The presence in the bovine mammary gland of a desaturase specific for the conversion of stearic to oleic acid allows the manipulation of the physical properties of milk fat by varying the dietary intake of c 1 6 and CIH fatty acids. In particular, feeding the dairy cow an oil with a high C1ti:Cni ratio should allow the proportion of milk fat that is liquid at 5 'C to be increased, with a consequent improvement in the low-temperature spreadability of butter. In this paper, the effect of feeding concentrate rations containing (a) soya oil as the free oil and as cracked soya beans; (b) various levels of soya oil, the ration being offered twiceor 24-times daily, on the thermal properties of the milk fat and on the yield of milk constitiients is examined in detail. Feeding ratios containing free soya oil greatly increased the proportion of milk fat liquid at 5 C , whereas cracked soya oil gave only slight increases. Proportion and yield of milk fat were decreased by feeding soya oil twice daily but were increased by continuous feeding.

The temperature at which raw milk was stored, within the range 4°‐8°C, affected the rate of growth of bacteria and the release of free fatty acids. The effects were of both statistical and practical significance and it was shown that, by maintaining milk temperatures at 4°C, a useful extension of the storage time of raw milk could be achieved. Lipolysis in stored milk was not closely related to the total concentration of psychrotrophic bacteria, but lipolytic rancidity was not observed when the psychrotroph count was below 5 × 106 colony forming units/ml.

SummaryFour Ayrshire cows in their second lactation were used in a 4×4 Latin square experiment to determine the effects of frequency of feeding on milk secretion. The animals were given a diet consisting of hay, sugar-beet pulp and dairy concentrates (34·9:19·7:45·5 on a dry-matter basis) either in 2 equal meals/d at 07.00 and 17.00 h or with the hay in 4 meals at 07.00, 12.00, 17.00 and 22.00 h and the sugar-beet pulp and concentrates in 24-h meals. Each feeding frequency was tested at 2 levels of feeding providing either 100 or 80% of the standard requirements for energy.There were no significant effects of frequency of feeding on milk yield or on the milk content of fat, protein, lactose, solids-not-fat or total solids, but underfeeding depressed the yield of milk and of all milk constituents. Neither the frequency nor level of feeding had appreciable effects on the fatty-acid composition of the milk fat or on the blood glucose concentration, but in animals fed frequently plasma urea levels were lower than in animals fed twice daily and there was no postprandial peak in urea concentration.

SummaryThe effects of supplementing a basal diet of grass silage with various oil-enriched mixtures on milk yield and composition, fatty acid composition and the melting characteristics of the milk fats are reported. In the first experiment, various dietary fats (soya oil, tallow and a palm oil/palmitic acid mixture) were fed at a fixed level (10% by weight of the concentrate ration). Soya oil had no effect on the proportion or yield of milk fat, and increased the proportion of low melting material in the milk fat; tallow increased the proportion of milk fat, and had little effect on the melting characteristics of the milk fat; the palm oil/palmitic acid mixture increased both the content and the yield of milk fat, and increased the proportion of high-melting material in the milk fat. In the second experiment, soya oil was fed at 3 levels (10, 15 and 20% by weight of the concentrate ration). No statistically significant effect on the yield of milk or of its constituents was observed; the proportion of low-melting material in the milk fat increased progressively with increasing intake of soya oil.

The quality of raw blended milk going to manufacture at five creameries in southwest Scotland has been surveyed over a two‐year period. Analyses of gross composition, somatic cell count, numbers and types of bacteria present and the concentration of free fatty acid in the milk show that milk quality was of a high standard. However, extended storage of the milk at 6°C indicated that the bacteria in the milk were actively multiplying and the results showed that extended storage of milk at creameries could seriously jeopardize milk quality. The development of lipolysis on extended storage of the milks was associated with the numbers and types of bacteria present in the milk before storage.