奶牛在泌乳早期这一特殊生理阶段,泌乳量增加使其对能量需要增多,但同时较低采食量限制能量摄入,奶牛必须动用体脂肪来满足泌乳对能量的需要,使机体处于能量负平衡状态[1-4]。奶牛在泌乳早期过多动用体脂肪,可引起乙酰乙酸、β-羟丁酸、游离脂肪酸等代谢物质在肝蓄积,导致产生脂肪肝、酮病等,从而直接影响经济效益[5-8]。日粮中补充脂肪可有效缓解奶牛泌乳早期能量负平衡问题[9-13]。棕榈酸是高度饱和惰性脂肪酸,不含可被瘤胃微生物加氢饱和的双键,因而在瘤胃中不发生变化,直接进入后段消化道被消化吸收[14-15]。目前,日粮补充棕榈酸可提高产奶量,但对乳成分、血液生化指标和激素浓度的影响报道结果并不一致[11, 16]。如张海波[11]在奶牛泌乳早期日粮中添加1.5%棕榈酸可提高奶牛产奶量、乳脂率、血糖和胰岛素浓度;Mathews等[16]给泌乳中期奶牛日粮中添加4.0%棕榈酸提高了奶牛产奶量,但对乳成分、血糖和胰岛素浓度影响不显著。
本试验旨在研究奶牛泌乳早期日粮中补充棕榈酸对泌乳性能、血液生化指标和能量代谢相关激素浓度的影响,为奶牛泌乳早期合理添加棕榈酸提供理论依据和基本参数。
1 材料与方法 1.1 试验设计与饲养管理本研究选用体重((598.49±30.98) kg)、胎次(2~4胎)、泌乳时期(产犊15~37天)和泌乳量((20.76±3.32) kg)相近的中国荷斯坦奶牛30头,按照单因素试验设计原则分为3个处理组,对照组饲喂基础日粮,试验组分别在基础日粮中补充2.0%和4.0%棕榈酸,每组设10个重复,每个重复1头牛,试验时间为60 d。棕榈酸添加剂量参考Zhang等[17]、Piantoni等[18]和Rico等[19]的试验。棕榈酸纯度大于97%,主要为高饱和脂肪酸,原产地马来西亚,其商品名称为棕榈酸1699。按照NRC(2001)产奶量20 kg、乳脂率3.5%和体重600 kg泌乳奶牛的需要量配制基础日粮[20],日粮组成及营养水平见表 1。试验牛散栏饲养,自由活动和饮水。每天早、中和晚各饲喂1次,饲喂时先粗料后精料,饲喂后挤奶1次。
每天记录投入饲料和剩余饲料重量,根据每周测定1次投入饲料和剩余饲料干物质含量计算出每周内日均干物质采食量,最后用平均每周内每天的干物质采食量计算出整个试验期内日均干物质采食量(DMI)。采用张丽英[21]的方法测定饲料中粗蛋白、酸性洗涤纤维、粗脂肪和中性洗涤纤维含量。
1.2.2 牛乳样品采集与分析在试验开始前测定初始产奶量和乳成分。每周固定一天测定早、中和晚产奶量,最后计算出日均产奶量。在测定产奶量的同时,按照4:3:3比例采集早、中和晚混合乳样50 mL,用于乳成分测定。用FOSS红外乳成分测定仪测定混合乳样中乳蛋白、乳非脂固形物、乳糖和乳脂含量。按“0.4×产奶量(kg)+15×乳脂产量(kg)”计算标准乳(4%FCM)产量。
1.2.3 血液样品采集与分析试验奶牛分别在试验开始和结束空腹时尾静脉采集血液15 mL,3 500 g离心15 min后收集血清,存入-20 ℃冰箱,用于血液指标测定。血清葡萄糖含量采用全自动生化分析仪分析测定。血清乙酰乙酸含量采用水杨醛比色法测定。血清β-羟丁酸、胰高血糖素、游离脂肪酸、胰岛素、瘦素和胰岛素样生长因子1(IGF-1)含量参照试剂盒说明书测定,试剂盒购买于南京建成生物工程公司。
1.3 数据处理与统计分析数据经SPSS 19.0统计软件中的单因素方差分析后,用Duncan氏方法进行多重比较。试验结果用“平均值±标准差”表示。P < 0.05表示差异显著。
2 结果 2.1 日粮补充棕榈酸对奶牛产奶量与饲料效率的影响由表 2可知,3个处理组之间DMI、初始产奶量和饲料效率差异不显著(P>0.05)。与对照组相比,2.0%和4.0%棕榈酸组显著提高产奶量和标准乳(4%FCM)产量(P < 0.05),而2.0%与4.0%棕榈酸组间差异不显著(P>0.05)。
由表 3可知,3个处理组之间的乳脂率、乳蛋白率、总固形物含量和乳糖含量的初始值差异不显著(P>0.05)。与对照组相比,2.0%和4.0%棕榈酸组显著提高试验后乳脂率(P < 0.05)。2.0%与4.0%棕榈酸组之间的乳蛋白率、乳非脂固形物率和乳糖含量差异不显著(P>0.05)。
由表 4可知,3个处理组之间游离脂肪酸、葡萄糖、乙酰乙酸和β-羟丁酸的初始值差异不显著(P>0.05)。与对照组相比,2.0%和4.0%棕榈酸组显著提高试验后葡萄糖含量(P < 0.05),显著降低试验后乙酰乙酸、β-羟丁酸和游离脂肪酸浓度(P < 0.05)。2.0%与4.0%棕榈酸组之间葡萄糖、乙酰乙酸、β-羟丁酸和游离脂肪酸浓度差异不显著(P>0.05)。
由表 5可知,3个处理组之间IGF-1、胰岛素、胰高血糖素和瘦素浓度的初始值差异不显著(P>0.05)。与对照组相比,2.0%和4.0%棕榈酸组显著提高试验后胰岛素和IGF-1浓度(P < 0.05),显著降低试验后胰高血糖素和瘦素浓度(P < 0.05)。2.0%与4.0%棕榈酸组之间的IGF-1、胰岛素、胰高血糖素和瘦素浓度差异不显著(P>0.05)。
奶牛泌乳早期日粮能量浓度对泌乳性能起至关重要作用。本试验中,相较于对照组,日粮补充2.0%和4.0%棕榈酸对DMI无明显影响,但提高泌乳量和标准乳量。其可能原因是日粮补充2.0%和4.0%棕榈酸对DMI无明显影响的前提下,提高了奶牛能量进食绝对量,可在一定程度上满足奶牛泌乳对能量的需求,进而提高了泌乳量和标准乳量。张海波[11]研究发现,在泌乳早期奶牛日粮中添加1.5%棕榈酸对DMI无明显影响,但显著提高了泌乳量和标准乳量。Rico等[19]研究结果表明,在泌乳中期奶牛日粮中补充4.0%棕榈酸可显著提高泌乳量和标准乳量。
乳脂合成受到日粮脂肪酸饱和度以及添加剂量调控。本试验中,与对照组相比,日粮补充2.0%和4.0%棕榈酸提高了乳脂率。其可能原因是棕榈酸是高度饱和脂肪酸,与瘤胃微生物不发生氢化反应,直接过瘤胃进入肠道,在肠道被分解为长链脂肪酸进入血液后被运输到乳腺,而长链脂肪酸是乳脂合成前体之一,最终表现为提高乳脂率。并且,相较于对照组,本试验也发现,日粮添加2.0%和4.0%棕榈酸对乳糖、乳蛋白和乳非脂固形物率无明显影响。Piantoni等[18]研究发现,在日粮中添加2.0%棕榈酸提高了奶牛乳脂率,对乳糖、乳蛋白和总固形物含量影响不显著。Rico等[19]研究结果表明,在泌乳中期奶牛日粮中补充4.0%棕榈酸可显著提高乳脂率。
3.2 日粮补充棕榈酸对奶牛血液生化指标的影响血清葡萄糖含量变化最直接反映机体能量代谢状况。由于泌乳早期奶牛采食量较低,使丙酸等糖异生作用合成血清葡萄糖减少,发生糖异生障碍,机体处于能量负平衡状态。本试验中,相较于对照组,日粮补充2.0%和4.0%棕榈酸提高了血清葡萄糖含量。Zhang等[17]也发现,可以通过添加1.5%棕榈酸提高泌乳早期奶牛血清葡萄糖含量。这可能是因为日粮补充棕榈酸在肠道被消化成长链脂肪酸后进入血液,而长链脂肪酸可以生成脂酰辅酶A从而降低乙酰辅酶A活性,减少乙酰辅酶A生成,进而降低葡萄糖氧化,起到节约葡萄糖作用,最终提高血清葡萄糖含量。并且,与对照组相比,本试验也发现,日粮添补充2.0%和4.0%棕榈酸减少了β-羟丁酸、游离脂肪酸和乙酰乙酸含量,可能是因为补充棕榈酸使脂酰辅酶A生成增加,乙酰辅酶A的生成减少,从而减少β-羟丁酸、游离脂肪酸和乙酰乙酸含量。以上研究结果表明,日粮补充棕榈酸能够缓解泌乳早期奶牛能量负平衡状态,减少酮体生成量,降低酮病发生率,促进奶牛体况恢复。
3.3 日粮补充棕榈酸对奶牛能量代谢相关激素浓度的影响胰岛素对奶牛体内能量代谢起重要调节作用,能够促进细胞高效利用葡萄糖,促进脂肪合成和抑制脂肪分解,促使脂肪沉积[22]。本试验中,与对照组相比,日粮补充2.0%和4.0%棕榈酸可提高胰岛素浓度。相关研究也发现,在奶牛日粮中添加1.5%棕榈酸[11]、长链脂肪酸钙[23]可提高胰岛素浓度。可能原因是日粮补充棕榈酸通过提高血清葡萄糖含量,使胰岛β细胞分泌胰岛素的功能增强,进而分泌更多胰岛素。并且,相较于对照组,本试验也发现,日粮添补充2.0%和4.0%棕榈酸提高了IGF-1浓度,这与日粮添加1.5%棕榈酸升高IGF-1浓度的结果一致[11]。IGF-1也对调控奶牛体内能量代谢起重要作用,具有类似于胰岛素的功能,主要是提高葡萄糖利用效率,有利于体脂肪沉积[24]。并且IGF-1通过与受体结合作用于调控乳腺发育成熟和促进乳汁分泌[25],与泌乳量呈正相关[26-28]。在泌乳早期奶牛体内,胰岛素和IGF-1浓度较低,葡萄糖利用效率不高,脂肪分解作用加强,机体处于能量负平衡状态,而本试验中,日粮补充2.0%和4.0%棕榈酸提高了胰岛素和IGF-1浓度,表明其提高了血清葡萄糖利用效率,减少脂肪动员,提高奶牛的体况。
胰高血糖素是调控奶牛体内能量代谢的激素之一,其作用与胰岛素相反,主要降低细胞利用葡萄糖的效率,促进脂肪分解[29]。本试验中,相较于对照组,日粮补充2.0%和4.0%棕榈酸降低了胰高血糖素浓度。可能原因是补充棕榈酸通过提高血清葡萄糖含量,使胰岛α细胞分泌胰高血糖素功能减弱,进而分泌胰高血糖素减少。瘦素参与体内能量代谢的调控,可负反馈调节胰岛素分泌,其主要功能是促进脂肪分解和抑制脂肪合成,增加能量消耗[30]。并且,与对照组相比,本试验也发现,日粮补充2.0%和4.0%棕榈酸降低了瘦素浓度。可能是因为补充棕榈酸通过促进胰岛素分泌来抑制瘦素分泌。在泌乳早期奶牛体内胰高血糖和瘦素浓度较高,葡萄糖利用效率降低,促进脂肪分解,而本试验中,日粮补充2.0%和4.0%棕榈酸降低了胰高血糖素和瘦素浓度,表明其提高了血清葡萄糖利用效率,使脂肪分解减少,对奶牛体况恢复有利。
4 结论日粮补充2.0%或4.0%棕榈酸提高了胰岛素和IGF-1浓度,降低了胰高血糖素和瘦素浓度,使葡萄糖利用率提高,酮体生成减少,促进奶牛体况恢复,提高产奶量,提升乳品质。本试验条件下,综合考虑日粮补充棕榈酸的最佳剂量为2.0%。
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