Summary
To obtain further insight into the endocrine control of energy metabolism in the lactating cow, diurnal variations in concentrations of hormones (prolactin, growth hormone (GH), insulin and thyroxine) and metabolites (glucose, non-esterified fatty acids (NEFA), 3-hydroxybutyric acid (BHB) and I-lactic acid) were measured in blood of high-yielding and low-yielding cows. Samples were taken on approximately days 40, 80, 120 and 180 during lactation and in the following dry period, at hourly intervals during 24 h when milking and feeding were at the same time. Additionally hormones were measured for a second 24 h when one meal was delayed 4-5 h after milking.
Significant diurnal variation was observed in the levels of prolactin, insulin, glucose, NEFA and BHB. The extent of the variation was different at different stages of lactation even though the diet remained constant. The changes in insulin and prolactin appeared to be related to feeding rather than to milking.
Insulin and NEFA concentrations were negatively correlated during the dry period but not during lactation. GH was not correlated with NEFA.
It was concluded that mobilization of body fat towards meeting energy requirements for milk production in early lactation depends on a high mean level of GH at that time. Short-term increases in insulin secretion in response to feeding reduce net lipolysis, and hence NEFA levels, after a meal.
Key-Words:
Insulin
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Growth Hormone
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Prolactin
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Thyroxine
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Glucose
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Non-Esterified Fatty Acids
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3-Hydroxybutyric Acid
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Lactic Acid
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Cows
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Blood Plasma
