The relationship between plasma lipid composition and milk fat secretion in cows given diets containing soybean oil

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Abstract
Summary The effects on the composition of the plasma lipids of the isocaloric replacement of starch in a low-fat concentrate mixture by 8 % soybean oil were investigated in a feeding experiment with 6 cows in mid-lactation. Two high-fat concentrate mixtures were given: in one, the soybean oil was incorporated into the diet by the inclusion of the appropriate amount of coarsely ground soybeans; in the other, the soybean oil was incorporated directly in the concentrate mixture. The concentrate mixtures were given with a high-roughage diet that supplied daily 5·5kg hay and 2·7kg of sugar-beet pulp. When the cows were given the 2 high-fat diets there were increases in the concentrations of total plasma fatty acids and in the concentrations of fatty acids circulating in the plasma as phospholipids and cholesteryl esters. There were increases in the concentrations of plasma unesterified fatty acids and triglyceride fatty acids when the cows were given the diet containing soybeans but there were decreases in the concentrations of these 2 fractions when the cows were given the diet containing soybean oil. The inclusion of soybeans in the diet increased the concentration of 18:0 in the plasma triglycerides and unesterified fatty acids whilst the inclusion of soybean oil in the diet increased the concentrations of 18:0 and 18:1 but decreased the concentration of 18:2 in the plasma unesterified fatty acids. When the cows were given the 2 high-fat diets there were decreases in the concentrations of 16:0 and 16:1 and increases in the concentration of 18:2 in the plasma cholesteryl esters. The inclusion of soybeans in the diet also reduced the concentrations of 18:1 and 20:3 in the plasma cholesteryl esters but increased the concentrations of 18:0 and 18:2 in the plasma phospholipids. The major fatty acid circulating in the plasma of the cows was 18:2 (46·4–55·3%) on all diets but only about 1% (0·40–1·04%) of the total plasma 18:2 occurred in the plasma triglycerides. The results are discussed in relation to the changes in the composition of the milk fatty acids produced by the cows when they were given the experimental diets.

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