Blood chemistry, nutrition, productivity and fertility were monitored and their interrelationship examined in groups of cows from 15 commercial dairy herds. Plasma urea, albumin, glucose, non-esterified fatty acids (NEFA), acetone, calcium, magnesium, inorganic phosphorus (IP), blood copper, haemoglobin and packed cell volume were examined in relation to the intakes of starch equivalent (SE) digestible crude protein (DCP), calcium, phosphorus, magnesium, sodium, potassium, copper and manganese. The most consistent correlations were found for the regressions of SE intake as a percentage of requirements (I/R per cent) on plasma non-esterified fatty acids, the ratio of SE intake over DCP intake on plasma urea, DCP I/R per cent on plasma urea and phosphorus I/R per cent on plasma IP, but factors other than nutrition accounted for a large part of the variation in all cases. The mean plasma glucose concentration within +/- three days before or after first service of cows which held was higher than that of cows which returned, but the difference was only approaching significance at the 5% level and it is doubtful whether it could be of practical value. No other differences in blood component levels were demonstrated for first or second service. It is concluded that, within the nutritional ranges encountered, the levels of the selected blood components did not show a consistent relationship to nutrient balance or potential fertility. In this context a multiple analytical scheme employing these components would appear to be of limited value, particularly if samples are taken on only one occasion. The technique is more appropriately regarded as an aid to the conventional approach involving the examination of feeding systems and feedstuffs, herd records, management and clinical conditions.
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