译文：

日粮中的阴阳离子差

对不同生理阶段奶牛的影响及应用

作者：David K. Beede
C.E. Meadows Chair Professor in the Department of Animal Science, Michigan State University

翻译：韩佳璇 奥特奇反刍大客户经理

定义

日粮中阴阳离子差的计算主要涉及两种阳离子（钾和钠）和两种阴离子（氯和硫）。阴阳离子差的计算公式如下：

它的单位是毫克当量（meq），离子浓度均以日粮干物质为基础。近20年来，日粮中的阴阳离子差已经被广泛应用于科研和牧场生产中 (NRC, 2001)。从生理学角度出发，日粮中的阴阳离子差会影响动物体内稳态的酸碱平衡、产犊期间的钙离子状态和矿物质元素的吸收利用。研究结果表明，日粮中的阴阳离子差会对体内钙离子代谢、怀孕后期干奶牛和围产期奶牛的健康状况产生显著影响(NRC, 2001; Goff and Horst, 1997; Block, 1994)。同时，日粮阴阳离子差还会直接影响产奶性能(Beede, 2005; Hu and Murphy, 2004)。

日粮中的阴阳离子差对怀孕后期干奶牛的影响

为了降低产犊后奶牛患低血钙和产乳热的风险，怀孕后期的主要目标是为奶牛提供一个低阴阳离子差的日粮（低于5毫克当量每100克日粮干物质）或者负阴阳离子差的日粮。降低低血钙和产乳热的发病率还可以降低其他相关代谢性疾病的发病风险，比如胎衣不下、真胃变位和子宫炎。在大多数情况下，降低日粮中钾离子和钠离子的浓度可以降低日粮中的阴阳离子差，从而提高围产期奶牛的生产性能。饲喂奶牛低阴阳离子差的日粮还可减少乳房水肿。如果降低钾和钠还不足以降低阴阳离子差，在日粮中添加阴离子（氯和硫）也可降低日粮中的阴阳离子差。当日粮中的阴阳离子差维持在-5至-10毫克当量范围时，奶牛的健康状况和生产性能均会得到改善。

阴离子盐常用来降低日粮中的阴阳离子差，例如氯化铵、氯化钙、氯化镁、硫化铵、硫化钙和硫化镁。值得注意的是，粉末状的硫（化学成分硫）没有生物学效价，也就是说不能被动物利用，因此不能降低日粮中的阴阳离子差，也不能达到改善奶牛生理、健康和生产性能的功效。阴离子盐的适口性较差，过多添加阴离子盐可能会降低采食量。商业化的阴离子盐通过某种加工工艺处理（如盐酸处理）能有效解决阴离子盐降低采食量的问题。

如果日粮中的阴阳离子差维持在-5至-10毫克当量每100克干物质，并且尿液pH 6.0-6.7，说明日粮中阴离子盐的添加是有效的。如果尿液中的pH低于6.0，说明没必要在日粮中添加阴离子盐。值得注意的一点是，不添加阴离子盐，并且饲喂常规日粮（阴阳离子差大于等于20毫克当量）的奶牛，其尿液pH 7.8-8.2。即使把钾离子和钠离子从日粮中移除，日粮中的阴阳离子差仍然会大于等于20毫克当量，并且即便日粮中不添加阴离子盐，尿液中的pH也会处于正常范围。因此，日粮中的阴阳离子差不能用于预测某种日粮是否会导致怀孕后期干奶牛易患低血钙或其他代谢疾病。只有当添加阴离子盐用于降低阴阳离子差的时候，尿液pH才能用来检验动物体内的酸碱平衡和钙离子状态。如果日粮中的阴阳离子差在-5至-10毫克当量范围内，并且围产期奶牛的采食不受阴离子盐添加的影响，3-7天后尿液pH将会降低，这可以说明阴离子盐对采食量没有影响。

日粮中的阴阳离子差对泌乳期奶牛的影响

泌乳牛日粮中添加阳离子盐（钠和钾）能够中和由于瘤胃发酵产生的大量酸以及系统代谢产生的酸，从而有利于奶牛的健康。通常可以通过减少阴离子或富含阴离子的饲料原料，或添加碳酸氢钠或碳酸钾来增加日粮阴阳离子差。

研究表明，25-30 毫克当量的阴阳离子差能最大地提高泌乳牛的采食量和产奶量(Beede, 2005)。20-40 毫克当量的阴阳离子差对产奶性能的影响不大，但是当阴阳离子差小于20或大于40毫克当量时会对产奶性能产生负面影响。只要阴阳离子差处于20-40 毫克当量的正常范围内，继续添加阳离子盐的效果将不会显著。如果想增加日粮中的阴阳离子差，添加钾离子和钠离子对产奶性能的影响差不多。因此，推荐使用两者中价格相对便宜的一种。目前只有少数几篇研究测定了高产奶牛日粮中的最佳阴阳离子差，这样的研究结果对实际生产将会有指导意义。

处于热应激阶段的奶牛应饲喂最佳阴阳离子差范围上限的日粮。举例来说，日粮中含1.5 %钾、0.5%钠、0.3% 氯和0.25%硫，该日粮的阴阳离子差为36毫克当量每100g 日粮干物质。如果日粮中的氯离子和硫离子过量，首先要做的就是通过替代富含阴离子的饲料原料来降低基础日粮中氯离子和硫离子的浓度。如果以上方法不可行，可以通过添加碳酸氢钠或碳酸钾来增加阴阳离子差。但是，这种方法对于奶业和环境都是不可持续的，因为过多的钠和钾会排放进入土壤、农作物、地表水和地下水。

从整体的角度出发，我们需要降低奶牛日粮中钾、钠、氯和硫的用量。日粮中阴阳离子超过奶牛需要的部分会被排泄进入环境。如果排泄的离子不能被周边的农作物有效利用，这种过量饲喂阴阳离子的饲养模式就会对整个牧场运行系统产生长久的负面影响。

原文：

Dietary Cation-Anion Difference for Dairy Rations

Definition

Dietary cation-anion difference (DCAD) typically includes two cations [potassium (K) and sodium (Na)] and two anions [chlorine (Cl) and sulfur (S)]. The DCAD equation most often applied in the field equals: milliequivalents (meq) [(%K divided by 0.039) + (%Na divided by 0.023)] – [(%Cl divided by 0.0355) + (%S divided by 0.016)]/100 grams of dietary dry matter (DM). Considerable research and field application of the DCAD concept have occurred in the last 2 decades (NRC, 2001). Physiologically, DCAD influences the animal’s acid-base homeostasis, Ca status around calving, and mineral element utilization. Much of the early work addressed effects of DCAD on Ca status and metabolic health of late pregnant, transition cows (NRC, 2001; Goff and Horst, 1997; Block, 1994). Also, direct effects of DCAD on lactational performance were examined (Beede, 2005; Hu and Murphy, 2004).

DCAD for late pregnant dry cows

The primary goal in late pregnancy is to provide a ration with a low (less than +5 meq/100 grams of dietary DM) or negative DCAD to reduce the risk of hypocalcemia (low blood Ca) and clinical milk fever around calving. Minimizing the incidence of hypocalcemia and milk fever reduces the incidence of other associated metabolic disorders such as retained fetal membranes, abomasal displacement, and metritis. In many cases, simply using feedstuffs with lower concentrations of K and Na will lower DCAD enough to improve transition cow performance. This also may reduce udder edema. When it is not possible to reduce dietary K and Na enough, supplementation with anions (chloride and sulfate) will reduce DCAD. A target DCAD of -5 to -10 meq may improve transition cow health and performance.

Anion sources to reduce DCAD include the so-called anionic salts such as ammonium chloride, calcium chloride, magnesium chloride, ammonium sulfate, calcium sulfate, and magnesium sulfate. Note: Flowers of Sulfur (elemental sulfur) is not bioavailable or bio-reactive and is not effective to reduce DCAD or affect the cow’s physiology, health, or performance. Anionic salts are not very palatable and may reduce feed intake if too much is supplemented. Commercial anion supplements such as those prepared by treatment of feedstuffs with hydrochloric acid or other anions can be effective with less risk to reduce feed intake than anionic salts.

With a DCAD of -5 to -10 meq/100 grams of dietary DM, urine pH of 6.0 to 6.7 indicates that anion supplementation is effective. Urine pH of less than 6.0 indicates it is not necessary to feed so much anion source. Note: Urine pH of dairy cows fed typical rations without anion supplementation (DCAD of +20 meq or greater) is between 7.8 and 8.2, a normal value for ruminants. Even if significant K and Na are removed from the ration and the DCAD is still +20 meq or greater, without anion supplementation, urine pH values will be within the normal range. In this case, the transition performance of cows fed low, positive DCAD rations may be normal. Therefore, DCAD can not be used to predict whether or not a particular ration will predispose late pregnant dry cows to hypocalcemia and other transition disorders. Only when supplemental anions are used to reduce DCAD can urine pH be an indicator of the effectiveness to affect acid-base and Ca status. If the ration is actually -5 to -10 meq and transition cows are eating well, reduced urine pH will be a good indicator of proper anion intake after 3 to 7 days of feeding the ration.

DCAD for lactating cows

Increasing DCAD of lactation rations with supplementation of cations [Na and (or) K] may be beneficial for lactating dairy cows to neutralize tremendous amounts of acids produced in ruminal fermentation and systemic metabolism. The DCAD may be increased by reducing anions or high anion-containing feed ingredients, or by supplementing with sodium bicarbonate or potassium carbonate.

In lactation rations, DCAD between +25 to +30 meq/100 grams of dietary DM is effective and sufficient to achieve maximum feed intake and milk yield (Beede, 2005). Magnitude and differences in lactational responses were small between +20 to +40 meq. Less than +20 meq or greater than +40 meq was quite detrimental to performance in some studies. As long as DCAD is between the optimal range of +20 to +40 meq, little (or no) benefit is expected by supplementing additional cations. Both Na and K are equally efficacious to lactational performance if a greater DCAD is desired. Thus, use of the cation source with the least cost on a milliequivalent basis is recommended. There are few reported studies trying to determine optimal DCAD with very high producing cows; these studies would be useful.

During heat stress the best DCAD may be at the upper end of the optimal DCAD range. For example, a ration with 1.5% K, 0.5% Na, 0.3% Cl, and 0.25% S, DM basis, has a DCAD of +36 meq/100 grams of dietary DM. If the Cl and S concentrations exceed those listed above, the first formulation step should be to try to reduce Cl and S concentrations in the basal ration by replacement of high anion-containing ingredients with other ingredients. If this is not possible, inclusion of more Na and (or) K from sodium bicarbonate and (or) potassium carbonate can be used to increase ration DCAD. However, doing this may not be sustainable for dairy production and environmental management because higher concentrations of these cations may occur in soils, crops (feeds), and surface and ground waters.

For overall farm nutrient balance, reducing the amounts of ration-supplemented K, Na, Cl and sulfate is a critical consideration. Amounts consumed that are in excess of the cows’ requirements are excreted, and must be recycled effectively via crops or exported, or problems can occur in dairy farming systems.

转自：牛人艺语