放牧奶牛乳脂肪酸组成及瘤胃脂肪酸代谢规律的研究
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摘要
大量研究表明放牧可以增加奶牛乳中十八碳烯酸-反-11(C18:1t11)、共轭亚油酸顺-9，反-11(CLAc9,t11)和α-亚麻酸(C18:3n-3)等对人体健康有益的脂肪酸含量，但针对完全放牧条件下奶牛乳脂肪酸组成的研究较少，而有关放牧奶牛瘤胃脂肪酸代谢方面的研究更少，这方面的信息资料的缺乏不仅会限制人们对放牧奶牛独特乳脂肪酸形成原因和机制的了解，而且不利于调控放牧奶牛乳成分有效措施的研究。为了进一步了解完全放牧奶牛的乳脂肪酸组成特点，并研究瘤胃脂肪酸代谢规律，阐明放牧奶牛独特乳脂肪酸组成的形成原因和机理，本研究以在新西兰林肯大学奶牛场(LUDF)的优质人工草场上放牧的奶牛为研究对象，通过研究牧草和乳脂肪酸组成的季节性变化特点、补饲大麦和青贮玉米对放牧奶牛乳脂肪酸组成和代谢的影响研究，并结合优质牧草放牧奶牛瘤胃脂肪酸组成的昼夜变化和PUFA代谢特点、瘤胃微生物的脂肪酸组成特点及牧草中富含的水溶性碳水化合物(WSC)和钾元素对体外C18脂肪酸代谢的影响研究，取得以下结果：
1.优质牧草放牧奶牛乳中C18:1t11、CLAc9,t11和C18:3n-3在在一个泌乳期内的含量别为4.0-5.0%、1.5-2.4%和1.2-1.8%，平均含量分别是资料报道TMR饲喂奶牛乳中的1.88、3.03和2.48倍，乳中C18:2n-6: C18:3n-3的平均比值为0.7:1。牧草中的总脂肪酸和C18:3n-3含量在春、秋季较高而夏季较低。乳脂肪酸的变化主要表现为春季泌乳早期(10月)与其他时间的不同：春季泌乳早期放牧奶牛的能量负平衡显著增加了乳中C18:0，C18:1c9和不饱和脂肪酸(UFA)的含量(P<0.05)，不影响C18:1t11和C18:3n-3的含量(P>0.05)，但降低了CLAc9,t11的含量(P<0.05)。回归和相关分析表明乳中CLAc9,t11含量与乳中C18:1t11含量显著线性正相关(P<0.05, R2=0.596)，与Δ9-去饱和酶指数呈弱相关关系(P<0.05, r=0.469)，但乳中C18:1t11、C18:3n-3和CLAc9,t11含量与牧草C18:3n-3含量间无显著相关性(P>0.05)。
2.给放牧奶牛补饲3kg DM/d大麦和4kg DM/d青贮玉米显著改变了瘤胃pH (P<0.05)、瘤胃发酵模式(P<0.05)及瘤胃和乳中的脂肪酸组成(P<0.05)，尤其是增加了乳中MCFA和C18:2n-6的含量(P<0.05)、降低有益脂肪酸C18:1t11和C18:3n-3和CLAc9,t11的含量(P<0.05)。乳腺脂肪酸代谢参数分析表明：上述结果的产生主要与放牧奶牛C18:3n-3摄入量的降低、C18:2n-6摄入量的增加、乳腺C18:1t11总量的降低及CLAc9,t11内源合成比例的下降有关。因此，日粮PUFA摄入量的不同是引起乳中C18:3n-3和C18:2n-6含量不同的主要原因，而乳腺可用于CLAc9,t11内源合成的C18:1t11总量的增加是引起放牧奶牛乳中富含CLAc9,t11的直接原因。
3.瘤胃内容物中的脂肪酸组成在24h的放牧周期内变化显著(P<0.05)，其中，OBCFA的变化模式清晰地反映了瘤胃微生物在新食入牧草上的定植过程，而C18脂肪酸的变化模式清楚的反映了牧草中的PUFA在瘤胃的氢化过程。在24h放牧周期内奶牛的瘤胃pH值低于6.0的总时间达14h，但放牧奶牛瘤胃C18:3n-3和C18:2n-6的氢化率分别达93.0%和89.6%，氢化速率分别为16.5%/h和10.8%/h。脂肪酸氢化反应的最后一步被抑制是放牧奶牛瘤胃PUFA代谢的主要特点，而瘤胃中较高的C18:1t11含量和较低的CLAc9,t11含量进一步证明由氢化抑制所致的瘤胃C18:1t11蓄积是造成放牧奶牛乳中富含CLAc9,t11的主要原因。而在18:00和20:00所观察到的较低的瘤胃pH和微生物数量及较高浓度的PUFA都表明傍晚时分瘤胃的发酵活性较低，这一结果表明，在瘤胃活性较低时补充外源PUFA可能会减少其在瘤胃的氢化量而使更多的PUFA通过瘤胃。
4.放牧奶牛瘤胃原虫、混合细菌、LAB和SAB间的脂肪酸含量差异显著(P<0.05)，同时受牧草种类的显著影响(P<0.05)。放牧奶牛的瘤胃原虫富含UFA，尤其是C18:3n-3、C18:2n-6和C18:1t11，但其CLAc9,t11含量与细菌间差异不显著(P>0.05)，说明瘤胃原虫不能增加放牧奶牛后端肠道CLAc9,t11的供给量。
5.初步研究表明了高WSC和高钾对瘤胃脂肪酸的氢化作用有一定影响，但其影响效果和作用机理有待进一步研究，确定WSC和高钾在放牧奶牛的瘤胃pH范围内的作用效果。
上述结果表明：采食优质牧草的放牧奶牛乳中尤其富含有益脂肪酸，但泌乳早期的能量负平衡不能增加乳中这些脂肪酸的含量，而改变放牧奶牛的日粮组成显著降低了其含量；脂肪酸氢化反应的最后一步被抑制是放牧奶牛瘤胃PUFA代谢的主要特点，而由此所致的C18:1t11蓄积是放牧奶牛乳中富含C18:1t11和CLAc9,t11的主要原因；放牧奶牛乳中较高的C18:3n-3含量主要与摄入量有关，而牧草中富含的WSC和钾可能对放牧奶牛独特乳脂肪酸的形成有一定影响。
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.
1.优质牧草放牧奶牛乳中C18:1t11、CLAc9,t11和C18:3n-3在在一个泌乳期内的含量别为4.0-5.0%、1.5-2.4%和1.2-1.8%，平均含量分别是资料报道TMR饲喂奶牛乳中的1.88、3.03和2.48倍，乳中C18:2n-6: C18:3n-3的平均比值为0.7:1。牧草中的总脂肪酸和C18:3n-3含量在春、秋季较高而夏季较低。乳脂肪酸的变化主要表现为春季泌乳早期(10月)与其他时间的不同：春季泌乳早期放牧奶牛的能量负平衡显著增加了乳中C18:0，C18:1c9和不饱和脂肪酸(UFA)的含量(P<0.05)，不影响C18:1t11和C18:3n-3的含量(P>0.05)，但降低了CLAc9,t11的含量(P<0.05)。回归和相关分析表明乳中CLAc9,t11含量与乳中C18:1t11含量显著线性正相关(P<0.05, R2=0.596)，与Δ9-去饱和酶指数呈弱相关关系(P<0.05, r=0.469)，但乳中C18:1t11、C18:3n-3和CLAc9,t11含量与牧草C18:3n-3含量间无显著相关性(P>0.05)。
2.给放牧奶牛补饲3kg DM/d大麦和4kg DM/d青贮玉米显著改变了瘤胃pH (P<0.05)、瘤胃发酵模式(P<0.05)及瘤胃和乳中的脂肪酸组成(P<0.05)，尤其是增加了乳中MCFA和C18:2n-6的含量(P<0.05)、降低有益脂肪酸C18:1t11和C18:3n-3和CLAc9,t11的含量(P<0.05)。乳腺脂肪酸代谢参数分析表明：上述结果的产生主要与放牧奶牛C18:3n-3摄入量的降低、C18:2n-6摄入量的增加、乳腺C18:1t11总量的降低及CLAc9,t11内源合成比例的下降有关。因此，日粮PUFA摄入量的不同是引起乳中C18:3n-3和C18:2n-6含量不同的主要原因，而乳腺可用于CLAc9,t11内源合成的C18:1t11总量的增加是引起放牧奶牛乳中富含CLAc9,t11的直接原因。
3.瘤胃内容物中的脂肪酸组成在24h的放牧周期内变化显著(P<0.05)，其中，OBCFA的变化模式清晰地反映了瘤胃微生物在新食入牧草上的定植过程，而C18脂肪酸的变化模式清楚的反映了牧草中的PUFA在瘤胃的氢化过程。在24h放牧周期内奶牛的瘤胃pH值低于6.0的总时间达14h，但放牧奶牛瘤胃C18:3n-3和C18:2n-6的氢化率分别达93.0%和89.6%，氢化速率分别为16.5%/h和10.8%/h。脂肪酸氢化反应的最后一步被抑制是放牧奶牛瘤胃PUFA代谢的主要特点，而瘤胃中较高的C18:1t11含量和较低的CLAc9,t11含量进一步证明由氢化抑制所致的瘤胃C18:1t11蓄积是造成放牧奶牛乳中富含CLAc9,t11的主要原因。而在18:00和20:00所观察到的较低的瘤胃pH和微生物数量及较高浓度的PUFA都表明傍晚时分瘤胃的发酵活性较低，这一结果表明，在瘤胃活性较低时补充外源PUFA可能会减少其在瘤胃的氢化量而使更多的PUFA通过瘤胃。
4.放牧奶牛瘤胃原虫、混合细菌、LAB和SAB间的脂肪酸含量差异显著(P<0.05)，同时受牧草种类的显著影响(P<0.05)。放牧奶牛的瘤胃原虫富含UFA，尤其是C18:3n-3、C18:2n-6和C18:1t11，但其CLAc9,t11含量与细菌间差异不显著(P>0.05)，说明瘤胃原虫不能增加放牧奶牛后端肠道CLAc9,t11的供给量。
5.初步研究表明了高WSC和高钾对瘤胃脂肪酸的氢化作用有一定影响，但其影响效果和作用机理有待进一步研究，确定WSC和高钾在放牧奶牛的瘤胃pH范围内的作用效果。
上述结果表明：采食优质牧草的放牧奶牛乳中尤其富含有益脂肪酸，但泌乳早期的能量负平衡不能增加乳中这些脂肪酸的含量，而改变放牧奶牛的日粮组成显著降低了其含量；脂肪酸氢化反应的最后一步被抑制是放牧奶牛瘤胃PUFA代谢的主要特点，而由此所致的C18:1t11蓄积是放牧奶牛乳中富含C18:1t11和CLAc9,t11的主要原因；放牧奶牛乳中较高的C18:3n-3含量主要与摄入量有关，而牧草中富含的WSC和钾可能对放牧奶牛独特乳脂肪酸的形成有一定影响。
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.
引文
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