Effects of 0, 1·7, 3·3 or 5·0 kg/day of a protected soya bean – tallow supplement, incorporated into a hay:concentrate diet (25:75) and fed ad libitumto Friesian cows, on intake and digestion of fatty acids, on output of milk fatty acids and on blood lipoprotein composition were measured.
Most of the increased intake of fatty acids, approximately 1 kg/day, was accounted for by increased intakes of C16:0, C18:0 and C18:1. At low intakes, amounts of all fatty acids apparently digested were linearly related to their respective intakes. At high intakes of C16 and C18 acids, curvilinear relationships were established.
Yield of total milk fat was related positively to dietary intakes of total fatty acid and carbohydrate and negatively to live-weight change. Yields of short and intermediate chain acids in milk, synthesized within the mammary gland, were negatively correlated and yields of C18 fatty acids positively correlated with respective dietary intakes of these acids. Decreased proportions of C4–16 and increased proportions of C18:0 and C18:1 fatty acids in milk were associated with increased protected tallow in the diet. Yields of C16:1 and C18:1 were positively related to corresponding outputs of saturated acids and negatively to weeks of lactation. The proportion of C18:1 in milk was positively related to the corresponding proportion of C18:0.
The increased intake of fatty acids resulted in increased concentrations of very low density lipoproteins (VLDL, d < 1·019 g/ml), low density lipoproteins (LDL1 + LDL2, 1·019 < d < 1·06 g/ml), high density lipoproteins (d > 1·060 g/ml) and serum free fatty acids. Most of the increase in low density lipoproteins was accounted for by a very large increase in LDL1, whose proportion increased from 17 to 75% (2 to 22% of total serum lipid). The proportion of triglyceride in the combined low density lipoprotein fraction decreased from 11 to 2% whilst phospholipids increased from 29 to 36%. These changes were attributed to the increased proportion of LDL1 present.
The proportions of VLDL and LDL triglyceride taken up by the mammary gland averaged 0·79 and 0·34 respectively. The proportion of VLDL+LDL triglyceride taken up by the gland decreased with increased amounts of fatty acid digested. Yields of C18 fatty acids in milk tended to be positively related to apparent uptakes of VLDL triglyceride and to VLDL C18 fatty acids, but negatively related to apparent uptakes of LDL triglycerides and LDL C18 fatty acids. It is suggested that the increased LDL1 resulted from the utilization of VLDL triglyceride for milk fat formation.
Protected lipid feeding increased the proportion of C14:0, C16:0 C16:1 and C18:1 and decreased the proportions of C14:1 and C18:0 fatty acids in jugular serum triglycerides. Similar changes were observed in jugular VLDL triglycerides. Differences in the compositions of VLDL and LDL triglycerides across the mammary gland were observed and attributed either to selective uptake or to interchange of fatty acids between triglycerides and free fatty acids.