The composition and functional properties of cow’s milk are of considerable importance to the dairy farmer, manufacturer, and consumer. Broadly, there are 3 options for altering the composition and/or functional properties of milk: cow nutrition and management, cow genetics, and dairy manufacturing technologies. This review considers the effects of nutrition and management on the composition and production of milk fat and protein, and the relevance of these effects to the feeding systems used in the Australian dairy industry. Dairy cows on herbage-based diets derive fatty acids for milk fat synthesis from the diet/rumen microorganisms (400–450 g/kg), from adipose tissues (<100 g/kg), and from de novo synthesis in the mammary gland (about 500 g/kg). However, the relative contributions of these sources of fatty acids to milk fat production are highly dependent upon feed intake, diet composition, and stage of lactation. Feed intake, the amount of starch relative to fibre, the amount and composition of long chain fatty acids in the diet, and energy balance are particularly important. Significant differences in these factors exist between pasture-based dairy production systems and those based on total mixed ration, leading to differences in milk fat composition between the two. High intakes of starch are associated with higher levels of de novo synthesis of fat in the mammary gland, resulting in milk fat with a higher concentration of saturated fatty acids. In contrast, higher intakes of polyunsaturated fatty acids from pasture and/or lipid supplements result in higher concentrations of unsaturated fatty acids, particularly oleate, trans-vaccenate, and conjugated linoleic acid (CLA) in milk fat. A decline in milk fat concentration associated with increased feeding with starch-based concentrates can be attributed to changes in the ratios of lipogenic to glucogenic volatile fatty acids produced in the rumen. Milk fat depression, however, is likely the result of increased rates of production of long chain fatty acids containing a trans-10 double bond in the rumen, in particular trans-10 18 : 1 and trans-10-cis-12 18 : 2 in response to diets that contain a high concentration of polyunsaturated fatty acids and/or starch. Low rumen fluid pH can also be a factor. The concentration and composition of protein in milk are largely unresponsive to variation in nutrition and management. Exceptions to this are the effects of very low intakes of metabolisable energy (ME) and/or metabolisable protein (MP) on the concentration of total protein in milk, and the effects of feeding with supplements that contain organic Se on the concentration of Se, as selenoprotein, in milk. In general, the first limitation for the synthesis of milk protein in Australian dairy production systems is availability of ME since pasture usually provides an excess of MP. However, low concentrations of protein in milk produced in Queensland and Western Australia, associated with seasonal variations in the nutritional value of herbage, may be a response to low intakes of both ME and MP. Stage of lactation is important in determining milk protein concentration, but has little influence on protein composition. The exception to this is in very late lactation where stage of lactation and low ME intake can interact to reduce the casein fraction and increase the whey fraction in milk and, consequently, reduce the yield of cheese per unit of milk. Milk and dairy products could also provide significant amounts of Se, as selenoproteins, in human diets. Feeding organic Se supplements to dairy cows grazing pastures that are low in Se may also benefit cow health. Research into targetted feeding strategies that make use of feed supplements including oil seeds, vegetable and fish oils, and organic Se supplements would increase the management options available to dairy farmers for the production of milks that differ in their composition. Given appropriate market signals, milk could be produced with lower concentrations of fat or higher levels of unsaturated fats, including CLA, and/or high concentrations of selenoproteins. This has the potential to allow the farmer to find a higher value market for milk and improve the competitiveness of the dairy manufacturer by enabling better matching of the supply of dairy products to the demands of the market.

Two experiments examining the effects of herbage mass and herbage allowance on the consumption of nutrients by lactating dairy cows were conducted on irrigated perennial pasture swards in northern Victoria. Experiment 1 was conducted in early lactation (spring) with a perennial ryegrass (Lolium perenne L.)–white clover (Trifolium repens L.) sward at herbage masses of 3.1 (low) or 4.9 (medium) t dry matter (DM)/ha and herbage allowances of about 20, 35, 50 and 70 kg DM/cow.day. Within each herbage mass treatment, there were no significant differences between herbage allowance treatments in nutritive characteristics of pregrazing herbage. Daily DM intake increased linearly from 7.1 to 16.2 (low mass) or 9.9 to 19.3 (medium mass) kg DM/cow, as herbage allowance increased which was equivalent to 2.29 kg DM/t DM increase in herbage mass and 0.18 kg DM/kg DM increase in herbage allowance. This was associated with a decrease in utilisation of herbage from 35 to 23% and from 52 to 29%. Also, milk production increased linearly from 21.8 to 27.1 (low mass) or 24.7 to 32.0 (medium mass) kg/cow.day as herbage allowance increased. Experiment 2 was conducted in mid lactation (summer) with a paspalum (Paspalum dilatatum L.)-dominant sward at herbage masses of 3.0 (low) or 4.7 (medium) t DM/ha and herbage allowances of about 25 and 45 kg DM/cow.day and either 0 or 5 kg DM concentrate/cow.day. Within each herbage mass treatment, there were no significant differences between herbage allowance treatments in nutritive characteristics of pregrazed herbage. Daily DM intake increased by 0.13 kg DM for every 1 kg DM increase in herbage allowance. Over the 2 herbage masses, 2 herbage allowance treatments and 2 concentrate treatments, cows consistently selected a diet about 1.03 higher in in vitro DM digestibility and 1.24 higher in crude protein than that in the herbage on offer. Substitution rate increased from 0.20 to 0.42 (low mass) and from 0.34 to 0.44 (medium mass) kg DM reduction in herbage intake/kg DM of concentrates consumed, with increasing herbage allowance. Along with these changes, marginal returns to supplements decreased from 1.38 to 0.95 (low mass) and 1.07 to 0.97 (medium mass) kg milk/kg DM of concentrates with increasing herbage allowance. Dairy farmers should consider the effects of herbage allowance and herbage mass on intake, nutrient selection and milk production when allocating pasture to dairy cows. Herbage mass and allowance had a greater effect on intake in spring than in summer, principally due to the decline in nutritive characteristics that occurs in summer. Cows selected nutrients to varying degrees depending on the composition of the pasture sward and this selection may result in seasonal nutrient imbalances. Finally, it appears that the best use of supplements will occur when pastures are short in height because this will minimise substitution of supplement for pasture and maximise marginal returns in milk production.

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Two experiments were conducted to establish responses in milk Se concentrations in grazing dairy cows to different amounts of dietary Se yeast, and to determine the effects of the Se concentration of the basal diet. The hypothesis tested was that the response in milk, blood, and tissue Se concentrations to supplemental Se would not be affected by whether the Se was from the basal diet or from Se yeast. In addition, by conducting a similar experiment in either early (spring; experiment 1) or late (autumn; experiment 2) lactation, we hypothesized that different Se input-output relationships would result. Both 6-wk experiments involved 60 multiparous Holstein-Friesian cows, all of which had calved in spring. They were allocated to 1 of 10 dietary Se treatments that included 2 types of crushed triticale grain (low Se, approximately 165 microg of Se/kg of DM; or high Se, approximately 580 microg/kg of DM) fed at 4 kg of DM/d, and 1 kg of DM/d of pellets formulated to carry 5 quantities of Se yeast (0, 4, 8, 12, or 16 mg of Se). Daily total Se intakes ranged from <2 to >18 mg/cow in both experiments. Milk Se concentrations plateaued after 15 and 7 d of supplementation in experiments 1 and 2, respectively, and then remained at plateau concentrations. Average milk Se concentrations for the plateau period increased as the amount of Se yeast increased, and low- and high-Se grain treatments were different at all quantities of Se yeast, although there was a tendency for this difference to diminish at the greatest concentrations of yeast. There were significant positive, linear relationships between Se intake and the concentrations of Se in milk, which were not affected by the source of Se, and the relationships were similar for both experiments. Therefore, the output of Se in milk in experiment 1 was greater than that in experiment 2 because the milk yield of the cows in early lactation was greater. The estimated proportions of Se partitioned to destinations other than milk and feces increased with the amount of Se in the diet and were greater in experiment 2 than in experiment 1, a result that was supported by Se concentrations in whole blood and plasma and in semitendinosus muscle tissue. If high-Se products are to be produced for human nutrition, it is important to be able to develop feeding systems that produce milk with consistent and predictable Se concentrations so that products can consistently meet specifications. The results indicate that this objective is achievable.

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