The effects on the fatty acid composition of milk fat of feeding various oilenriched diets (soya oil, red palm oil/palmitic acid and tallow) to dairy cows receiving a fat-deficient basal diet are reported. It was found that a linear relation was obtained between the dietary intake of C 18 fatty acids and their output in milk fat. A similar relation, applicable only to the oil-rich diets, was found for the C 16 acids. The significance of these relations is discussed. Fatty acid compositions and yields are discussed and compared with literature examples in which similar oil-rich diets were fed, but in which the low amount of fat in the basal diet did not markedly depress milk yield. The proportion of trans-isomev in the octadecenoic acid was found to increase appreciably on feeding soya oil, and to a lesser extent with dietary tallow. A linear relation between the 18:0 and 18:1 contents of milk fat was found, and its significance is discussed.

SummaryThe effects of supplementing a basal diet, in which the low level of fatty acids limited milk production, with soya oil, a palm oil/palmitic acid mixture and tallow, on the yield of milk and of its constituents, and on the composition of the milk, are reported. The yields of milk and milk fat were greatly increased by all the oil-supplemented diets; the mean daily yield of solids-not-fat (SNF) was also increased, but supplementation with soya oil caused the yield of crude protein (CP) to decrease, whereas the other fat-rich concentrate mixtures gave the same mean yield of protein as did the low-fat, control diet. All 3 oil-supplemented diets lowered the proportion of CP in the milk, but the SNF content was unchanged by any treatment. Dietary soya oil tended to lower the proportion of fat in the milk, whilst the palm oil/palmitic acid mixture raised it, with the tallow exerting no effect. The results are discussed in relation to previous work in which these dietary oils have been used, but in which the intake of fatty acids from the control diets did not limit milk production to the extent reported here.

A series of milk fats of different fatty acid composition has been obtained by supplementing the diet of the dairy cow with tallow, a mixture of palm oil/palmitic acid or soya oil. The melting behaviour of the milk fats has been studied using the differential scanning calorimeter. Dietary soya oil yielded a butterfat which was fairly soft (50 per cent of the crystalline material melting at 8oC), whilst palm oil/palmitic acid gave a relatively hard butterfat (50 per cent of the crystalline material melting at 18‐3oC), with the product derived from feeding tallow intermediate between these extremes, but closer in its behaviour to the butterfat obtained by feeding palm oil/palmitic acid. In order to eliminate the effect on the melting behaviour of milk fat triglycerides having non‐random structures, thermograms were also obtained for samples of the experimental butterfats after they had been inter‐esterified. The results suggest that, for the milk fats having the compositions used in this work, melting behaviour is dominated by the proportion of palmitic acid present.