigate circulating INS and GLU of shaded and un-shaded Bos indicus and Bos taurus feedlot cattle during summer. Thirty-six steers (12 Angus, Charolais and Brahman) with an initial BW of 318.5 ± 6.7 kg were used in a 180 d feedlot study with 2 treatments: un-shaded and shaded (3 m2/animal; 90% solar block shade cloth). There were 6 steers (2/breed) per pen (162 m2) and 3 pens/treatment. Blood samples were collected via jugular venepuncture on five occasions (period 1 to period 5). Plasma concentrations of INS and GLU were analyzed using a repeated measures model (PROC MIXED; SAS Inst. Inc. Cary, NC). Plasma INS were highly variable across breed × treatment groups, therefore concentrations were Log10 transformed analysis. The model included breed (P = 0.03; P = 0.03), treatment (P = 0.81; P = 0.97), period (P < 0.0001; P = 0.0003), treatment × breed (P = 0.38; P = 0.39), treatment × period (P = 0.06; P = 0.09), breed × period (P = 0.03; P < 0.0001) and treatment × breed × period (P = 0.50; P = 0.48) as fixed effects for Log10 INS and GLU (mmol/L), repectively. Pen nested within treatment and treatment × breed × animal ID nested within pen were included as random effects. Overall the Brahmans had higher plasma GLU than Angus, particularly during periods 1 (P < 0.0001) and 2 (P = 0.003). However as days on feed increased, the variability of GLU on a breed × treatment group basis decreased. There was a trend for increasing INS and GLU over time, which can be partly explained by the high starch, high energy diet that was fed. In other species, feeding diets with a high glycaemic index is known to be associated with insulin resistance characterised by persistent hyperglycemia despite increased insulin secretion. In the current study, plasma GLU concentrations increased but GLU homeostasis was maintained but with a marked increase in circulating INS. Together these results may indicate the development of insulin sensitivity in feedlot cattle during summer on a diet designed for weight gain and fat deposition.
Keywords: Feedlot cattle, Glucose, Heat stress, Insulin