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To obtain information on the physiological mechanisms controlling the partition of dietary energy between body weight and milk production in lactating cattle, the concentrations of hormones (prolactin, growth hormone, insulin and thyroxine) and metabolites (glucose, non-esterified fatty acids, β-hydroxybutyric acid and l-lactic acid) in plasma obtained from eight high-yielding and seven low-yielding cows, matched for diet and stage of lactation, were compared. Blood samples were taken via a jugular catheter, throughout four 48 h periods at various times during lactation (days 40, 80, 120 and 180) and also when the animals were dry.
The milk yield rose for 7 weeks after parturition, reaching peak values of 24·8 and 10·1 kg/day in the high- and low-yielding groups respectively. The body weights of the low-yielding animals rose steadily, whereas cows in the high-yielding group lost weight up to week 14 of lactation and thereafter gradually gained weight. No differences were found between the groups in the digestibility of the diets, although analysis of the rumen fluid indicated small differences in the proportions of volatile fatty acids. Milk analysis suggested that the protein content of milk obtained from low-yielding cows was greater than that of milk from high-yielding animals.
Throughout lactation, the concentrations of growth hormone (P < 0·001), non-esterified fatty acids (P < 0·01) and β-hydroxybutyric acid (P < 0·05) were higher in the high-yielding than in the low-yielding group, whereas the concentration of insulin was higher (P < 0·01) in the low-yielding animals. The level of thyroxine was higher (P < 0·05) in the low-yielding group when the animals were dry. A comparison of peak lactation (days 40 and 80) with the dry period demonstrated that the concentration of prolactin was higher in both groups high-yielding, P < 0·01; low-yielding, P < 0·001) when the animals were dry and the concentration of thyroxine in the low-yielding cows was higher (P < 0·01) when the animals were dry. Significant reductions in the concentrations of growth hormone (P < 0·001) and non-esterified fatty acids (P < 0·001) were accompanied by an increase in the concentration of glucose (P < 0·01) when the high-yielding animals were dry.
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