Journal of Dairy Research

From 6 weeks before calving to the end of lactation 2 groups of 8 cows were fed a ration incorporating a dairy nut containing 29 g/kg urea or an isocaloric isonitrogenous nut containing fish-meal and groundnut. During the 18 weeks after calving, concentrates were fed to appetite. 305-d yields of milk (7023 and 5796 kg) and lactose (345.7 and 273.4 kg) were significantly lower in the urea-fed cows (P < 0·05). Intakes of dry matter, metabolizable energy and digestible crude protein were similar in both groups until 18 weeks after calving when feeding according to yield commenced, but by 6 weeks after calving the urea-fed cows lost only 5% of their immediate post-calving weight compared with 7% in the controls. There was no evidence of ammonia toxicity in the urea-fed cows. Blood glucose concentrations were similar in both groups, but higher serum urea concentrations in the urea-fed cows suggested that there was poor utilization of ingested N. A model proposed by Roy et al. (1977) has been used to calculate the requirements of rumen-degradable protein and undegraded protein (UDP) of the 2 groups from their energy intakes and milk yields. Between 4 and 11 weeks post-calving, dietary requirement for UDP closely matched intake of UDP. It is concluded that UDP intake was a major factor in determining the peak milk yield. Mechanisms by which the degradability of the dietary protein might influence milk yield are discussed.

The effect of season on milk yield, milk composition, oestrus manifestation, oestrus cycle characteristics and conception rate was studied in high-yielding Israeli-Friesian cows. ‘Summer’ cows were allotted to control (SC) and forced ventilation (SV) groups. ‘Winter’ cows (W) were kept in one group. During July-September mean rectal temperatures at 16.00 h in primiparous and multiparous cows were 39·6 and 39·8°C, respectively in cows of the SC group, and 39·4 and 39·3°C, respectively in cows of the SV group. Mean rectal temperatures at 16.00 h of the animals in the W group during December-March were 38·9°C in both primiparous and multiparous cows. The mean 122-d milk yields in primiparous cows were 3508, 3251 and 3571 kg/cow in the SC, SV and W groups respectively (not significant (NS)). In the multiparous cows the respective yields were 4183, 4416 and 4538 kg/cow (P < 0·05). Differences in milk composition were small and statistically NS.

Observations on oestrus manifestation, performed 4 times daily, indicated that standing oestrus lasted 9·2 and 10·6 h in primiparous and multiparous cows respectively (P < 0·05); differences between groups were statistically not significant. Oestrous cycle lengths of primiparous and multiparous cows were 21·1 and 22·5 d respectively (P < 0·05) between ovulations and 23·0 and 28·05 d respectively (P < 0·01) between standing heats. Cows were inseminated following standing heat only. Conception rates (on all services within experimental periods) in primiparous cows were 50, 35 and 72% in the SC, SV and W groups respectively (NS). In the multiparous cows, the respective rates were 22, 52 and 80% (P < 0·05).

It is concluded that in the present experiment hyperthermy decreased milk production to a very small extent, in spite of very high milk yields, but fertility was severely affected. It is also suggested that primiparous cows are less affected by heat stress than multiparous cows.

The modified technique for transplanting one mammary gland of a lactating goat to the neck, with the mammary (pudic) artery and vein anastomosed to the carotid artery and jugular vein respectively, is described. This technique was successful in 6 operations and there were no significant differences between the milk yield of the transplanted and control (in situ) glands at any stage after operation. These results are compared with those from an earlier series in which no special provision was made for lymphatic drainage and in which anticoagulant therapy and treatment of the gland prior to attachment were not standardized.

Dynamic responses of the teat end, measured as changes in external diameter and milk flow-rate, due to step changes in pressure, uniformly applied to the whole teat, are presented. The experiment involved a front and a rear teat of each of 6 Holstein–Friesian cows. The measured variables were: (1) step change in pressure, (2) milk flow-rate, (3) step change in external teat end diameter, (4) rise times of pressure and teat end diameter changes, (5) fall times of pressure and teat end diameter changes, and (6) delay times between pressure and teat end diameter changes both at start of rise and of fall times. A statistical analysis showed that front teats expanded more than rear teats (P < 0·05). Several other findings of importance to any system of milk removal through the streak canal of the teat are discussed.

Mammary lipoprotein lipase of lactating goats was extracted by 3 methods: homogenization of tissue acetone-ether powders; aqueous homogenization of crude tissue using an Ultra-Turrax apparatus; aqueous homogenization of crude tissue using a Sorvall Omni-mixer microhomogenizer. Although there were differences between absolute values obtained by the 3 methods, each type of homogenate had a lipolytic activity with lipoprotein lipase characteristics (i.e. more than 90% inhibition by serum omission or NaCl addition). Furthermore, the 3 methods were highly correlated and presented similar variations with the stage of lactation, in parallel with long-chain fatty acid secretion into milk. Repeatability of the measure of homogenate lipolytic activities was about 8%, whereas day-to-day repeatability of enzyme extraction and assay was about 20% for each method.

Twelve adult West African dwarf (Fouta djallon) does, about 2 years old and weighing from 25 to 28 kg were kept for lactation studies lasting two 19-week periods. During these periods the does were hand-milked twice daily and the daily samples were bulked for each animal for subsequent analysis.

The results showed that the colostrum was much richer in its content (g/kg) of Na 1·44 ± 0·17, Κ 3·38 ± 0·22 and Cl 4·83 ± 0·29 than the mature milk which contained (g/kg) Na 0·65 ± 0·09, Κ 1·57 ± 0·19 and Cl 2·46 ± 0·58. The corresponding values obtained for Ca (0·65 ± 0·02) and P (0·36 ± 0·10) in the colostrum were, however, lower than 2·01 ± 0·98 and 1·18± 0·28 g/kg obtained for Ca and P respectively in the mature milk. The composition of these elements in the colostrum approached that of the normal goat's milk on the sixth d after parturition. The results showed a rise in Ca, P, Na and Cl levels with stage of lactation and a fall in Κ content of the milk with advancing lactation with the trends being highly significant (P < 0·01).

A frequency response of the cow's teat has been determined by applying sinusoidal changes in vacuum level to the teat. Twelve frequencies were applied ranging from 0.06 to 12 Hz. The responses investigated were the external teat end diameter and the teat milk flow-rate. The measured variables were: (1) frequency of applied pressure and teat end diameter, (2) peak-to-peak pressure and teat end diameter, (3) average vacuum level and teat end diameter, (4) time delay between maxima of vacuum level and teat end diameter, (5) time delay between minima of vacuum level and teat end diameter, and (6) teat milk flow-rate. The results showed that within the range of the investigation milk flow-rates were lower between 0.8 and 2.5 Hz than at frequencies both below and above this range. A statistical analysis of the flow rate and teat end diameter data supports the common practice of applying the same frequency to both front and rear teats during milking. Other findings show good agreement with results reported previously on dynamic responses of the teat to step changes in pressure.

A high correlation was found between soluble calcium and citrate concentrations during a study of seasonal changes in the composition of the commercial milk supply to South-west Scotland. A simple physicochemical model of the ion equilibria in the aqueous phase of milk suggests that the observed correlation is causal. This hypothesis was corroborated by an analysis of data on individual cow milks and by experiments involving the addition of citrate to milk at constant pH.

Recent years have seen a great increase in the knowledge and understanding of milk proteins. Arising from several origins including the blood stream and various cellular sources, many of the proteins found in milk are products of the secretory cells directly involved in the synthesis and secretion processes of various milk components. The lactation-specific proteins present in major amounts are synthesized in the rough endoplasmic reticulum (RER) under genetic control and undergo further post-translational modifications in their secretory route from the RER through the Golgi apparatus and secretory vesicles before ejection into the lumen with other milk components. Various molecular aspects of these mechanisms and their control are now understood, but many remain to be described.

Twelve of twenty-one udder quarters with colonized teat ducts became infected within 13 d when milked with a machine producing large cyclic and irregular vacuum fluctuations in the teatcup; of 23 quarters milked under the same conditions, but with metal shields fitted inside the liners to protect the teat apex (Thiel, 1974), only 2 quarters became infected in the same period. The introduction of a post-milking teat disinfectant teat dip resulted in the elimination of orifice colonization from 15 teats dipped in Na hypochlorite solution (40 g/l available chlorine) and from all but 2 of 15 teats dipped in an iodophor solution (5 g/l available iodine).

The lactose synthetase activity of homogenates and particulate fractions prepared from the udders of goats killed at various stages of pregnancy were determined. Both components of the enzyme, galactosyltransferase and α-lactalbumin, became detectable about half way through the gestation period, at the time when rapid proliferation of mammary epithelial tissue commences. Throughout the second half of pregnancy the udder possessed a high level of lactose synthetase activity though the lactose content of the tissue did not increase. These findings are discussed in relation to the endocrine control of lactogenesis in the goat and possible mechanisms for the induction of milk production at parturition.

The radiolabelled primary translation products of ovine mammary mRNAs synthesized in a wheat germ cell-free system were isolated by immuno-precipitation and analysed by automated Edman degradation. The 3 ‘Ca-sensitive’ caseins (α, α andβ ), k-casein, β-lactoglobulin and α-lactalbumin were found to be synthesized as precursors with amino terminal extensions of 15, 21, 18 and 19 amino acid residues respectively. The extra pieces of these various lactoproteins are similar to ‘signal’ peptides of other secretory proteins in their length and hydro-phobicity. The occurrence of an alanyl residue at the C-termini of the extra pieces of the 6 ovine prelactoproteins suggests that the mammary proteinase responsible for the cleavage of the signal peptides may have an elastase-like specificity.

When mammary mRNAs were translated in a wheat germ cell-free system in the presence of mammary microsomal membranes, pre-β-casein was converted into authentic β-casein as demonstrated by amino terminal sequence analyses. Additionally, pre-β-casein was post-translationally converted into authentic β-casein by a specific proteinase(s) extracted from rough microsomes with Na deoxycholate.

Examination of data pertaining to milk yield and litter growth rate of guinea-pigs at peak lactation showed that milk yield increased with increasing number of pups carried by the dam during pregnancy and also with increasing maternal weight. There was a significant linear correlation (P < 0.05) of mammary gland weight (expressed per unit of maternal body weight) with litter size. Increasing litter size, within 18 h of parturition, from 1 pup to 5 pups did not elicit a response in milk yield. Decreasing litter size from 5 pups to 1 pup reduced milk yield to the level expected of a monotocous dam. Data are presented showing the ability of the guinea-pig pup to survive from birth without access to milk and the inadequacy of milk supply to meet litter requirements when the litter size is more than 2 pups. The mechanisms controlling lactation in the guinea-pig are discussed in relation to data obtained in other species.

A simplified moving boundary electrophoresis technique has been developed for the measurement of the electrophoretic mobility of casein micelles. The zeta potentials of casein micelles from different skim-milk samples were calculated using Henry's equation and shown to decrease with decrease in pH between pH 6.9 and 5.3 and to increase with increase in temperature between 10 and 45 °C. Neither severe heat treatment (up to 135 °C for 51 min) nor centrifugal fractionation of micelles into different micelle size ranges had any significant effect on zeta potential. The ionic composition of the serum phase has been shown to be extremely important in determining the electrophoretic mobility. Casein micelles electrophoresed through milk ultrafiltrate consistently gave a lower mobilities than the same micelles centrifuged through milk centrifugate. The results are discussed in relation to present theories of casein micelle structure; these theories do not accommodate all of the observations.

Milk-ejection activity was determined in the blood plasma of ewes during normal milking and during milking when adrenaline was injected intravenously before or after udder stimulation. It was found that administration of adrenaline either before or after udder washing, decreased the oxytocin concentration and milk yield but increased the yield by hand-stripping. Adrenaline also retards the average time for peak oxytocin concentration. These results and the use of a β-receptor blocker to inhibit the effect of adrenaline at the myoepithelial cell level indicate that in ewes adrenaline can prevent the release of oxytocin from neurohypophysis.

Casein kinase from lactating bovine mammary gland catalyses the transfer of the terminal phosphoryl group of ATP to specific serine residues in dephosphorylated caseins. Best substrates for casein kinase are the dephosphorylated proteins (bovine αs1 - and β-caseins and pepsin), unphosphorylated human β-casein and the dephosphorylated peptide (residues 1–25) from bovine β-casein. Results obtained with bovine and human β-caseins indicate that the two serines underlined in the cluster Ser-Leu-Ser-Ser-Ser are particularly susceptible to the action of casein kinase. Since a similar sequence is found in dephosphorylated αs1-casein, it is probable that serines in this region of αs1-casein are also phosphorylated. The results support the concept that certain serines in casein are particularly susceptible to phosphorylation by casein kinase.

The effect of lactation and diet on milk lipoprotein lipase activity (LPL), cell count, serum albumin concentration and free fatty acid levels, expressed in terms of acid degree value (ADV), was examined in 2 experiments. In addition, the ability of blood plasma from cows on different diets to activate LPL was also investigated.

Changes in ADV were not consistently associated with changes in LPL, cell count or serum albumin throughout lactation, although results indicated that ADV appeared to be influenced by cell count in early lactation, and by the amount of blood plasma proteins reaching the milk during the remainder of lactation. LPL activity was low in colostrum, but increased rapidly between d 2 and 12 post-partum, reaching a peak at 5 months. Activity was unaffected by diet.

There was no effect on ADV of feeding concentrate: hay ratios of 75:25 and 60:40, but ADV was higher in cows fed a low protein diet (crude protein 9%) between 10 and 60 d post-partum, than in cows fed a high protein diet (crude protein 18 %) during the same period. In addition, the blood plasma for cows on the low protein diet had a greater ability to activate LPL although, on the basis of the millk: blood serum albumin ratio, a smaller proportion of blood constituents had reached the milk. The ability of blood plasma to activate LPL was also found to increase with increasing levels of lipoprotein lipid in the blood.

The effect that changes in composition which occur in milks possessing high cell counts have on milk lipolysis has been investigated. High cell counts were produced either by intramammary infusion of Escherichia coli endotoxin or Streptococcus agalactiae or by addition of washed cells which were isolated from milk obtained from quarters infused with endotoxin. Free fatty acid levels in milk were measured in terms of acid degree value (ADV) either as initial ADV measured immediately after milking or ADV developed after a prescribed incubation period.

There was an increase in initial ADV after the infusion either of endotoxin or of Str. agalactiae relative to a control quarter. This increase appeared to be associated with changes in cell count, but in absolute terms the influence of cells on ADV became less as cell count increased. Neither type of infusion had any effect on lipoprotein lipase activity. The addition of washed cells to normal milk resulted in an increase in developed ADV, but the increment was not as large as that produced by the addition of 1% blood serum. When cream and skim-milk from endotoxin-treated quarters and control quarters were mixed in different combinations with and without additional cells, developed ADV was higher in those samples containing endotoxin cream and those with added cells. Milk from a quarter treated with endotoxin was diluted with its own skim-milk to produce different cell counts and ADV was determined after various time intervals at 4 and 37 °C. Lipolysis increased with increasing cell count, but a depression in lipolytic rate was noted after incubation for 6 h at 4 °C and 20 min at 37 °C.

The proportion of skim-milk lipoprotein lipase activity in milk serum was larger both in milks possessing high cell counts and in normal milk adjusted to between 5 and 20 mM-NaCl by addition of solid NaCl. These levels of NaCl inhibited lipolysis.

The possible direct and indirect effects of high cell count on milk lipolysis are discussed.