Changes in mammary blood flow, arterial and venous plasma concentrations of glucose and individual amino acids, udder volume, and milk yield and composition were measured at intervals throughout lactation in four Friesland ewes. Milk yields peaked 50–80 d post partum and declined by 40% within 3 months. Neither mammary blood flow (43·3 ± 5·8 (s.e.m.) ml/100 cm3, min) nor udder volume changed significantly throughout the period of study, but for three ewes the ‘mammary blood flow:milk yield’ ratio increased from 300 (peak yield) to 570 (late lactation). Mammary glucose uptake remained essentially constant throughout lactation despite a 50% decline in lactose output. Arterial concentrations of glucose were much lower at peak yield than in late lactation. Mammary amino acid uptake conformed quite closely to ‘essential’ and ‘non essential’ categories previously defined for goats and cows, the degree of balance with output in milk protein being similar at all stages of lactation. For several amino acids arterial concentrations and arteriovenous differences were significantly positively correlated: the changes in arterial concentrations with lactation stage were also correlated for some amino acids. Apart from the intrinsic value of such studies on a breed of ewe increasingly used for dairy purposes, the Friesland ewe appears well suited for use in quantitative metabolic studies on lactation.

The records of 453260 British Friesian cows, accumulated by the Milk Marketing Board of England and Wales in 1981/82, were used to estimate the effect of the calving interval on milk production in the subsequent lactation. There was a clear relationship between the length of the dry period and the calving interval in which it occurred, such that the dry period was extended by 2·5 d for every 10 d extension of the calving interval. The relationship between the interval and milk production in the following lactation appeared more likely to be due to variation in the dry period than to variation in the calving interval itself. A dry period between 50 and 60 d was associated with maximum production of milk, fat and protein. Among cows with dry periods shorter than 55 d, fat and protein production increased by 8 and 3·5 kg/d respectively. For cows with dry periods longer than 55 d, production of fat and protein was reduced by 0·2 and 0·12 kg/d respectively. The modal calving interval was 355 d including a dry period of 51 d. The interval tended to increase with the age of the cow, at the rate of 3 d/month of age within parity. The best estimate of future calving intervals was the mean of all previous intervals of the cow.

A method is described for preparing immunologically homogeneous human milk β-casein, against which monospecific rabbit antiserum was prepared. The antiserum was used to quantify β-casein, the major human casein, by rocket Immunoelectrophoresis in individual milk samples. However, it was found that in most samples β-casein occurred together with degradation products originating from its proteolysis by plasmin. Immunological quantification of human β-casein, treated with plasmin for various time periods, showed that rocket height was not affected by proteolysis up to degradation states clearly more advanced than those observed in all samples of fresh human milk tested. Assays of 150 individual milk samples from 80 women, covering a lactation period of up to 730 d, gave an average concentration of β-casein (native+degraded) of 4·67±0·89 standard deviation (g/1); extremes at 2·1 and 7·3 g/1 did not vary significantly during the period under study. Comparison of this average value with an accepted casein content of 4·4 g/1 (Macy & Kelly, 1961) showed that the casein content of human milk is underestimated when obtained by N determinations on milk and on its supernatants at pH 4·6 (whey). Caseins other than β-casein occurred only in minute amounts, if at all.

The effects of serotonin, injected into one udder artery, on teat smooth muscle function were investigated in four lactating cows. Motility of longitudinal smooth muscles was recorded by a plethysmographic technique, and sphincter function by measuring milk leakage from the full udder. Serotonin (40, 120 and 360 μg) activated teat muscle tonicity and reduced the volume of milk leakage. The effects on longitudinal smooth muscles were reduced by mianserin and ketanserin (0·375, 1·5 and 6 mg) and by methysergide (1·5 mg). These blocking substances were also effective (0·2, 0·6 and 1·8 mg respectively) in antagonizing the inhibiting action of serotonin on milk leakage. It is suggested that serotonin effects are mediated by receptors of the S2-type.

Guinea pig mammary glands which were either lactating, involuting or dry were infused with colloidal carbon or killed staphylococci. At different time intervals following infusion, animals were killed and the superficial inguinal lymph nodes examined for the presence of carbon. Sides which had nodes with visible carbon were designated ‘positive’. The time intervals from infusion to positive for the three groups were compared using logistic regression. The times required for 50 % of the sides to be positive were estimated to be ˜4 h for lactating glands, 32 h for those in involution, and 13 min for dry glands. Histological differences in distribution of carbon in the mammary tissue suggest that differences in transit time may have been due to different mechanisms of transport through the glands in the three different physiological states. The distribution of bacteria was similar to that of the carbon in the corresponding tissues.

A New Zealand dairy herd of 73 cows was observed at pasture during the breeding season. Individual cow milk yields and composition analyses were obtained before, during and after oestrus. There was an overall reduction in yield at the first, and a rebound enhancement at the second milking after onset of oestrus. Changes in fat content followed the same time course and there was a slight increase in lactose content. Protein and somatic cell content were unaffected. There were considerable individual differences in both the duration and intensity of oestrous behaviour on the one hand, and the extent and timing of the effects on milk production on the other. However no relationship was found between the degree of behavioural excitement shown by a cow in oestrus and its change in milk production.

The yield and composition of milk from Friesian cows grazing either perennial ryegrass (Lolium perenne) (G, ten cows) or white clover (Trifolium repens) (C, nine cows) were evaluated between d 21 and 129 post partum. The two forages, of similar digestible energy content, were the sole source of nutrients and were offered ad lib. (exp. 1). Digestion and flow at the duodenum were measured on 13 occasions in early lactation with comparable cows fitted with rumen and duodenal cannulae and grazing similar forages (exp. 2). The gross milk yield (22·2, G; 25·0, C, kg/d P < 0·05) and the yield of protein (0·66, G; 0·77, C, kg/d, P < 0·01) were higher, but the protein content was similar and the fat content lower for cows fed C compared with G. Cows fed G were heavier at the beginning of the experiment and lost weight more rapidly than cows fed C. Milk energy output, adjusted for tissue energy gain or loss, was 83·9 for cows fed C compared with 71·8 MJ/d for cows fed G (P < 0·001), during the period of tissue energy repletion (weeks 11–18). From week 18 to the end of lactation all cows from exp. 1 were fed silage ad lib. and 502 kg dry matter of concentrates. The total (305 d) difference in lactation response to grazing C compared with G was 931 1 (5657 1, C; 4726 1, G); this was a direct response during the experiment of 301 1 and a residual response of 630 1. In exp. 2, more organic matter (6·47, G; 8·01, C, kg/ d, P < 0·001), and more non-ammonia N (433, G; 575, C, g/d, P < 0·001) entered the duodenum of cows grazing C compared with G.

Although β-lactoglobulin (β-lg) has been considered to be absent from human milk, recent results of other workers, based on immunological reactions between human milk and rabbit antiserum to bovine β-lg, suggest that this protein may be present. Although our results show similar immunological reactions, we consider that lactoferrin is responsible for these, as it was the only reactive protein species which could be prepared to homogeneity. Indeed two types of antibodies were found by ELISA test in the antisera to bovine β-lg. One of them would be able to bind loosely to human lactoferrin, but its binding sites would not be antigenic in the rabbit.

The activities of glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, isocitrate dehydrogenase, malic enzyme, lactate dehydrogenase and malate dehydrogenase have been determined in secreted milk from sows, rats and rabbits. Within each species, although there was considerable variation in the absolute activities of these enzymes, the relative activities were similar to those observed for, or previously published for mammary homogenates. The only exception was milk glucose 6-phosphate dehydrogenase which tended to lose activity upon prolonged storage in the mammary gland. These results suggest that the pattern of milk enzymes can be an accurate reflection of that occurring in the mammary gland.

Total bacterial counts of the milk from individual cows were measured for three groups of ten winter housed cows at three milkings. The teats were either (i) left unwashed or (ii) washed with disinfected water (60 ppm available iodine) and dried with individual paper towels or (iii) washed with plain water and then dried with a single fabric cloth impregnated with a polymeric bisguanide and a quaternary ammonium compound. The mean total bacterial counts/ml for the groups were 5820, 2108 and 1116 respectively. Treatments (ii) and (iii) were also compared for their ability to prevent the inter-teat transfer of bacteria. Before teat washing and drying, one teat of each cow was deliberately contaminated with Streptococcus agalactiae. Significantly fewer teats (5/30) became contaminated with Str. agalactiae when treatment (iii) was used for teat washing and drying compared with treatment (ii) (20/30).

Fibronectins were isolated from both bovine colostrum and fresh milk, and they were found to be very similar to plasma fibronectin. The concentration of fibronectin in milk decreased post partum: 30 µg/ml after 0 d and 2 µg/ml after 7–851 d. No fibronectin could be detected in any brands of ultra heat treated liquid milk on the market (≤0·006 µg/ml).

Penetrability of the bovine teat duct to Escherichia coli endotoxin solution was measured before and after reaming the duct with a polypropylene tube, a steel twist drill bit, or a fluted drill point. There was some evidence of a transient increase in penetrability for one or two milkings following reaming but most ducts were not rendered penetrable to endotoxin. Histological examination of ducts after reaming showed that although the loosely packed superficial keratin was effectively removed a continuous smooth keratin layer remained covering the epithelium. These observations are discussed with respect to the possible mechanisms responsible for maintaining the defensive barrier of the duct to bacterial penetration.

Milks from two groups of dairy cows grazing either white clover or ryegrass swards were compared to determine whether their acceptability in terms of flavour, composition and rennet coagulation properties was affected by these differences in the cows' diet.

The concentrations of Ca and Mg were slightly higher and that of inorganic phosphorus was slightly lower in milks from the animals on the ryegrass pasture compared with those grazing clover, but the levels of Na, K and citrate were the same. The clover milks contained more αs- and β-casein and a greater proportion of small micelles but had similar contents of κ-casein to the ryegrass milks. Following addition of rennet at pH 6·4, the clover milks gave rise to firmer curds but the clotting times and rates of syneresis were not consistently different from the ryegrass milks. The clover milks contained a greater proportion of short-chain (C4–C10) fatty acids, but lower proportions of long-chain (C18+) and unsaturated acids than the ryegrass milk.

The different pastures did not give rise to any differences in the flavour of the milk.

The effect of separating conditions on lipase activity and free fatty acid levels in preheated milk, cream and skim milk was measured on nine occasions during a 12-week period covering the seasonal change from winter feeding to summertime grazing. This change consisted of four periods each representing a different type of forage intake, namely: silage, kale, daytime grazing and 24 h grazing. Milk was separated at 30, 40, 50 and 60°C with preheating times of 10, 25 and 55 s. Results were expressed both as absolute values and in terms of changes relative to the original unheated milk. Lipase activity and free fatty acid concentration were significantly reduced as separation temperature increased but were not influenced by holding time. The loss of activity in cream was progressive so that at 60°C only 40% of the original activity remained. Up to 50°C little change occurred in preheated milk or skim milk activity, while at 60°C 83 and 76% respectively of the original activity remained. The amount of activity calculated to be associated with the fat fraction of the cream also decreased with temperature. Activity varied significantly with date; maximum values were observed during the first 3 weeks of summertime grazing. Relative activity values indicated that the susceptibility of milk lipase to heat inactivation also varied with date. Lipolysis was also significantly affected by date. Cream free fatty acid levels were lower during the period of daytime grazing and were significantly higher than those in preheated milk. The correlation between lipase activity and free fatty acid levels was generally poor, accounting for between 0 and 34% of the variance. Possible reasons for the effect of separating temperature on lipolysis in cream are discussed.

The variation in plasmin activity and proteose-peptone (PP) content in milks from 81 cows of four breeds was statistically analysed. Plasmin activity increased with parity and stage of lactation and differed between breeds, but the breed effect was removed when adjustment was made for differences in milk casein content. The negative correlation found between plasmin activity and casein content seemed to be expressed only under assay conditions and was thus probably an artefact. The linear regressions of proteose-peptones in fresh (PP) and cold-stored milks (ΔPP) on plasmin activity were highly significant (P < 0·001). However, the variation in plasmin activity explained only 38 and 33%, respectively, of the variation in PP and ΔPP. At apparent zero plasmin activity, PP and ΔPP were significantly different from zero (P < 0·001) and the intercepts represented 77 and 46%, respectively, of the averages. PP and ΔPP were significantly higher in milks containing the BB genotype of ²-lactoglobulin than in milks with the AA genotype.

The lactoperoxidase/SCN-/H2O2 system (LPS) was found to inhibit lipoprotein lipase (LPL) in milk. Inhibition was related to the amount of SCN-/H2O2 added to milk. Lipolysis was unaltered until LPL activity was decreased to <40% of the original activity. When LPL was inhibited further, decreased lipolysis was observed. Purified LPL was also inhibited by LPS, at similar concentrations of SCN-/H2O2 as in milk. It was shown that non enzymically prepared OSCN- inhibited LPL at concentrations comparable to those produced by the LPS. Cysteine could eliminate the effect of LPS on LPL. The sulphydryl reagents dithiobisnitrobenzoic acid and Na tetrathionate had no effect on LPL activity.

Extended X-ray absorption fine structure spectra, above the Ca K absorption edge, were recorded from lyophilized casein micelles obtained from the milk of cow, pig, goat, rabbit and rat. A spectrum was also obtained from the calcium phosphate prepared from the casein micelles of human milk. Comparison of these spectra showed that the short-range environment of calcium in the micellar calcium phosphate was the same in all six species. Furthermore, these spectra closely resembled that of brushite (CaHP04. 2H20). However, X-ray diffraction showed no evidence for an extensive crystal lattice in micellar calcium phosphate.

The chemical composition and rennet coagulation properties at pH 6·4 of bulked milks from two groups of dairy cows grazing ryegrass or white clover swards were monitored over a period of 14 weeks commencing at the fifth week of lactation. There was considerable variation in the concentrations of many components and large variations in the rennet clotting time (RCT), coagulum strength and rate of syneresis. The relative proportions of αs-, β- and κ-casein were also subject to considerable variation. Many statistically significant correlations were found between concentrations of protein fractions, minerals and citrate, some of which may be of physiological significance whereas others may be incidental. The coagulum strength displayed significant correlations with concentrations of total casein, αs-casein, inorganic phosphate (Pi), whey protein and RCT, but not with β-casein or η-casein. The syneresis time was significantly related to the concentrations of fat, citrate, η-casein and the original milk pH. RCT displayed a significant correlation with the concentration of Pi.