The predominant fatty acids in milk are the long-chain fatty acids myristic, palmitic and stearic. These saturated fatty acids account for 75% of the total fatty acids, with a further 21% occurring as monounsaturated fatty acids of which the most prevalent is oleic acid. Only 4 g/100 g of the milk fatty acids are polyunsaturated, occurring mainly as linoleic and linolenic acids. All milk fatty acids are derived, almost equally, from either de novo synthesis or directly from preformed fatty acids in the diet. There are four main dietary sources of fatty acids: forages, oilseeds, fish oil and fat supplements. The digestive tract exerts a profound influence on the fate of dietary fatty acids. The short-chain saturated free fatty acids are absorbed through the walls of the rumen or abomasum into the bloodstream. The medium- and longer-chain saturated fatty acids pass into the small intestine, diffuse across the membrane wall where they are incorporated into lipoproteins and enter the bloodstream via the lymphatic system. The majority of unsaturated fatty acids are extensively hydrogenated in the rumen. However, recent work has shown that the levels of certain saturated fatty acids can be reduced and the levels of oleic, linoleic and linolenic fatty acids increased by feeding oilseeds rich in mono- or polyunsaturated fatty acids. In addition, work reported here has confirmed that eicosapentaenoic and docosahexaenoic acids can be transferred to milk when a diet containing fish oil is fed, but the transfer efficiencies are low.

Since estimated dietary selenium intake in the UK has declined steadily from around 60 µg day −1 in 1975 to 34 µg day −1 in 1997, there is a need to increase selenium intake from staple foods such as milk and milk products. An experiment was therefore done to investigate the relationship between dietary source and concentration of selenium and the selenium content of bovine milk. In a 3 × 3 factorial design, 90 mid-lactation Holstein dairy cows were supplemented over 8 weeks with either sodium selenite (S), a chelated selenium product (Selenium Metasolate) (C) or a selenium yeast (Sel-Plex) (Y) at three different dietary inclusion levels of 0.38 (L), 0.76 (M) and 1.14 (H) mg kg −1 dry matter (DM). Significant increases in milk selenium concentration were observed for all three sources with increasing inclusion level in the diet, but Y gave a much greater response (up to +65 µg l −1 ) than the other two sources of selenium (S and C up to +4 and +6 µg l −1 respectively). The Y source also resulted in a substantially higher apparent efficiency of transfer of selenium from diet to milk than S or C. Feeding Y at the lowest dietary concentration, and thus within the maximum level permitted under EU regulations, resulted in milk with a selenium concentration of 28 µg l −1 . If the selenium concentration of milk in the UK was increased to this value, it would, at current consumption rates, provide an extra 8.7 µg selenium day −1 , or 11 and 14% of daily recommended national intake for men and women respectively.

The aim of this work was to test the robustness of the 0.68 estimate of the efficiency of conversion of metabolisable protein into true milk protein (Agriculture and Food Research Council (AFRC), 1993) for protein-limiting diets and to determine whether a different value is appropriate for practical rationing. Seventy-two multiparous cows were blocked on the basis of milk energy output per unit of dry matter intake (DMI), and allocated at random to one of four treatments. Treatments supplied metabolisable energy (ME) at a fixed level to individuals within a block, but varied metabolisable protein (MP) supply from 25% below the estimated requirements, through 212.5% and 112.5% up to 25% above requirements for the average performance of animals within blocks at the start of the study. Cows were offered diets to meet their predicted ME requirements for each 3-week period with measurements performed in the last week of each period. Milk protein output was regressed against the estimated MP available for production for each cow and the efficiency of conversion of MP into milk true protein was calculated, assuming a maintenance requirement according to the MP system. The efficiency of conversion of MP into milk true protein decreased with the increasing supply of MP from 0.77 to 0.50. Using an iterative approach to determine the best fit of the data when supply matched requirement resulted in a range of efficiency values between 0.62 and 0.64 g of true milk protein per g of MP.

-With the aim of reducing the degree of saturation and increasing the C18:1 cis fatty acid content of milk fat, the effects of feeding high levels of whole cracked rapeseed to dairy cows was investigated together with the effect of increasing dietary intake of vitamin E on the vitamin E content of milk. Using a 3 × 3 factorial design, 90 Holstein dairy cows were fed one of three levels of whole cracked rapeseed (0 (ZR), 134 (MR) and 270 g·kg -1 diet dry matter (DM) (HR)) in combination with one of three intakes of supplementary vitamin E (0 (ZE), 2 (ME) and 4 g·cow -1 ·d -1 (HE)). Supplementing with up to almost 2 kg·d -1 of rapeseed oil (diet HR) significantly (P < 0.001) increased C18:1cis in milk fat, from 181 (ZR) to over 400 g·kg -1 (HR) of total milk fatty acids. Concentrations of C18:0, C18:2 and C18:3 fatty acids were also increased (P < 0.001) but by a much lesser degree, and the saturated fatty acids C4:0 to C16:0 decreased substantially. Vitamin E supplementation increased (P < 0.01) milk vitamin E concentrations from 1.29 (ZE) to 1.68 mg·kg -1 whole milk (HE). Thus substantial changes in milk fat composition with potentially beneficial effects on human health were achieved and without any adverse effects on milk taste. However, these improvements must be offset against the substantial reductions (P < 0.001) observed in voluntary feed DM consumption (ZR, 20.6; HR, 15.2 kg DM·d -1 ), milk yield (ZR, 22.9; HR, 13.2 kg·d -1 ) and milk fat concentration (ZR, 42.1; HR, 33.4 g·kg -1 ) which would not be commercially sustainable unless a considerable premium was paid for this modified milk. It seems likely that the optimum dose of dietary rapeseed is lower than used in this study. Résumé -Augmentation de la concentration en acide oléique et en vitamine E du lait de bovin en incorporant dans la ration des graines de colza entières et de la vitamine E. Afin de réduire le degré de saturation des acides gras et d'augmenter la teneur en C18:1 cis du lait, les effets d'une alimentation avec des niveaux élevés de graines de colza concassées entières ont été étudiés chez des vaches laitières, en relation avec les effets de l'augmentation de l'ingestion de la vitamine E sur la teneur en vitamine E du lait. Au total, 90 vaches laitières de race Holstein ont été alimentées selon un schéma factoriel 3 × 3 avec trois niveaux de graines de colza concassées (0 (ZR), 134 (MR) et 270 g·kg -1 de matière sèche (HR)) et trois teneurs en vitamine E (0 (ZE), 2 (ME) et 4 g·vache -1 ·j -1 (HE)). La supplé-mentation avec près de 2 kg·j -1 d'huile de graines de colza (régime HR) a augmenté significativement (P < 0,001) la concentration du C18:1 cis du lait, de 181 (ZR) à plus de 400 g·kg -1 d'acides gras totaux du lait (HR). Les concentrations en C18:0, C18:2 et C18:3 ont été également augmentées (P < 0,001), mais dans des proportions moindres. Dans le même temps, la concentration des acides gras saturés C4:0 à C16:0 a diminué sensiblement. La supplémentation en vitamine E a augmenté les concentrations en vitamine E du lait ...

1. Experiments were carried out in vivo to investigate the pathways of ammonia incorporation into rumen bacteria, bacterial fractions and free amino acids within the bacteria.2. Steers were alternately given two isoenergetic, isonitrogenous diets containing the nitrogen mainly as either urea or decorticated groundnut meal (DCGM). At the end of each period on a given diet, a solution of l5NH,C1 was infused into the rumen and samples of rumen contents were removed at 2, 10, 20 and 90 min and 5, 10 and 24 h afterwards. Concentrations of ammonia and its 16N enrichment were determined and samples of mixed rumen bacteria were prepared. Bacteria were disrupted ultrasonically and separated into bacterial protein, cell wall and protein-free cell supernatant fractions. Amino acids were separated after hydrolysis and their 15N contents determined.3. A rumen fluid circulation pump was developed so that representative samples could be taken at very short time intervals after the introduction of the 15N label.4. Rumen pH changes, rumen fluid dilution rates and patterns of rumen ammonia concentrations were consistent with normal rumen metabolism. Net bacterial synthesis (as calculated from the net outflow of bacteria from the rumen) was significantly (P < 0.05) greater with the DCGM diet (12.4 g bacterial N/d) than with the urea diet (9.24 g bacterial N/d).5. With both diets the 16N label rapidly left the rumen ammonia pool and entered the rumen bacteria. Analysis of the bacterial fractions indicated that the label appeared rapidly in the protein-free cell supernatant fraction and more slowly in the bacterial protein and cell wall fractions.6. With the DCGM diet bacteria apparently utilized intracellular label less efficiently than with the urea diet. The proportion of N in the protein-free cell supernatant was higher with the DCGM diet, suggesting increased levels of intracellular amino acids and peptides, following extracellular protein degradation.7. Levels of enrichment of the amino acids alanine and glutamate in the protein-free cell supernatant fraction suggested that the enzymes alanine dehydrogenase (EC 1 .4.1.1) and glutamate dehydrogenase (EC 1 . 4 . 1 . 2 and 1 .4.1.4) may be the major enzymes for assimilating ammonia when concentrations of soluble carbohydrate and rumen ammonia are high in the rumen.8. The high levels of intracellular alanine are discussed with reference to published work on the excretion of alanine by rumen bacteria.

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-The potential to increase the concentrations of n-3 polyunsaturated fatty acids (PUFAs) in milk fat was investigated by studying the effects of feeding a xylose-treated, whole cracked linseed supplement (rich in α-linolenic acid) to dairy cows. Also the effect of increasing the dietary intake of vitamin E on the vitamin E status of milk was investigated. The effect of pasteurisation on milk fatty acid composition was also examined. Using a 3 × 2 factorial design, a total of 60 Holstein dairy cows were fed a total mixed ration based on grass silage supplemented with one of three levels of whole cracked linseed (78, 142 or 209 g·kg -1 diet dry matter (DM); designated LL, ML or HL, respectively) in combination with one of two levels of additional dietary vitamin E intake (6 or 12 g vitamin E·animal -1 ·day -1 ; designated LE or HE, respectively). Increasing lipid supplementation reduced (P < 0.01) diet DM intake and milk yield, and increased (P < 0.001) the overall content of oleic, vaccenic, α-linolenic and conjugated linoleic acids, and total PUFAs and monounsaturated fatty acids (MUFA). Myristic and palmitic acids in milk fat were reduced (P < 0.001) through increased lipid supplementation. While α-linolenic acid concentrations were substantially increased this acid only accounted for 0.02 of total fatty acids in milk at the highest level of supplementation (630 g α-linolenic acid·animal -1 ·day -1 for HL). Conjugated linoleic acid concentrations in milk fat were almost doubled by increasing the level of lipid supplementation (8.9, 10.4 and 16.1 g·kg -1 fatty acids for LL, ML and HL, respectively). Although milk vitamin E contents were generally increased there was no benefit (P > 0.05) of increasing vitamin E intake from 6 to 12 g·animal -1 ·day -1 . The fatty acid composition of milk was generally not affected by pasteurisation. milk fat / fatty acids / α-linolenic acid / vitamin E / pasteurisation Résumé -Augmentation de la concentration en acide α-linolénique et en vitamine E dans le lait de vache par complémentation de la ration avec des graines de lin entières et de la vitamine E. La possibilité d'augmenter la concentration en acides gras polyinsaturés n-3 (PUFAs) de la matière grasse du lait a été étudiée en suivant les effets de l'addition de graines entières de lin concassées, traitées au xylose (riches en acide α-linolénique) à la ration de vaches laitières. L'effet de l'augmentation de l'ingestion de la vitamine E sur la concentration de la vitamine E dans le lait, * Corresponding author: e.r.deaville@reading.ac.uk 4 E.R. Deaville et al.et l'effet de la pasteurisation sur la composition en acides gras du lait ont également été examinés. En utilisant un schéma factoriel 3 × 2, un total de 60 vaches laitières Holstein ont reçu une ration complète à base d'ensilage d'herbe complétée par un des trois niveaux de graines de lin entières concassées (78, 142 ou 209 g·kg -1 matière sèche (MS), respectivement, LL, ML ou HL), en combinaison avec un des deux niveaux d'apport alimentaire de vitamine E (6...

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