Improve Calcium Homeostasis with a Close-up Diet Formulated With Appropriate Inclusion of Anions, Calcium Magnesium, and Phosphorus.

By
Jim Eadie
-
1540

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Milk Fever and subclinical hypocalcemia, caused by the sudden increase in mammary gland uptake of blood Ca at the onset of lactation, are gateway disorders of the recently calved cow.  Prolonged subclinical hypocalcemia increases the risk of retained placenta, ketosis and displaced abomasum.  It compromises immune cell function, increasing the incidence of metritis and mastitis as well.  Most cows suffer only a small and temporary decline in blood Ca the first day of lactation.  Ordinarily, cows adapt to the sudden demand for Ca at the onset of lactation by secreting a hormone from glands in the neck called parathyroid hormone (PTH) whenever blood Ca falls below normal levels.  PTH stimulates release of Ca from bone and stimulates the kidney to convert vitamin D to a hormone, called 1,25-(OH)2 vitamin D, that enhances intestinal Ca absorption.  These actions will generally restore blood Ca concentrations into, or close to, the normal range within 24 hours after calving.  Unfortunately, for many cows, particularly older cows, Ca homeostasis is impaired resulting in prolonged hypocalcemia.  High potassium (K) diets are now understood to be a major factor contributing to failure of Ca homeostasis.  When cations such as K that are abundant in forages are absorbed across the intestine they alkalinize the blood and tissues.  Alkalosis induced by forage K, reduces the ability of PTH to bind to its receptors on bone and kidney cells.  This interferes with the ability of PTH to stimulate bone Ca release and diet Ca absorption. The result is hypocalcemia that can cause milk fever and/or subclinical hypocalcemia that persists for several days after calving.

In most herbivore diets the amount of K consumed will cause blood and tissue pH to be slightly alkaline.  Urine pH generally reflects blood pH and so most herbivores normally have urine pH above 8.  In contrast, carnivores consume less K and higher amounts of chloride and sulfate.  They are in a state of slight acidosis, with urine pH closer to 6.5.

The good news is that we can overcome metabolic alkalosis in the close-up cow induced by K cations in forage by adding anions such as chloride and sulfate to the diet.  These dietary anions acidify the body- but only if they are absorbed into the blood.  The chloride anion is absorbed with nearly 100% efficiency from the diet.  Sulfate anion absorption is variable.  It is about 90% when fed at low levels and about 30% when fed at high levels.  On average, one Equivalent of sulfate is ~60% as acidifying as one Equivalent of chloride.   Diet Cation-Anion Difference is often calculated using the equation (mEq Na + mEq K) – (mEq Cl + mEq Sulfate) to discern the effect a diet will have on blood and urine pH.  While useful, keep in mind that this equation fails to account for the reduced acidifying action of sulfate compared to chloride anions and totally neglects the alkalinizing action diet Ca and Mg cations can have when they are absorbed.

The second most common factor interfering with calcium homeostasis is lack of magnesium.  It turns out Mg is critical for PTH to work as well.  Magnesium can only be absorbed across the rumen wall so your Mg source must be soluble in rumen fluid.  Generally the commercial anion products include Mg in a very available form.  The problem arises after calving when Mg oxide is used in the lactation diet.  This is done to meet the Mg needs of the cow AND to help buffer the rumen!!  Unfortunately the quality of Mg oxide on the market is highly variable and hypomagnesemia in the first days of lactation contributes to hypocalcemia after calving

Guidelines for formulating the close-up diet mineral profile for optimal DCAD

  1. Keep diet potassium as close to 1 % as possible.
    1. Use warm season grasses (including some corn silage)
    2. Use cool season grasses and legumes only if mature and coming from fields with no manure or potash applied. (ex. Wheat but not oat straw).
    3. More mature forages have lower potassium.
  2. Add enough chloride and / or sulfate to induce a mild metabolic acidosis in the cow.
    1. Check urine pH. Target is average of 6.3.   EXPECT TO SEE VARIATION -with average of 6.3, you must expect to see cows between 5.75 and 6.75 .  We only need to be on the acid side of life to reduce milk fever
    2. The effect of chloride on urine pH is more predictable than is the sulfate anion. USING SOYCHLOR WE HAVE ENOUGH DATA DEMONSTRATING THAT DCAD SHOULD BE BETWEEN -75 AND -125 mEQ/KG.   Set at -75 if Ca is kept below 0.5%, -100 if Ca is below 1%, and -125  mEq/kg if you choose to raise diet Ca above 1.4%.
    3. If using anion sources that rely on a mix of chloride and sulfate you will have to reach a DCAD level around -150 mEq/kg to reach a target average urine pH of 6.3.
  3. Keep diet sodium as close to 0.12 %, the requirement of the cow, as possible. 
    1. Salt (NaCl) can be used. NOT Na bicarb.  Most US diets will be sodium deficient.   Exceeding 0.2% Na has contributed to udder edema in some studies
  4. Keep diet phosphorus as close to 0.23%, the cow’s requirement, as possible.
    1. Above 0.33% P contributes to hypocalcemia by blocking kidney production of 1,25-dihydroxyvitamin D preventing efficient diet Ca absorption
    2. No added mineral phosphorus- ever!!
    3. Beware. Canola, brewers, corn gluten, wheat mids and DDG contribute high diet P.  Wheat straw, beet pulp, and soy hulls are low! Soy is a lower phos protein source.
  5. Let dietary Ca stay at whatever level it is from the feedstuffs and the added Soychlor.
    1. Often this will be 0.65-085% Ca, well above requirements for a dry cow
    2. Absorbed Ca cations are alkalinizing. Limestone can also reduce dry matter intake.  DO NOT ADD LIMESTONE UNLESS YOU WANT TO REDUCE DMI AND BUY MORE SOYCHLOR or other anion products!
  6. Set close-up and early lactation diet Mg to at least 0.35% Mg.
    1. Soychlor brings in very available Mg, but depending on inclusion rate this may not reach 0.35% Mg.
    2. Of particular importance is using a high quality MgO in the lactation diet.
  7. Heifers do not benefit noticeably from low DCAD. BUT if you must feed heifers too, then avoiding an over-acidified diet is of even greater importance to maintain DMI.
  8. NON-DCAD considerations for close-up cow diet- my opinion.
    1. NDF >40% preferred
    2. Metabolizable protein at least 1200 g for Holstein heifers and cows
      1. High straw / prairie grass diets will likely require added RUP or even bypass AA.
        1. Ability to use NPN for microbial protein compromised due to low fermentability of typical high forage close-up approach.
      2. Fresh cows are already immune stressed. Haylages can sometimes bring in mycotoxins that this cow just does not need.

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