粗料来源对奶牛乳蛋白前体物生成与生产性能的影响与机制研究
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摘要
近年来我国奶业得到了快速的发展,奶牛生产水平有了较大的提高,但是乳品质量仍有待改善。优质饲草的缺乏以及对现有资源的利用不合理是我国奶业可持续发展的瓶颈问题。我国每年大量进口优质牧草-苜蓿草,但同时有大量的秸秆资源并没有得到合理的利用。因此,本论文从我国现有的饲料资源着手,研究不同粗料来源对奶牛乳蛋白前体物生成和泌乳性能的影响与机理,旨在为我国现有资源的合理利用提供理论与实践指导依据。研究分别以苜蓿、羊草和玉米秸作为粗料来源对奶牛乳蛋白前体物的生成和泌乳性能的影响,揭示其影响机理(第一部分)；研究玉米秸日粮中补充淀粉和氨基酸对奶牛乳蛋白前体物生成量和泌乳性能的影响(第二部分)
1、不同粗料来源对奶牛乳蛋白前体物的生成和泌乳性能的影响。
1.1不同粗料来源对奶牛泌乳性能与氮利用效率的影响(试验一)。为比较研究苜蓿草,羊草和玉米秸为粗料来源对奶牛泌乳性能、氮利用效率和血液生化指标的影响,选择12头处于泌乳早期的健康中国荷斯坦奶牛,根据泌乳量和泌乳天数随机分成3组,每组4头,进行。试验采用3x3复拉丁方设计。三组试验日粮试验日粮等能等氮且精粗比为55：45(DM),主要区别为粗料来源不同(%干物质,DM)：(1)玉米秸19(CS),(2)羊草21(CWR),(3)苜蓿干草17(AH)。结果发现,粗料来源对奶牛干物质采食量无显著影响(P>0.05)。奶牛产奶量饲喂AH日粮有高于CS日粮奶牛(P<0.10)的趋势, CWR居中。AH日粮奶牛乳蛋白含量高于饲喂CWR日粮(P<0.05),而CS日粮与另外两组差异均不显著。与CS和CWR日粮相比,AH的血浆、尿和乳中尿素氮含量较低,氮利用效率最高(P<0.01)。各试验组之间血浆白蛋白、葡萄糖和游离脂肪酸(NEFA)含量差异不显著。上述结果表明,苜蓿为粗料来源时其奶产量、乳蛋白产量和氮利用效率高于玉米秸,而玉米秸日粮与羊草日粮差异不显著。
1.2不同粗料来源影响奶牛泌乳性能及乳蛋白前体物生成量的机理(试验二)。为了揭示不同粗料来源影响奶牛乳蛋白前体物生成的机理,分别从瘤胃发酵特性、代谢蛋白质生成量、动静脉血液游离氨基酸之差等方面进行了研究。从试验一的奶牛口腔取瘤胃液,检测瘤胃发酵参数；尼龙袋法检测日粮有机物(OM)和蛋白质(CP)的降解率,用尿中嘌呤衍生物法估测微生物蛋白质生成量；尾动静脉采血检测血浆游离氨基酸。结果发现,各试验组之间瘤胃pH差异不显著(P>0.05),氨态氮浓度CS日粮较高(P<0.01)。总挥发性脂肪酸(VFA)浓度,AH日粮高于CS日粮(P<0.05),CWR居中,且与AH和CS日粮之间差异均不显著；可见AH日粮可为奶牛机体提供较多的能量,导致其奶产量较高。日粮AH的OM瘤胃降解率高于CS,主要原因在于其含有较高的快速降解部分；日粮AH的微生物蛋白产量高于(P<0.05)CS和CWR,与其较高的VFA浓度和较低的氨态氮浓度结果一致。各试验组间来源于非降解饲料蛋白的小肠可消化部分(1ADP)(P>0.05)差异不显著,表明AH日粮较高的代谢蛋白质(MP)供给量主要是由于较高的小肠可消化微生物蛋白质(IMCP)；微生物蛋白产量与乳蛋白产量之间存在高度相关性(P<0.01)。饲喂AH日粮奶牛乳腺对必需氨基酸和总氨基酸的摄取均高于CS日粮,提示AH日粮的MP中氨基酸平衡性较好,利于吸收利用。以上结果表明,相比玉米秸日粮,苜蓿日粮含有较高的易发酵碳水化合物,合成较多的瘤胃微生物蛋白质,以致贡献较多的乳蛋白前体物—小肠代谢蛋白质,并且氨基酸利用效率较高,最终奶产量和乳蛋白产量高于其他两组。因此,有必要研究在玉米秸日粮补充可发酵碳水化合物和必需氨基酸对奶牛乳蛋白前体物和泌乳性能的影响。
2、玉米秸日粮补充淀粉和氨基酸对奶牛乳蛋白前体物生成量和泌乳性能的影响。
2.1玉米秸日粮补充淀粉和氨基酸对奶牛泌乳性能和氮利用效率的影响(试验三)。为研究玉米秸日粮补充淀粉和氨基酸对奶牛泌乳性能、氮利用效率和血液生化指标的影响,选择12头健康的处于泌乳中期的健康中国荷斯坦奶牛,根据泌乳量、泌乳天数和胎次随机分成3组,每组4头,进行。试验分3期,采用3x3复拉丁方设计试验。试验日粮等能等氮且精粗比为55：45(DM)的,主要区别为(%DM)：(1)苜蓿干草19(AH)；(2)淀粉8,玉米秸22(CSS)；(3)在日粮CSS的基础上每天添加19.7g的过瘤胃保护Met和40.1g的过瘤胃保护Lys(CSSAA),用CPM软件估测日粮CSSAA的MP中Met和Lys的含量分别为2.4和7.2%。试验日粮等能等氮且精粗比为55：45(DM)。结果表明发现,各处理之组间干物质采食量和产奶量差异均不显著。与相比CSS日粮相比,添加氨基酸后增加1.4kg/d的产奶量,有提高乳转化率的趋势(P<0.10)。各试验组之间乳蛋白、乳脂和乳糖含量差异不显著(P>0.05)。而日粮CSS的乳蛋白产量有低于AH和CSSAA日粮的趋势(P<0.10)。尿中尿素氮的含量CSS日粮显著高于(P<0.05)AH,CSSAA居中,与其它两组之间差异不显著。乳尿氮和血浆尿素氮CSSAA日粮显著低于CSS(P<0.05),AH组居中。日粮CSS氮利用率低于AH和CSSAA日粮(P<0.01),AH和CSSAA日粮之间差异不显著。各试验组之间血浆总蛋白、白蛋白、葡萄糖、NEFA和β-羟丁酸之间含量差异不显著。上述结果表明,玉米秸日粮中适量补充可溶性糖分淀粉后其泌乳性能与达到苜蓿日粮相近似水平,而进一步补充氨基酸可提高奶牛饲料转化率和氮利用率。
2.2玉米秸日粮补充淀粉和氨基酸影响乳蛋白前体物生成量的机制(试验四)。为揭示玉米秸日粮补充淀粉和氨基酸影响奶牛乳蛋白产量的机理,从瘤胃发酵特性、代谢蛋白质生成量、血浆游离氨基酸等方面进行研究。瘤胃发酵参数的检测、日粮OM和CP的降解率测定、微生物蛋白质产量的估测、血液的采集和血浆游离氨基酸的测定同试验二。结果发现,各试验组之间pH值和总VFA浓度差异均不显著,表明补充淀粉后CSS日粮提供能量增加,与上述两组间奶产量相近的结果一致。氨态氮浓度日粮CSS日粮显著高于AH(P<0.01)。日粮CSS的OM瘤胃有效降解率和微生物蛋白产量与AH日粮均没有显著差异(P>0.05),说明淀粉的补充使CSS日粮提供的瘤胃可利用能达到了AH日粮的水平,导致两个日粮间瘤胃微生物蛋白合成量相近；各试验组之间IADP差异也不显著(P>0.05)。添加氨基酸对奶牛瘤胃发酵参数、微生物蛋白合成量和MP产量均无显著影响,但增加了血浆必需氨基酸和总氨基酸的浓度(P<0.05)。上述结果表明,玉米秸日粮补充淀粉提高日粮乳蛋白产量的主要原因在于,补充淀粉可为瘤胃微生物提供更多可利用能,增加微生物蛋白产量,为乳蛋白合成提供更多的前体物；补充氨基酸则不仅可改善乳蛋白前体物的供应,而且可优化氨基酸的平衡性。
综上所述,苜蓿日粮比玉米秸日粮可合成较多的瘤胃微生物蛋白质,为乳腺供应较多的乳蛋白前体物,导致奶产量和乳蛋白产量高于玉米秸日粮。玉米秸日粮乳蛋白含量较低的原因之一是日粮可发酵碳水化合物含量不足,影响了瘤胃微生物蛋白的合成量；玉米秸日粮同时补充易发酵碳水化合物和必需氨基酸时,可促进了瘤胃微生物蛋白质合成,改善机体氨基酸平衡和代谢蛋白质利用效率,导致乳蛋白产量得到显著提高。
With the development of dairy industry, the production level of dairy cows has been rapidly improved in China during recent years. However, the milk quality still needs to be improved. Deficiency of high quality forage and the unreasonable utilization of existing resources are the two important problems. A large amount of alfalfa has been imported annually, while huge amount of cereal straws and stover has not been fully utilized in animal feeds. It is necessary to establish the utilization system of feed resources that is suitable for China's national conditions. Therefore, the objective of this study was to evaluate the effects of dietary forage sources on precursor of milk protein and lactation performance in lactation dairy cows and approach to the mechanism. This study contained two parts including four trials. In the first part, the effects of dietary forage sources (alfalfa hay, Chinese wild rye grass, and corn stover) were evaluated on milk performance and precursor of milk protein, and the possible mechanism were approached. In the second part, the effects were studied of supplementing starch and rumen-protected (RP) AA on lactation performance and prodcurtion of milk protein precursor in lactating dairy cows fed corn stover.
Part one:
Evaluation of the effects of dietary forage sources on production of milk protein precursor and lactation performance in early-lactation dairy cows and approach to the mechanism (Trial1and2)
1. Effects of dietary forage sources on milk performance and N utilization in lactation dairy cows (Trial.1). In order to evaluate the effects of forage sources on milk performance, N utilization efficiency and blood parameters of lactation dairy cows, twelve primiparous Chinese Holstein dairy cows were used in a multiple3×3Latin square design. Cows were divided into3groups according to similarity in milk production and day in milk. Diets were isonitrogenous and isocaloric, with a forage to concentrate ratio of45:55(dry matter (DM) basis) and contained similar concentrate mixtures. Different forage sources were:(1) corn stover19(CS),(2) Chinese wild rye hay21(CWR), and (3) alfalfa hay17(AH). Dry matter intake (DMI) was not affected by the sources of dietary forage (P>0.05). Milk yield was higher for cows fed AH than those on CS (P＜0.1), with an intermediate value for CWR. Milk protein content was higher in the cows fed AH than CWR, with CS at an intermediate position. The contents of milk fat and lactose were not different among the treatments. The concentrations of urea N in the urine, blood and milk were decreased for cows fed AH, indicating an increased N conversion (P＜0.01). Plasma albumin, glucose, and nonesterified fatty acid (NEFA) was not affected by treatments (P＞0.05). It is indicated that feeding alfalfa could improve milk production, milk protein yield, and N utilization efficiency. No significant differences were detected on milk performance and N utilization efficiency between Chinese wild rye grass and corn stover as the main forage sources.
2. Approach to the mechanism with which the dietary forage sources affect peoduction of milk protein precursor in dairy cows (Trial.2). In order to reveal the mechanisms with which the forage sources affect production of milk protein precursor in dairy cows, rumen fermentation characters, production of metabolizable protein (MP), and arteriovenous difference in free AA were investigated from the samples taken in Trial1. Rumen fluid was collected from oral cavity, and rumen fermentation characters were detected; rumen OM and CP degradation of experiment diets was determined by nylon bag method; Urinary purine derivatives were used to estimate the microbial protein yield; blood samples were collected from coccygeal. The rumen pH was not affected by treatments (P>0.05). Ammonia N concentration was higher for cows fed diet CS than those fed CWR and AH (P＜0.05), with no difference between CWR and AH (P＞0.05). Concentrations of total VFA in cows consumed diet AH were higher than those fed CS (P＜0.05), with CS being intermediate.it is indicateding that AH could improve the rumen fermentation, supply more energy to the cows, and benefit to the milk production. The MCP yield for AH was higher than that for CS and CWR (P＜0.05). This was consistent with the higher VFA concentration and lower ammonia concentration in AH. And there existed a significant regression of the milk protein yield to the microbial protein yield in lactation cows. Effective degradability value of OM and CP for AH was higher (P＜0.05) compared with the CS and CWR. The higher rumen OM degradation was largely attributed to higher part proportion of fast degradable fraction. It is indicated that high proportion of alfalfa hay in the diet could improve MCP synthesis in the rumen due to its high sufficient supply of rumen available energy to the rumen microbes. No effects of the treatments were observed on the intestinally absorbable dietary protein (IADP)(P＞0.05). Thus, compared with diet CS and CWR, the higher MP value was attributed to its higher MCP yield. There existed a significant regression of the milk protein yield to the microbial protein yield in lactation cows. Compared with diet CS, AH improved the absorption of essential and total AA. indicated suggesting the better AA balance in AH was better than in CS. Overall, a high proportion of alfalfa hay in the diet is beneficial for milk protein yield and milk protein production by increasing microbial protein yield. This can be attributed to the improving supply of rumen available energy and the utilization efficiency of AA. It is necessary to evaluate the effect of supplementing starch and AA on production of milk protein precursor and lactation performance in dairy cows fed corn stover.
Part two:
Effects of supplementing starch and AA on precursor of milk protein and lactation performance of lactating dairy cows fed corn stover and approach to the mechanism (Trial3and4)
1. Effects of supplementing starch and AA on milk performance and N utilization in lactation dairy cows fed corn stover (Trial.3). In order to evaluate the effects of supplementing starch and AA on milk performance, N utilization efficiency and blood parameters of lactation cows fed corn stover, twelve multiparous (3.5±1.0parity) Chinese Holstein dairy cows were used in a3x3Latin square design. Cows were divided into3groups according to similarity in milk production, day in milk and parity. The basal diets were isonitrogenous and isocaloric, with a ratio of forage to concentrate of45:55(DM basis). Three diets contained the following forage ingredients (%on a DM basis):(1) alfalfa19, and Chinese wild rye grass6(AH);(2) corn stover22, alfalfa6, supplemented with starch8(CSS); and (3) CSS supplemented with RP Met and RP Lys (CSSAA). In diet CSSAA the proportion of Lys and Met in MP estimated by CPM dairy soft was7.2and2.4%, respectively. Dry matter intake (P>0.05) and milk yield (P>0.05) did not differ among the treatments. Supplementation of RPAA increased the1.4kg milk yield thus increased the milk efficiency (P＜0.10). The contents of milk protein, fat, and lactose were not different among the treatments. Milk protein yield was higher (P＜0.10) for cows fed AH than those fed CSS, with CSSAA at an intermediate position. The urinary concentration of urea N for CSS diet was higher (P<0.05) than for AH, whereas the CSSAA diet had a lower urea N concentration in the milk (P＜0.05) and blood (P＜0.05) than CSS. Lower N conversion (P＜0.01) was detected for CSS than AH and CSSAA, with no difference between CSSAA and AH. Plasma total protein, albumin, glucose, NEFA, and β-hydroxybutyrate was not affected by treatments (P＞0.05). The results indicated that the supplementation of starch to a corn stover diet could achieve a similar lactation performance as cows fed alfalfa, while RPAA supplementation further improved the efficiency of milk and N utilization.
2. Approach to the mechanism with which supplementing starch and AA affects peoduction of milk protein precursor in dairy cows fed corn stover (Trial.4). The effects of supplementing of starch and AA on rumen fermentation characters, MP, plasma AA concentration were studied. Rumen fluid was collected from oral cavity of cows fed in trail3. Detection of rumen fermentation characters, rumen OM and CP degradation, estimation of the MCP yield, blood collection and plasma free AA concentration detection were similar to trail2. Ruminal pH and total VFA concentration did not differ among the treatments (P＞0.05). Ammonia N concentration was higher for cows fed diet CSS than those fed AH (P＜0.05), indicating that starch supplementation in corn stover diet improved energy supply to the cows, resulting in similar milk yield with those cows fed AH. The OM degradation and MCP yield were not different between CSS and AH (P＞0.05), suggesting that with starch supplementation the available energy supplied from CSS approached to diet AH, resulting in similar MCP yield. No difference was detected in IADP between diet CSS and AH (P＞0.05). Thus, the MP supply was not different between diets CSS and AH (P＞0.05). No significant difference was detected with AA supplementation on rumen fermentation, MCP yield, and MP supply. However, AA supplementation increased the plasma concentration of essential AA and total AA. From Tthe above results, it is indicated that the enhanced production of milk protein precusors in corn stover diet supplemented with starch is attributed to the increased supply of energy available to the rumen microbes and subsequently increased microbial protein synthesis. Supplementations of AA further increased the supply of milk protein precusors to the cows, but also improve the balance of amino acids within the cows'body.
In summary, high proportion of alfalfa hay in the diet is beneficial for milk protein yield and milk protein production by increasing microbial protein yield. This can be attributed to the improving supply of rumen available energy and the utilization efficiency of AA. Insufficient of rumen fermentation carbohydrate in corn stover induced low milk protein yield. Corn stover with both rumen fermentable carbohydrate and AA supplementation could increase the MCP synthesis and improve MP utilization, resulting in higher milk protein yield.
1、不同粗料来源对奶牛乳蛋白前体物的生成和泌乳性能的影响。
1.1不同粗料来源对奶牛泌乳性能与氮利用效率的影响(试验一)。为比较研究苜蓿草,羊草和玉米秸为粗料来源对奶牛泌乳性能、氮利用效率和血液生化指标的影响,选择12头处于泌乳早期的健康中国荷斯坦奶牛,根据泌乳量和泌乳天数随机分成3组,每组4头,进行。试验采用3x3复拉丁方设计。三组试验日粮试验日粮等能等氮且精粗比为55：45(DM),主要区别为粗料来源不同(%干物质,DM)：(1)玉米秸19(CS),(2)羊草21(CWR),(3)苜蓿干草17(AH)。结果发现,粗料来源对奶牛干物质采食量无显著影响(P>0.05)。奶牛产奶量饲喂AH日粮有高于CS日粮奶牛(P<0.10)的趋势, CWR居中。AH日粮奶牛乳蛋白含量高于饲喂CWR日粮(P<0.05),而CS日粮与另外两组差异均不显著。与CS和CWR日粮相比,AH的血浆、尿和乳中尿素氮含量较低,氮利用效率最高(P<0.01)。各试验组之间血浆白蛋白、葡萄糖和游离脂肪酸(NEFA)含量差异不显著。上述结果表明,苜蓿为粗料来源时其奶产量、乳蛋白产量和氮利用效率高于玉米秸,而玉米秸日粮与羊草日粮差异不显著。
1.2不同粗料来源影响奶牛泌乳性能及乳蛋白前体物生成量的机理(试验二)。为了揭示不同粗料来源影响奶牛乳蛋白前体物生成的机理,分别从瘤胃发酵特性、代谢蛋白质生成量、动静脉血液游离氨基酸之差等方面进行了研究。从试验一的奶牛口腔取瘤胃液,检测瘤胃发酵参数；尼龙袋法检测日粮有机物(OM)和蛋白质(CP)的降解率,用尿中嘌呤衍生物法估测微生物蛋白质生成量；尾动静脉采血检测血浆游离氨基酸。结果发现,各试验组之间瘤胃pH差异不显著(P>0.05),氨态氮浓度CS日粮较高(P<0.01)。总挥发性脂肪酸(VFA)浓度,AH日粮高于CS日粮(P<0.05),CWR居中,且与AH和CS日粮之间差异均不显著；可见AH日粮可为奶牛机体提供较多的能量,导致其奶产量较高。日粮AH的OM瘤胃降解率高于CS,主要原因在于其含有较高的快速降解部分；日粮AH的微生物蛋白产量高于(P<0.05)CS和CWR,与其较高的VFA浓度和较低的氨态氮浓度结果一致。各试验组间来源于非降解饲料蛋白的小肠可消化部分(1ADP)(P>0.05)差异不显著,表明AH日粮较高的代谢蛋白质(MP)供给量主要是由于较高的小肠可消化微生物蛋白质(IMCP)；微生物蛋白产量与乳蛋白产量之间存在高度相关性(P<0.01)。饲喂AH日粮奶牛乳腺对必需氨基酸和总氨基酸的摄取均高于CS日粮,提示AH日粮的MP中氨基酸平衡性较好,利于吸收利用。以上结果表明,相比玉米秸日粮,苜蓿日粮含有较高的易发酵碳水化合物,合成较多的瘤胃微生物蛋白质,以致贡献较多的乳蛋白前体物—小肠代谢蛋白质,并且氨基酸利用效率较高,最终奶产量和乳蛋白产量高于其他两组。因此,有必要研究在玉米秸日粮补充可发酵碳水化合物和必需氨基酸对奶牛乳蛋白前体物和泌乳性能的影响。
2、玉米秸日粮补充淀粉和氨基酸对奶牛乳蛋白前体物生成量和泌乳性能的影响。
2.1玉米秸日粮补充淀粉和氨基酸对奶牛泌乳性能和氮利用效率的影响(试验三)。为研究玉米秸日粮补充淀粉和氨基酸对奶牛泌乳性能、氮利用效率和血液生化指标的影响,选择12头健康的处于泌乳中期的健康中国荷斯坦奶牛,根据泌乳量、泌乳天数和胎次随机分成3组,每组4头,进行。试验分3期,采用3x3复拉丁方设计试验。试验日粮等能等氮且精粗比为55：45(DM)的,主要区别为(%DM)：(1)苜蓿干草19(AH)；(2)淀粉8,玉米秸22(CSS)；(3)在日粮CSS的基础上每天添加19.7g的过瘤胃保护Met和40.1g的过瘤胃保护Lys(CSSAA),用CPM软件估测日粮CSSAA的MP中Met和Lys的含量分别为2.4和7.2%。试验日粮等能等氮且精粗比为55：45(DM)。结果表明发现,各处理之组间干物质采食量和产奶量差异均不显著。与相比CSS日粮相比,添加氨基酸后增加1.4kg/d的产奶量,有提高乳转化率的趋势(P<0.10)。各试验组之间乳蛋白、乳脂和乳糖含量差异不显著(P>0.05)。而日粮CSS的乳蛋白产量有低于AH和CSSAA日粮的趋势(P<0.10)。尿中尿素氮的含量CSS日粮显著高于(P<0.05)AH,CSSAA居中,与其它两组之间差异不显著。乳尿氮和血浆尿素氮CSSAA日粮显著低于CSS(P<0.05),AH组居中。日粮CSS氮利用率低于AH和CSSAA日粮(P<0.01),AH和CSSAA日粮之间差异不显著。各试验组之间血浆总蛋白、白蛋白、葡萄糖、NEFA和β-羟丁酸之间含量差异不显著。上述结果表明,玉米秸日粮中适量补充可溶性糖分淀粉后其泌乳性能与达到苜蓿日粮相近似水平,而进一步补充氨基酸可提高奶牛饲料转化率和氮利用率。
2.2玉米秸日粮补充淀粉和氨基酸影响乳蛋白前体物生成量的机制(试验四)。为揭示玉米秸日粮补充淀粉和氨基酸影响奶牛乳蛋白产量的机理,从瘤胃发酵特性、代谢蛋白质生成量、血浆游离氨基酸等方面进行研究。瘤胃发酵参数的检测、日粮OM和CP的降解率测定、微生物蛋白质产量的估测、血液的采集和血浆游离氨基酸的测定同试验二。结果发现,各试验组之间pH值和总VFA浓度差异均不显著,表明补充淀粉后CSS日粮提供能量增加,与上述两组间奶产量相近的结果一致。氨态氮浓度日粮CSS日粮显著高于AH(P<0.01)。日粮CSS的OM瘤胃有效降解率和微生物蛋白产量与AH日粮均没有显著差异(P>0.05),说明淀粉的补充使CSS日粮提供的瘤胃可利用能达到了AH日粮的水平,导致两个日粮间瘤胃微生物蛋白合成量相近；各试验组之间IADP差异也不显著(P>0.05)。添加氨基酸对奶牛瘤胃发酵参数、微生物蛋白合成量和MP产量均无显著影响,但增加了血浆必需氨基酸和总氨基酸的浓度(P<0.05)。上述结果表明,玉米秸日粮补充淀粉提高日粮乳蛋白产量的主要原因在于,补充淀粉可为瘤胃微生物提供更多可利用能,增加微生物蛋白产量,为乳蛋白合成提供更多的前体物；补充氨基酸则不仅可改善乳蛋白前体物的供应,而且可优化氨基酸的平衡性。
综上所述,苜蓿日粮比玉米秸日粮可合成较多的瘤胃微生物蛋白质,为乳腺供应较多的乳蛋白前体物,导致奶产量和乳蛋白产量高于玉米秸日粮。玉米秸日粮乳蛋白含量较低的原因之一是日粮可发酵碳水化合物含量不足,影响了瘤胃微生物蛋白的合成量；玉米秸日粮同时补充易发酵碳水化合物和必需氨基酸时,可促进了瘤胃微生物蛋白质合成,改善机体氨基酸平衡和代谢蛋白质利用效率,导致乳蛋白产量得到显著提高。
With the development of dairy industry, the production level of dairy cows has been rapidly improved in China during recent years. However, the milk quality still needs to be improved. Deficiency of high quality forage and the unreasonable utilization of existing resources are the two important problems. A large amount of alfalfa has been imported annually, while huge amount of cereal straws and stover has not been fully utilized in animal feeds. It is necessary to establish the utilization system of feed resources that is suitable for China's national conditions. Therefore, the objective of this study was to evaluate the effects of dietary forage sources on precursor of milk protein and lactation performance in lactation dairy cows and approach to the mechanism. This study contained two parts including four trials. In the first part, the effects of dietary forage sources (alfalfa hay, Chinese wild rye grass, and corn stover) were evaluated on milk performance and precursor of milk protein, and the possible mechanism were approached. In the second part, the effects were studied of supplementing starch and rumen-protected (RP) AA on lactation performance and prodcurtion of milk protein precursor in lactating dairy cows fed corn stover.
Part one:
Evaluation of the effects of dietary forage sources on production of milk protein precursor and lactation performance in early-lactation dairy cows and approach to the mechanism (Trial1and2)
1. Effects of dietary forage sources on milk performance and N utilization in lactation dairy cows (Trial.1). In order to evaluate the effects of forage sources on milk performance, N utilization efficiency and blood parameters of lactation dairy cows, twelve primiparous Chinese Holstein dairy cows were used in a multiple3×3Latin square design. Cows were divided into3groups according to similarity in milk production and day in milk. Diets were isonitrogenous and isocaloric, with a forage to concentrate ratio of45:55(dry matter (DM) basis) and contained similar concentrate mixtures. Different forage sources were:(1) corn stover19(CS),(2) Chinese wild rye hay21(CWR), and (3) alfalfa hay17(AH). Dry matter intake (DMI) was not affected by the sources of dietary forage (P>0.05). Milk yield was higher for cows fed AH than those on CS (P＜0.1), with an intermediate value for CWR. Milk protein content was higher in the cows fed AH than CWR, with CS at an intermediate position. The contents of milk fat and lactose were not different among the treatments. The concentrations of urea N in the urine, blood and milk were decreased for cows fed AH, indicating an increased N conversion (P＜0.01). Plasma albumin, glucose, and nonesterified fatty acid (NEFA) was not affected by treatments (P＞0.05). It is indicated that feeding alfalfa could improve milk production, milk protein yield, and N utilization efficiency. No significant differences were detected on milk performance and N utilization efficiency between Chinese wild rye grass and corn stover as the main forage sources.
2. Approach to the mechanism with which the dietary forage sources affect peoduction of milk protein precursor in dairy cows (Trial.2). In order to reveal the mechanisms with which the forage sources affect production of milk protein precursor in dairy cows, rumen fermentation characters, production of metabolizable protein (MP), and arteriovenous difference in free AA were investigated from the samples taken in Trial1. Rumen fluid was collected from oral cavity, and rumen fermentation characters were detected; rumen OM and CP degradation of experiment diets was determined by nylon bag method; Urinary purine derivatives were used to estimate the microbial protein yield; blood samples were collected from coccygeal. The rumen pH was not affected by treatments (P>0.05). Ammonia N concentration was higher for cows fed diet CS than those fed CWR and AH (P＜0.05), with no difference between CWR and AH (P＞0.05). Concentrations of total VFA in cows consumed diet AH were higher than those fed CS (P＜0.05), with CS being intermediate.it is indicateding that AH could improve the rumen fermentation, supply more energy to the cows, and benefit to the milk production. The MCP yield for AH was higher than that for CS and CWR (P＜0.05). This was consistent with the higher VFA concentration and lower ammonia concentration in AH. And there existed a significant regression of the milk protein yield to the microbial protein yield in lactation cows. Effective degradability value of OM and CP for AH was higher (P＜0.05) compared with the CS and CWR. The higher rumen OM degradation was largely attributed to higher part proportion of fast degradable fraction. It is indicated that high proportion of alfalfa hay in the diet could improve MCP synthesis in the rumen due to its high sufficient supply of rumen available energy to the rumen microbes. No effects of the treatments were observed on the intestinally absorbable dietary protein (IADP)(P＞0.05). Thus, compared with diet CS and CWR, the higher MP value was attributed to its higher MCP yield. There existed a significant regression of the milk protein yield to the microbial protein yield in lactation cows. Compared with diet CS, AH improved the absorption of essential and total AA. indicated suggesting the better AA balance in AH was better than in CS. Overall, a high proportion of alfalfa hay in the diet is beneficial for milk protein yield and milk protein production by increasing microbial protein yield. This can be attributed to the improving supply of rumen available energy and the utilization efficiency of AA. It is necessary to evaluate the effect of supplementing starch and AA on production of milk protein precursor and lactation performance in dairy cows fed corn stover.
Part two:
Effects of supplementing starch and AA on precursor of milk protein and lactation performance of lactating dairy cows fed corn stover and approach to the mechanism (Trial3and4)
1. Effects of supplementing starch and AA on milk performance and N utilization in lactation dairy cows fed corn stover (Trial.3). In order to evaluate the effects of supplementing starch and AA on milk performance, N utilization efficiency and blood parameters of lactation cows fed corn stover, twelve multiparous (3.5±1.0parity) Chinese Holstein dairy cows were used in a3x3Latin square design. Cows were divided into3groups according to similarity in milk production, day in milk and parity. The basal diets were isonitrogenous and isocaloric, with a ratio of forage to concentrate of45:55(DM basis). Three diets contained the following forage ingredients (%on a DM basis):(1) alfalfa19, and Chinese wild rye grass6(AH);(2) corn stover22, alfalfa6, supplemented with starch8(CSS); and (3) CSS supplemented with RP Met and RP Lys (CSSAA). In diet CSSAA the proportion of Lys and Met in MP estimated by CPM dairy soft was7.2and2.4%, respectively. Dry matter intake (P>0.05) and milk yield (P>0.05) did not differ among the treatments. Supplementation of RPAA increased the1.4kg milk yield thus increased the milk efficiency (P＜0.10). The contents of milk protein, fat, and lactose were not different among the treatments. Milk protein yield was higher (P＜0.10) for cows fed AH than those fed CSS, with CSSAA at an intermediate position. The urinary concentration of urea N for CSS diet was higher (P<0.05) than for AH, whereas the CSSAA diet had a lower urea N concentration in the milk (P＜0.05) and blood (P＜0.05) than CSS. Lower N conversion (P＜0.01) was detected for CSS than AH and CSSAA, with no difference between CSSAA and AH. Plasma total protein, albumin, glucose, NEFA, and β-hydroxybutyrate was not affected by treatments (P＞0.05). The results indicated that the supplementation of starch to a corn stover diet could achieve a similar lactation performance as cows fed alfalfa, while RPAA supplementation further improved the efficiency of milk and N utilization.
2. Approach to the mechanism with which supplementing starch and AA affects peoduction of milk protein precursor in dairy cows fed corn stover (Trial.4). The effects of supplementing of starch and AA on rumen fermentation characters, MP, plasma AA concentration were studied. Rumen fluid was collected from oral cavity of cows fed in trail3. Detection of rumen fermentation characters, rumen OM and CP degradation, estimation of the MCP yield, blood collection and plasma free AA concentration detection were similar to trail2. Ruminal pH and total VFA concentration did not differ among the treatments (P＞0.05). Ammonia N concentration was higher for cows fed diet CSS than those fed AH (P＜0.05), indicating that starch supplementation in corn stover diet improved energy supply to the cows, resulting in similar milk yield with those cows fed AH. The OM degradation and MCP yield were not different between CSS and AH (P＞0.05), suggesting that with starch supplementation the available energy supplied from CSS approached to diet AH, resulting in similar MCP yield. No difference was detected in IADP between diet CSS and AH (P＞0.05). Thus, the MP supply was not different between diets CSS and AH (P＞0.05). No significant difference was detected with AA supplementation on rumen fermentation, MCP yield, and MP supply. However, AA supplementation increased the plasma concentration of essential AA and total AA. From Tthe above results, it is indicated that the enhanced production of milk protein precusors in corn stover diet supplemented with starch is attributed to the increased supply of energy available to the rumen microbes and subsequently increased microbial protein synthesis. Supplementations of AA further increased the supply of milk protein precusors to the cows, but also improve the balance of amino acids within the cows'body.
In summary, high proportion of alfalfa hay in the diet is beneficial for milk protein yield and milk protein production by increasing microbial protein yield. This can be attributed to the improving supply of rumen available energy and the utilization efficiency of AA. Insufficient of rumen fermentation carbohydrate in corn stover induced low milk protein yield. Corn stover with both rumen fermentable carbohydrate and AA supplementation could increase the MCP synthesis and improve MP utilization, resulting in higher milk protein yield.
引文
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