The experiment determined the diurnal variation of a blood plasma hormone and three metabolites (insulin, glucose, NEFA and triglyceride) and how these varied with different frequencies of supplementary feeding. Twenty-four cows in early lactation (DIM 81 ± 20) were divided randomly into three treatments; grazed pasture only and pasture supplemented with 2 kg/cow/day of ruminally protected oilseed supplement (metabolisable energy 19 MJ/kg DM) either once a day after morning milking or twice a day after morning and afternoon milking. On day 12 of the experiment, 12 cows (4 cows/treatment) were jugular catheterised and commencing at 1600 h, blood sampled at 2-hourly intervals for 24 h. This protocol was repeated for the remaining 12 cows on day 13 Concentrations of the selected blood plasma hormone and metabolites varied diurnally (P<0.001). The difference between the average peak and nadir concentrations over the 24 hours were 54%, 66%, 224% and 63% for plasma insulin, glucose, NEFA and triglyceride respectively. Feeding the lipid supplement increased blood plasma NEFA (P<0.01), and triglycerides (P<0.001) but had no effect on insulin, or glucose concentrations. Frequency of supplementary feeding influenced (P<0.05) the diurnal variation of NEFA but not, insulin, glucose, or triglyceride. Concentrations of NEFA, glucose, and triglyceride were variable between 0600 h and 1000 h and stabilised between 1600 h to 2000 h. Plasma insulin concentrations were low and stable between 0400 h to 0800 h. Consideration of diurnal variation and the stability of the blood plasma components measured led to the recommendation that the best time to blood sample for insulin is between 0400 - 0800 h and for glucose, NEFA, and triglyceride, between 1600 2000h.
Proceedings of the New Zealand Society of Animal Production, Volume 63, Queenstown, 107-111, 2003
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