Rumen biohydrogenation of polyunsaturated fatty acids in high quality pasture
The intake of polyunsaturated fatty acids, such as omega-3 fatty acids and conjugated linoleic acids, can prevent many diseases such as coronary heart disease, cancer, diabetes and hypertension. This work looked at aspects of the control of polyunsaturated fatty acid levels in milk. Pasture contains a high proportion of its fat as polyunsaturated fatty acids and, as a result of the desire to reduce the saturated fatty acid content of milk, feeding high-quality pasture in diets to increase the unsaturation of milk fat was studied. This thesis investigated the reasons for the higher levels of C18:3 and the lower levels C18:0 and C18:2 fatty acids observed in milk from cows fed high-quality pasture. In a survey of milk and forage samples collected from commercial dairy farms in Canterbury, the results of pasture analysis indicated that levels of water-soluble carbohydrates and crude protein were highest in spring and autumn, while the content of NDF was lowest during the same periods. The most notable feature of milk fatty acid composition was the higher level of a-linolenic acid (C18:3) and lower percentage of C18:2 in milk produced from cows consuming high-quality Canterbury pasture. The highest monthly mean value for milk C18:3 was 1.82% of total milk fatty acids for herds fed low levels of supplement and in October (spring). High-quality pasture was used in a second experiment to determine the effect of low rumen pH on biohydrogenation of C18 fatty acids. A standard ryegrass sample was incubated in nylon bags in the rumen of a dry cow fed low-quality lucerne silage, or in two milking cows fed with high-quality LUDF pasture, during four experimental periods. The ruminal pH of dairy cows fed high quality pasture was below values recommended to optimise digestion (5.8) in comparison with the much higher average value in the dry cow (6.6). There was no effect of treatment or sampling time on the proportions of different volatile fatty acids. Dry matter (DM) degradability of the high-quality ryegrass pasture, estimated after eight hours incubation in the rumen was high for both treatments (77.5 and 74.3%, respectively). The levels of C18:3 and trans-11 C18:1 were very high in the residue in the experiment with nylon bags, moreover, low C18:0 and C18:2 contents were also observed. This provides strong evidence for the viewpoint that reduction of the yield and activity of rumen bacteria (Group B) is related to the effect of the high quality of pasture on rumen pH. Meanwhile, the other interesting finding was a low concentration of C18:2 in residues in nylon bags from cows with low rumen pH. This suggests that the population or activity of some specific Group A bacteria increased when the low rumen pH occurred, resulting in a change in the biochemical pathway of hydrogenation of C18:2.
polyunsaturated fatty acids, milk, pasture, biohydrogenation, rumen
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