Agriculture and Food Systems - Theses

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    Influence of different systems for feeding supplements to grazing dairy cows on milk and yoghurt composition
    Milk and dairy products contain various nutrients and bioactive compounds. Proportions of milk components can be altered by feeding strategies. Profiling milk compounds provides a view to the nutritional value and physicochemical properties of milk and could ensure that different feeding strategies will not compromise milk quality. Traditionally, grazed pasture has been the main source of nutrients for dairy cattle in southern Australia and New Zealand. Below average rainfall and reduced pasture availability in the last decade, have led to the consideration of alternative feeding systems to meet the nutritional requirements of the dairy cows. Feeding systems that maintain grazed pasture but take advantage of feeding a mixed ration, could offer enormous potential for pasture-based industries. Partial mixed ration (PMR), defined as total mixed ration (TMR) incorporated into the diets of grazing dairy cows, have the potential to be used in Australia. Researchers at the Department of Primary Industries in Victoria, Australia, have implemented a research program that has the aim of evaluating the efficiency of feeding supplementary forage and grain to grazing cows as a PMR. The influence of offering high and varying amounts of supplement (grain and forage) with different carbohydrate sources as a PMR to cows grazing a restricted allowance of pasture on milk components has not been tested. In this thesis, the effects of feeding two different mixed rations to grazing dairy cows (PMR1 and PMR2) on milk composition were evaluated and compared to the traditional system for feeding supplements to dairy cows (Control). Control and PMR1 diets contained readily fermentable carbohydrate source and PMR2 contained slowly fermentable carbohydrate source. The three dietary treatments were offered to the cows at different amounts ranging from 6 to 14 kg DM total supplement/cow per d. Milk fatty acids were considered as the main criteria to evaluate the influence of new feeding systems on milk composition, quality and nutritional value. Fatty acids are the most variable compounds of milk and can influence human health and quality of food products. Other compounds containing fatty acids in their structure, such as polar lipids or those might be derived from fatty acids, such as flavour compounds were studied. Yoghurt is a popular fermented dairy product with numerous purported health benefits. To evaluate the quality of dairy products produced from milk obtained from cows consuming new diets, yoghurt was selected for the study. The influence of feeding systems on compounds involved in yoghurt quality (flavour compounds and organic acids) and nutritional value (fatty acids) was also studied. The effects of feeding systems on milk and yoghurt components were speculated to be a result of the differences between carbohydrate sources of the diets. Milk from cows fed Control and PMR1 diets (with similar carbohydrate source) had similar fatty acid, polar lipid and organic acid proportions. Yoghurt produced from milk obtained from cows fed Control and PMR1 diets also contained similar proportions of fatty acids, organic acid and flavour compounds. The proportions of these compounds in PMR2 milk and yoghurt were different from Control and PMR1 milk and yoghurt. From a nutritional standpoint, milk from cows fed PMR2 contained higher proportions of 4:0, while milk from cows fed Control and PMR1 diets contained higher proportions of poly unsaturated fatty acids. These fatty acids are known to have health promoting effects. The proportions of medium chain fatty acids (with adverse health effects) and poly unsaturated fatty acids increased and 4:0 decreased as supplement level increased. From a technological standpoint, yoghurt manufacturing process did not mask the differences between groups in terms of the fatty acids and organic acids in the original milk. Different cow diets did not lead to the formation of different flavour compounds, although the proportions of some flavour compounds were altered. Differences between milk fat content and fatty acid composition affected the flavour of yoghurt. In conclusion, the form in which the supplements were offered to grazing dairy cows did not influence milk fatty acid composition. However, milk fatty acid composition was influenced by the carbohydrate source of the feeding systems (readily fermentable versus slowly fermentable). The same pattern of changes observed in milk fatty acids were also detected in other milk fat derived compounds such as, polar lipids as well as yoghurt fatty acids and flavour compounds with fat origin.