1. Measurements were made of arterial and coccygeal concentrations of plasma constituents and of arteriovenous differences across the mammary gland in two anaesthetized lactating sows, and of coccygeal-mammary-venous differences in three conscious sows when lactating and again later when ;dry'. 2. With the possible exception of acetate concentration, the compositions of arterial and coccygeal plasma were similar, and arteriovenous differences in the anaesthetized lactating sow corresponded closely to coccygeal-venous differences in the conscious animal. 3. In the ;dry' sow coccygeal-venous differences were in all instances small. 4. In the lactating sow there were large arteriovenous (or coccygeal-venous) differences (mean value as a percentage of arterial or coccygeal concentration) in glucose (31%), acetate (46%), arginine (27%), glutamate (42%), histidine (26%), isoleucine (36%), lysine (25%), leucine (39%), methionine (38%), phenylalanine (32%), proline (31%), threonine (22%), tyrosine (32%) and valine (27%), and in palmitate (19%), oleate (23%), linoleate (21%) and stearate (16%) of the plasma triglycerides. The values for the following constituents were in all instances small: beta-hydroxybutyrate, acetone+acetoacetate, citrate, lactate, alanine, glycine, aspartate, palmitoleate of the plasma triglycerides, phospholipids, cholesterol, cholesteryl esters and free fatty acids. 5. Of the total recorded uptake of plasma constituents by the lactating gland, 59% was accounted for by glucose, 28% by amino acids, 11% by plasma triglycerides and 2% by acetate. The relative uptakes of glucose and amino acids were higher in the sow than values reported previously for the goat, and the relative uptakes of acetate and triglycerides much less.

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I . Two experiments are described in which the effects of dietary fat on the synthesis of milk fat in the dairy cow have been investigated. In the first experiment a change-over design was used with two cows to study the effect of removing a mixed-oil supplement to a basal diet low in fat on the concentration and composition of plasma lipids and on the yield and composition of milk fat.2. Reducing the dietary fat intake from about 430 to 170 g/day caused falls in the concentrations in plasma of phospholipid and of free and esterified cholesterol and, in one cow, also of triglyceride. The changes in composition and concentration of the fatty acids in the plasma triglyceride fraction also reflected changes in dietary intake of fatty acids. No effect of dietary fat intake on the tots1 synthesis of milk fat was observed, but the composition of the milk fat reflected that of the dietary fat, and the yields of lauiic, myristic, stearic and oleic acids were decreased in association with a decreased dietary intake of these acids.3. I n the second experiment the effect of supplementing a basal diet low in fat with either coconut, red palm or groundnut oil on the composition and yield of milk fat was studied in four cows using a 4 x 4 Latin square design balanced with respect to residual effects.4. With each oil, increasing the dietary fat intake from about IOO to 400 g/day significantly increased the total yield of milk fat. Also, coconut and red palm oils significantly increased the fat content of milk, and groundnut oil the yield of milk.5 . W,ith the exception of linoleic and palmitic acids, the increased dietary intake of the major fatty acids characteristic of the various oil supplements led to increased yields of these acids in the milk.Tritium-labelled stearic acid administered orally, as either free acid or triglyceride, to lactating goats and cows appears in the blood plasma mainly in the triglycerides of the /3-lipoproteins and is then selectively removed from the blood by the udder and secreted in milk fat (Glascock, Duncombe & Reinius, 1956;Glascock, McWeeny & Smith, 1957;Glascock, Welch, Bishop, Davies, Wright & Noble, 1966). The intravenous infusion of tritium-labelled chylomicra into conscious lactating goats, and the perfusion of isolated goat mammary glands with similarly labelled chylomicra, have confirmed that the udder of the lactating ruminant rapidly removes, for the formation of milk fat, a large proportion of the chylomicra entering the circulation (Lascelles, Hardwick, Linzell & Mepham, 1964). Furthermore, there is a considerable difference in the arteriovenous concentrations of plasma triglycerides across the udder of lactating, but not of dry cows (Hartmann & Lascelles, 1964), and in the goat similar more detailed studies have shown that it is mainly stearic and palmitic acids in the triglycerides of the chylomicra and lipoproteins of density 1.005-1.019 which are taken up by the mammary gland (Barry, Bartley, Linzell & Robinson, 1963 Although there is, therefore, conclusive evidence that the m...

1. A study was made of the effects of feeding regimen and the composition of the diet on the ruminal production of V.f.a.'S in cattle.2. The day-to-day variations in the pattern of rumen fermentation in cows given a diet of hay and concentrates indicated that within-cow differences in the molar proportions of the acids from diet to diet are of significance if they exceed about 3% (of the mean value) for acetic acid, about 10% for propionic and butyric acids, and about 25% for valeric acid. The results emphasized also the important differences to be found between cows given the same diet.3. A change in the frequency of feeding from once to four times daily at a constant daily drymatter intake had little effect on the daily mean values for pH, concentration of total V.f.a.'S, or the molar percentages of the individual acids, but almost invariably decreased the range of values observed between feeding.4. With diets of hay and of hay and concentrates an increase in daily dry-matter intake was associated with a fall in pH and an increase in the concentration of total V.f.a.'S; with the diet of hay and concentrates there was a considerable decrease in the molar percentage of acetic acid and a corresponding increase in N-butyric, but there was little change in the molar percentages of the acids with the diet of hay.

The effect of 6-, 12-, 18-, 24-, 30-and 36-h milking intervals on milk secretion has been investigated. The effect of the previous milking intervals was eliminated by interposing a recovery period consisting of 12-h milking intervals between the experimental intervals. Bias due to the carry-over of residual milk was reduced by removing the residual milk at the end of each milking, after an injection of oxytocin.The rate of secretion of milk and of the individual constituents decreased curvilinearly, with duration of the interval, but the degree of curvilinearity differed between constituents. The rates of decrease were in the following increasing order: sodium, chloride, fat, whey proteins, casein N, water, non-protein N, lactose and potassium. The effects of the treatments persisted for some time after the end of the experimental intervals, but the original rates of secretion were regained by the end of the recovery period. The differing effects on the secretion of individual constituents resulted in increases in the concentrations of sodium, chloride and whey proteins in the milk and decreases in those of lactose and potassium.The earliest work on the rate of secretion of milk was carried out by Ragsdale, Turner & Brody (1924), who concluded that there was a progressive decline in the rate of milk secretion with increase in the length of milking interval, and that there was an even more marked effect on the secretion of fat. In the interpretation of the results the authors assumed that the milk obtained at a milking was that which had been secreted in the previous interval. Johansson (1940) demonstrated, however, that there is a considerable carry-over of milk and of fat from one milking interval to the next. He found that the carry-over of residual fat was in direct proportion to the amount of milk in the udder before milking. This was confirmed by Bailey, Clough & Dodd (1955), and Turner (1953, 1955 showed that it was true also for residual milk. In the type of experiment devised by Ragsdale et al. (1924) the carryover would therefore bias the results so that the quantity of milk removed from the udder after short intervals would be greater than that secreted during the interval and, conversely, after a long milking interval the quantity removed would be less than the amount secreted. The effects would be even more marked for fat than for milk yield.• Present address: Department of Agriculture, The University, Leeds.