Diurnal variation in the chemical composition of plasma from lactating beef cows on three dietary energy intakes

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Abstract
Summary: Twenty-two beef cows, producing on average 5·5 kg 4% fat corrected milk per day, were offered diets of rolled barley and silage such that eight animals were consuming 70%, seven 95% and seven 120% of their estimated energy requirements. Blood samples were taken at intervals during the day and the plasma concentrations of glucose, free fatty acids (FFA), ketone bodies, total protein, albumin, globulin, urea nitrogen, calcium, inorganic phosphate, magnesium and copper determined.Diurnal variation was found in the plasma concentration of glucose, FFA, ketone bodies, urea nitrogen, albumin, calcium and magnesium; it was particularly marked for the first four constituents and appeared to be related to feeding.There were no overall significant differences between groups in the mean daily concentrations of the measures of energy status (glucose, FFA, ketone bodies), but significant (P < 0·001) prefeeding group differences were found in glucose and ketone body concentrations. No significant differences between groups were found for the remaining constituents.There was a postprandial depression in plasma glucose concentration, which was positively correlated with FFA concentration.