Milk Fatty Acid Composition And Rumen Fatty Acid Metabolism Of Grazing Dairy Cows

It has been well documented that beneficial fatty acids, such as trans vaccenic acid (C18:1t11),conjugated linoleic acid (CLAc9,t11) and α-linolenic acid (C18:3n-3), in milk fat increased with theincrease of pasture intake. However, there is limited information related to the milk fatty acid compositionof pasture only grazing cows, while information of the rumen fatty acid metabolism of grazing cows iseven less, which both limits the understanding of the formation of this unique milk FA composition andprecludes the development of strategies for further improvement. To further understand the milk fatty acidcomposition of pasture only grazing cows, and to investigate the rumen fatty acid metabolism, as well asthe factors and the possible mechanisms resulting in this unique milk fatty acid composition of grazingcows, five experiments were carried out using the dairy cows grazing high quality pasture on LincolnUniversity Dairy Farm (LUDF), New Zealand, concerning the seasonal variation in milk fatty acidcomposition, effects of barley grain and maize silage supplementation on milk fatty acid composition andmetabolism, diurnal variation in rumen fatty acid composition and poly unsaturated fatty acid (PUFA)metabolism, comparison of rumen microbial fatty acid composition, and effects of high water solublecarbohydrate (WSC) and potassium on rumen C18fatty acid metabolism in vitro, respectively. The resultswere shown as follows:1. Proportions of C18:1t11, CLAc9,t11and C18:3n-3in milk from high quality pasture grazing cowswere4.5%,1.5-2.4%and1.2-1.8%over the lactation season, respectively, and their mean value were1.88,3.03and2.48folders of the proportions reported in milk from TMR fed cows. The ratio ofC18:2n-6:C18:3n-3was0.7:1in the present study. Concentrations of total fatty acid and C18:3n-3inpasture were greater in spring and autumn, however, lower in summer. The main seasonal variation in milkfatty acids happened in spring (October), when the cows were in their earlier lactations. It was showed thatthe negative energy balance happened during the early lactation in spring resulted in the increase of C18:0,C18:1c9and unsaturated fatty acid (UFA) in milk fat (P<0.05), proportions of C18:1t11and C18:3n-3werenot affected (P>0.05) but CLAc9,t11decreased (P<0.05). Regression and correlation analysis showed thatthe proportion of milk CLAc9,t11was correlated linearly with milk C18:1t11(P<0.05, R2=0.596), andcorrelated weakly with theΔ9-desaturase index (P<0.05, r=0.469). However, the proportions of milkC18:1t11, C18:3n-3and CLAc9,t11were not correlated with the proportion of pasture C18:3n-3(P>0.05).2. Supplementing3kg DM/d barley grain and4kg DM/d maize silage, respectively, to the grazing cows significantly decreased the pasture dry matter intake (DMI)(P<0.05), influenced the rumen pH,rumen fermentation pattern (P<0.05), and the fatty acid composition in rumen digesta and milk (P<0.05).Effects on milk fatty acid composition are characterized as decrease in the beneficial fatty acids (C18:1t11,C18:3n-3and CLAc9,t11), however, increase in middle chain saturated fatty acids (MCFA) and C18:2n-6.Results of fatty acid metabolism in the mammary gland indicated that the above observations were resultedfrom the reduced intake of dietary C18:3n-3while increased intake of C18:2n-6, and decrease in the totalC18:1t11available for CLAc9,t11endogenous synthesis and the proportion of CLAc9,t11synthesized inthe mammary gland. Therefore, differences in dietary PUFA intake is the major reason causing thedifferences in milk C18:3n-3and C18:2n-6, while the increased C18:1t11for CLAc9,t11synthesis bygrazing is the main reason resulting in the higher level of CLAc9,t11in the milk of pasture grazing cows.3. Fatty acid profiles in rumen digeasta varied considerably over the24h grazing cycle (P<0.05), withthe most dramatic changes occurred in the evening and overnight. Diurnal variation in odd-and branched-chain fatty acids (OBCFA) clearly reflected the microbial colonization of the newly ingested pasture whilethe variation in C18fatty acids reflected the rumen biohydrogenation of dietary poly unsaturated fatty acids(PUFA). Rumen pH of cows grazing high quality pasture is lower than6.0for approximately14h over agrazing cycle, however, the biohydrogenation of C18:3n-3and C18:2n-6were as high as93.0%and89.6%，respectively, and their biohydrogenation rate were16.5%/h and10.8%/h, respectively. Inhibition ofthe final step of biohydrogenation is the major characteristic of the rumen fatty acid metabolism of pasturegrazing cows. The greater proportion of C18:1t11and lower proportion of CLAc9, t11in rumen digestaconfirmed that the accumulation of C18:1t11by the incomplete biohydrogenation is the main reasonresulting in the greater CLAc9,t11in the milk of grazing cows. The lower rumen buffering capacity(decreased rumen pH), dramatic decline in rumen microbial population (assessed by total OBCFA)observed between1800h and2000h, as well as the large increase in plant-derived PUFA implied areduced rumen biohydrogenation activity during the evening, which might provide an approach to reducethe biohydrogenation of protected PUFA supplements by strategic feeding to coincide with reduced rumenactivity.4. Fatty acid composition differed significantly among rumen protozoa, mixed bacteria, liquid associatedbacteria (LAB) and solid associated bacteria (SAB)(P<0.05), and also affected significantly by the type ofpastures (P<0.05). Rumen protozoa were enriched in UFA, especially C18:3n-3, C18:2n-6and C18:1t11compared with mixed bacteria (P<0.05). However, proportion of CLAc9,t11did not differ betweenprotozoa and mixed bacteria (P>0.05), indicating that the presence of rumen protozoa in grazing cowscould not increase the supply of CLAc9,t11to the lower intestinal tract.5. The preliminary studies indicated that the high concentrations of WSC and potassium in pasturehad some effects on rumen fatty acid metabolism, however, their effects and the possible mechanism needs further investigation, especially under the rumen pH condition of pasture grazing dairy cows.The above results indicated that milk from high quality pasture grazing dairy cows are particularlyrich in beneficial fatty acids, which could not be increased by the negative energy balance in early lactation,however, decreased significantly by dietary variation. Inhibition of the last step of biohydrogenation is themajor characteristic of rumen fatty acid metabolism of pasture grazing cows, and the accumulation ofC18:1t11resulted from which causing the greater C18:1t11and CLAc9,t11in the milk of grazing cows,while the greater C18:3n-3in the milk of grazing cows is related to its higher intake. The higherconcentration of WSC and potassium in pasture could also have some effects on the formation of thisunique milk fatty acid composition of grazing dairy cows.