Cows were fed either 75 or 100% of the recommended intake levels for protein and 100% of recommended energy levels (Agricultural Research Council, 1965) from 8 weeks pre-calving until 14 weeks post calving. From 14 weeks post calving and to the end of lactation all the cows received 100% of the recommended protein and energy intakes.

The mean of the 305-d milk yields of the 2 groups was not significantly different and although cows on the lower protein intake produced less lactose during the first 14 weeks of lactation there was no significant difference in total lactose, fat, protein or total solids production between the groups. In both groups blood packed-cell volume, red cell count and haemoglobin decreased during the first 10 weeks of lactation and then began to increase in the high-protein group. The cows receiving the low-protein diet showed a similar increase only when they received the high-protein ration from 14 weeks post calving. The mean interval from calving to conception was 27·5 weeks in the high-protein group and 20 weeks in the low-protein group.

It is concluded that feeding 75% of protein requirements to dairy cows during the first 14 weeks of lactation does not reduce milk yield or quality significantly and probably has no adverse effect on fertility.

Four 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.

Acetate uptake by isolated perfused guinea-pig mammary glands was approximately l·0 mg g−1 h−1 when perfusate acetate concentrations were in the physiological range (5–11 mg/100 ml plasma). At perfusate concentrations below this range (mean 3·62 mg/100 ml plasma) the uptake was not significantly different, but in one experiment in which the mean acetate concentration was 14·5 mg/100 ml plasma the uptake was markedly elevated. Radioactivity from Na [1-14C]acetate was incorporated into CO2 and milk and tissue fat, being largely present in C16 and C18 fatty acids. The guinea-pig is atypical of non-ruminants and similar to ruminants in respect of the blood concentration and mammary utilization of acetate.

Perphenazine (0·8–1mg) caused udder growth and artificial induction of lactation, when stereotaxically implanted into the median eminence of virgin female goats. Sham implantations into the median eminence or implantation of perphenazine into sites of the hypothalamus not in the immediate proximity of the median eminence were ineffective. Fourteen–twenty-two d after implantation the goats were milked for the first time. Maximal milk yields of 250–1050 ml/d were achieved 3–5 months later. Udder development and induction of lactation during the premilking period were exclusively due to the action of perphenazine. It is likely that after implantation the milking stimulus played a synergic role with perphenazine in the progressive increase and the maintenance of lactation. Apparently no side effects were caused by the drug. Implantation of small doses of perphenazine in the hypothalamus might be a suitable technique for studying the physiological effects of increased levels of endogenous prolactin in goats.

This paper is a sequel to a previous one in which a number of factors likely to influence the accuracy of counting somatic cells in milk was assessed; in the present work the effects of 5 other factors are investigated. In a study of storage time and temperature of milk samples fixed in formalin, a significant increase in cell count occurred after 5–7 d when samples were stored at room temperature (17–23°C), compared with those maintained at 4 °C. When manual and mechanical mixing of fixed samples were compared only marginal differences in cell counts were observed. An increase in cell counts followed manual dilution of milk samples in comparison with automatic dilution. The temperature of samples prepared for counting was also studied and no significant variations occurred between mean temperatures of 12·7 and 32·9 °C. The final factor evaluated was that of holding time before counting; using 4 cell-count levels it was observed that counts were acceptable up to 1½ h.

Chemical analyses, polyacrylamide-gel electrophoresis and isoelectric focusing of milk-fat globule membrane proteins (FGMP) obtained from the milk of 2 Murrah buffaloes were done to determine if any change in composition occurred during lactation. Changes in the levels of sialic acid, hexose, hexosamine, N and P were found in the FGMP obtained at different stages of lactation. On the day of parturition, 8 major proteins in FGMP were determined by sodium dodecyl sulphate polyacrylamide-gel electrophoresis whereas 6 major proteins were obtained in FGMP of middle and late lactation milks. Isoelectric focusing of FGMP showed 8–9, 9–13 and 13–16 proteins from colostrum, middle and late lactation milks, respectively and the isoelectric pH of the proteins varied from 5·25 to 7·80, 5·85 to 8·30 and 5·75 to 8·61 respectively.

Small numbers of Escherichia coli were infused into both lactating and non-lactating udders. Twelve of the 17 lactating quarters infused became infected, and all 12 showed clinical symptoms of udder disease. The 5 lactating quarters which did not become infected all had pre-infusion somatic cell counts > 300000 cells/ml milk, whilst all the quarters which became infected had cell counts < 300000 cells/ml milk. E. coli was subsequently recovered from only 6 of the 16 non-lactating quarters infused. In only 2 of these quarters did clinical infection follow, both quarters being in a cow infused 2 d before calving. The remaining 4 quarters from which E. coli was recovered were all negative within 5 d of infusion. These differences in susceptibility are discussed, particularly with reference to the frequent occurrence of coliform mastitis at and shortly after calving.

The pH of optimum activity of alkaline phosphatase from cow's milk depended on the substrate, being 10·1 for p-nitrophenylphosphate, 8·6 for phosphoserine, 8·0 for phosvitin and 6·8 for casein. Individual casein components were dephosphorylated more rapidly than mixtures of αs- and β-caseins or of αs-, β- and κ-caseins and micellar casein. Mixtures of 2 components involving κ-casein were more readily dephosphorylated than αs- and β-casein mixtures. At pH 6·8, lactose, whey proteins and phosphate ions had an inhibitory effect. β-Lactoglobulin had an inhibitory effect only when the pH of the reaction was lower than the optimum pH value of the enzyme. Mg2+ and Zn2+ were not inhibitory. The optimum conditions for dephosphorylation of casein are described.

The electrokinetic potential of bovine casein micelles was determined under a variety of conditions using moving boundary electrophoresis. The technique used allowed sharp stable boundaries to be formed without addition of density gradient-forming materials. Casein micelles are negatively charged and in any one milk sample all micelles had similar electrophoretic mobilities. The electrokinetic potential of micelles was markedly reduced by the action of rennet and to a lesser extent by a decrease in temperature from 30 to 6°C. Addition of sucrose to milk caused a small increase in the electrokinetic potential. Addition of anionic detergent to milk increased both the electrokinetic potential and the rennet coagulation time. Addition of cationic detergent caused a reduction in both these quantities, promoted syneresis of the rennet coagulum and caused coagulation in the absence of rennet if present in concentrations exceeding about 10 mM. Alteration of cation activities by addition of salts to milk caused complex changes, either increasing or decreasing the electrokinetic potential. The data obtained are most readily explained by a model for the casein micelle in which some, but not necessarily all, of the κ-casein is located at the micelle surface.

Properties of whole milk and milk fractions from cows fed a diet that gave a greatly increased proportion of unsaturated fatty acid residues (especially of linoleic acid) in the milk lipids were studied, and this milk (high-linoleic milk) was compared with milk from cows on a control diet (control milk). The milk fractions were isolated by high-speed centrifugation of whole milk or cream and were examined by chemical analysis and electron microscopy. During centrifugation the globules of milk fat were disrupted and the membranes (fat-globule ‘ghosts’) floated as a layer beneath the free lipid. Membrane proteins from the 2 sorts of milk gave the same electrophoretic pattern and the amino acid compositions were the same. Lipid analysis of the membrane fraction from high-linoleic milk showed the expected increase in the proportion of unsaturated fatty acid residues in the neutral lipids, but there was an unexpected decrease in the proportion of unsaturated residues in the membrane phospholipids. No differences were found between high-linoleic and control milk in the ultrastructure of the milk-fat globules or the isolated membranes.

The 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.

Two distinct types of chymosin-sensitive polypeptide were found in the soluble fraction obtained when a solution of crude κ-casein in 0·4 m-NaCl was adjusted to pH 3. One corresponded to a segment of κ-casein comprising residues 94–169. The other was very similar to the section Gln77-Val169 in the protein, but contained a phosphate group plus isoleucine and glycine in the region between the N-terminus and the chymosin-sensitive bond. If these polypeptides form as a result of a specific cleavage of κ-casein, the data indicate that there is a variant of this protein with a number of amino acid replacements on the N-terminal side of the chymosin-sensitive bond. The shorter polypeptide and κ-casein are both rapidly cleaved by chymosin at pH 6·7 which suggests that the contribution of residues 1–93 to the sensitivity of the Phe-Met bond in κ-casein is a minor one.

The effects on the fatty acid composition of milk fat of feeding various oil-enriched 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 C18 fatty acids and their output in milk fat. A similar relation, applicable only to the oil-rich diets, was found for the C16 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-isomer 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.

Milk-fat globule membrane (MFGM) proteins were solubilized by treatment with SDS. Four of the major proteins were isolated as SDS complexes using column chromatography. The purity of each isolate was determined by SDS polyacrylamide gel electrophoresis and sufficient of each protein was obtained for amino acid analysis. The amino acid compositions of the isolated MFGM proteins and a total MFGM protein extract were determined. Differences in amino acid composition were found in particular between the major MFGM glycoprotein and the other 3 membrane proteins. The relationships of the amino acid composition to protein properties and structure are discussed.

The effects of heat and pH on sulphydryl (–SH) and disulphide (–SS–) groups of β-lactoglobulin (β-lg) A have been studied by heating at different temperatures and pH values in air and at pH 6·9 in the absence of air. At pH 6·9 under aerobic conditions a decrease of –SH groups and an increase of –SS– groups was observed with increasing time and temperature. Only small changes were found under anaerobic conditions. At pH from 3·0 to 9·8 the –SH level decreased while the –SS– level increased up to pH 6·9 and then dropped rapidly. In addition to –SH/–SS– interchange there were reactions to other sulphur-containing compounds as seen from the losses in the total amount of –SH plus –SS– sulphur. The results of gelchromatographic investigations suggest that –SH-initiated –SS– exchange-reactions play an important role in the formation of high molecular weight polymers of β-lg A during heat treatment, and that the formation of intermediates depends on the presence of air.

Slow, irreversible aggregation of whey proteins in acidified milk or whey at pH 3·4–4·6 held for up to 10 d at 35–45°C was revealed by the reduction of discrete bands in disc electrophoresis. The aggregation was confirmed by precipitation of protein observed in stored, acid whey. The rate of aggregation of all protein fractions increased with the acidity and the storage temperature. It was enhanced by the presence of casein, but was unaffected by the presence of milk fat or by pasteurization of the fresh, unacidified milk at 70°C. The effect may contribute to the physical properties of certain fresh cheeses and other cultured dairy products.

The carotene and immunoglobulin concentrations in colostra and milks of 6 Jersey, 4 Friesian and 6 Jersey × Friesian cows were determined during the first 21 d of lactation. Carotene concentrations of between 50 and 300 μg/g milk fat were found at first milking and these declined to normal concentrations, in the range 9–21 μg/g milk fat, by the eighth to tenth day of lactation. Immunoglobulin concentrations at first milking were in the range 4–40 g/l milk and decreased to normal concentrations, in the range 0·3–0·5 g/l, by the fifth to seventh day of lactation. For both substances the data from each cow's milk fitted an exponential decay model. The fractional rates of decrease in concentration of carotene were 0·19–0·57/d and of immunoglobulin were 0·40–0·73/d. Breed-group differences were observed in both carotene and immunoglobulin concentrations in milks after the fourteenth day of lactation. In Jersey milks, the average carotene and immunoglobulin concentrations were respectively 18·5 μg/g milk fat and 0·32 g/l of milk, while in Friesian milks the concentrations were respectively 11·4 μg/g milk fat and 0·46 g/l milk. There was also a difference between breed groups in the daily rates of decrease of the immunoglobulin concentrations. The average rate of decrease for the Friesians was about 1·5 times the rate for the Jerseys.

The heat denaturation of α-lactalbumin (α-la) in NaCl and KCl solutions, milk ultrafiltrate and milk was studied using the method of micro complement fixation. It was established that this protein was very resistant to heat denaturation and that it was more stable in milk ultrafiltrate than in the other media studied at temperatures up to 70 °C. Of the various milk proteins added to α-la, only β-lactoglobulin (β-lg) formed a heat-induced complex with this protein. This complex was identical in milk ultrafiltrate or in milk and depended on the molar ratio between both proteins; it was not modified by any other milk proteins. The binding of a-la to β-lg changed the ability of the latter protein to bind κ-casein.

Using a standard method, the electronic cell count of formalin-treated milk usually increased during storage. This increase varied with the milk and was found to be due to a corresponding decrease in the effectiveness of the fat-dispersal procedure before counting. A more severe fat-dispersal procedure incorporated into the standard method resulted in a stable cell count being obtained. It was found that formalin must be added to milk within 24 h of milking to prevent lysis of the cells.