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How to calculate the relative absorption coefficient of trace minerals for dairy cows?

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Dr Bill Weiss, Vice Chair of the new NASEM 2021 Dairy Requirements committee, explains the NASEM 2021 guidelines for dairy.

Dr. B. Weiss, Retired Professor of Dairy Cattle Nutrition in the Department of Animal Sciences at the Ohio State University has served as the Vice Chair of the new NASEM 2021 Dairy Requirements committee. During a webinar on October 4, 2021, he reviewed the NASEM 2021 Nutrient Requirements of Dairy Cattle for trace minerals, macro elements and vitamins. He shared his views on the correct formulation of these essential nutrients in dairy cattle diets. Dr. Weiss covered a number of topics in his presentation.

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Trace mineral requirement (Req) vs. Adequate trace mineral intake (AI)

The daily requirement is defined as the average trace mineral nutrient intake required to meet the needs of half of the healthy individual animals in a specific gender group at a certain stage of the production cycle. In humans, this is defined as the average plus 2 times the standard deviation as this covers the needs of 98% of the population. As the standard deviation is usually unknown in animals, the average is multiplied by a factor 1.2 to arrive at the final requirement for dairy trace minerals.

Adequate trace mineral intake (AI) is defined as the average daily nutrient intake that a group of experts has defined in case only a limited amount of experiment data is available. Based on expert opinion: “if most cows eat this much, they will probably be ok”.

How to calculate the requirements of a dairy cow?

Requirements vs. response were discussed, the formula on how to calculate requirements was reviewed and the difference between apparent absorption and absorption coefficient (AC) was explained.

The guidelines take sulfates as the reference. Understanding the methodology to calculate the relative absorption coefficient for a different source of trace mineral is crucial. Dr. Weiss stated: “If your ‘reference value’ is wrong so is everything else”.

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Requirements for macro-elements for dairy cows

The requirements for calcium and phosphorus did not change a lot. The requirements for magnesium went up significantly. Effects of common diet conditions were included in the analysis, the potential benefits of high magnesium levels on hypocalcemia of dairy cows were not.

Correct levels of electrolytes and DCAD application in dairy cows

Requirements for potassium did not change a lot. The equations for sodium changed a lot but they resulted only in little overall change in dietary requirements for dairy cows. The same is true for the chloride requirements.

DCAD requirement was defined as (Na+K) – (Cl+S) and set at about 175 mEq/kg DM. The DCAD for optimal prevention of milk fever in dairy cows is probably a lot lower, but the economic optimal DCAD is considerably higher depending on the value of increased milk and milk fat yield.

Trace mineral requirements of dairy cows

Trace mineral equirements for dairy cows were set for copper and zinc, Adequate intakes were defined for cobalt, iron, manganese and selenium.

Compared to the 2001 NRC Guidelines for dairy cows, the recommendations for Fe and Se didn’t change. Minor changes were made to the recommendations for Cr, I and Co, whereas the recommendations for Cu, Zn and Mn changed considerably.

Vitamin requirements for dairy cows

Water soluble vitamin requirements for dairy cows were reviewed extensively but an Adequate Intake could not be established. Adequate Intakes were established for vitamins A, D, and E.

Questions asked during the webinar...

Question Answer
For the copper requirement, did the Mo/S ratio remained the same, or was there a change in the Mo/S ratio from 2001 to 2021? The Mo and S ratio will not affect the requirement, but it will affect the AC. We did not have confidence in current data to quantify the antagonism in the software, however the text discusses possible adjustments to the AC based on diet S and Mo.
Based on what criterion or criteria is the adequacy of dietary P supply assessed in dry dairy cows? The P requirement for dry cows is calculated using the factorial method and would include endogenous fecal (a function of DMI) and P in the conceptus. Those requirements are well established and for the average dry cow will be accurate. The AC of P is also known pretty well, so estimates of dietary P requirement are accurate. Low plasma P ( <2 mg P/dL) can indicate inadequate P but that is often complicated with hypocalcemia which can make the interpretation of blood data difficult. If cows are clinically normal but have low blood P then they probably should be fed a little more P but in the vast majority of cases, NASEM requirements for P should be adequate.
What do you think about the study of Bouwstra 2008 regarding Vitamin E regeneration where 3000 IU has been found hazardous due to lack of co-oxidants to regenerate Vitamin E itself? The Bouwstra study found increased mastitis with high E (3000 IU/d) for the entire dry period. I have some issues with how they defined mastitis, but it was a large study (lots of farms and cows). The study clearly shows no benefit from high E supplementation for the whole dry period but additional studies are needed to confirm the increased mastitis incidence. Most studies with vit E supplementation show reduced mastitis but Bouwstra fed very high amounts for a long period of time. Any antioxidant can also function as a pro-oxidant if excess is fed and the same is true for vit E.
Why is there a difference in AC between inorganic P and organic P in the small intestine as organic feed P may transfer to inorganic P by microbial breakdown Some dietary inorganic P will be converted to organic P in the rumen. The AC shown in the talk assumes a certain amount of conversion based on a published model of P metabolism (JDS 98:7194).
How much ratio between minerals, is it the same for Zn and Cu and other minerals? The old NRC, or new NASEM dairy does not establish any set ratios between various essential trace minerals. Dairy animal requirements are based on a factorial approach that considers how trace minerals are being absorbed, retained, and excreted by the dairy animal to arrive at an approximate requirement for individual trace elements.
In a past publications, it has been recommended that it may be beneficial to increase NRC 2001 trace mineral requirements a bit (typically 10 – 20%). Do you continue to believe that the new NASEM 2021 requirements should be adjusted upward as well? As mentioned in the presentation, trace metal requirements for dairy animals have been arrived at via a factorial approach to derive a requirement that is based on the mean of all data reviewed, plus a correction factor of 1.2. If dairy animals are experiencing periods of heavy stress (environmental, nutritional, health, etc.) it may be prudent to consultant your nutritionist concerning the need to increase trace mineral feeding levels or an improved source of trace mineral.
Can you comment on how requirements for Cu supplementation will change in the presence of high levels of antagonists such as Mo and S in grazing swards? Higher levels of Mo and S can significantly reduce the availability of Cu in the animal due to the formation of thiomolybdates, which binds the Cu, Mo, and S in one indigestible molecule. The best action to counter this situation is to reduce the level of both Mo and S in the diet by eliminating high Mo/S ingredients and or diluting the diet's Mo and S levels via the use of alternative ingredients.
If a producer, or their nutritionist who is following the new NASEM Dairy 2021 recommendations decides to move from sulfate trace minerals to an improved trace mineral source, hydroxy, or organic, should the amount of metal supplied to the diet be reduced to compensate for product's increased bioavailability? The new NASEM Dairy 2021 uses sulfate trace mineral sources as its default source of supplemental trace mineral supplementation. As a result, the amount of supplemental trace mineral required is formulated based on its assigned AC (copper sulfate = approximately 5%). If an improved source of trace mineral is utilized to replace the sulfate source, its higher AC may be used, resulting in a reduction in the actual amount of supplemented trace mineral originating from the improved trace mineral source supplied to the animal.
How will the basal diet influence trace mineral requirements, and the interaction between minerals? The majority of trace mineral that exist within various basal ingredients are in an organic trace mineral form preventing any major antagonisms between basal ingredient trace minerals. It is only after they are digested within the cow that trace minerals become susceptible to various antagonisms. As the new NASEM points out the trace metal levels of all basal ingredients must be considered when formulating dairy diets. Where possible it is best to have basal ingredients assayed using wet chemistry to determine actual trace metal levels in high volume basal ingredients (silages, hay, etc.), allowing for greater trace metal formulation accuracy.
How can injectable Vit E and Se administration be adjusted in addition to the premix inclusions? The recommendations developed within the new NASEM for the supplementation of Se and Vitamin E are sufficient to meet the needs of a typical dairy cow. This is not to say that a cow experiencing periods of stress may not benefit from an additional injection of Se / Vit E. Best to consult your veterinarian or nutritionists to determine the correct protocol if additional supplementation is warranted.
Recent research has indicated the negative impact of more soluble, sulfate-based trace minerals on rumen fiber digestibility. Based on this data should sulfate trace minerals be avoided in dairy diets? This data was not considered by the NASEM committee due to its recent release, however recent research does indicate that sulfate-based trace minerals (Zn and Cu) appear to be negatively affecting rumen fibre digestibility. This appears to be due to the antimicrobial effects of zinc and copper sulfate when entering the rumen thereby negatively affecting rumen fiber digestibility.

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For every trace mineral, there is an optimum

Trace mineral nutrition in dairy cattle is an essential component of any Healthy Life program given its involvement in hundreds of physiological processes involving basic maintenance, reproduction, immune competency, milk production, growth, and structural integrity. Our recommendations related to trace mineral nutrition of dairy cows are designed to ensure that your cows are receiving the correct level and source of trace minerals to drive optimized profitability and cow well-being throughout their entire lifetime.

During the webinar Bill Weiss, Ph.D, Professor, Department of Animal Sciences, The Ohio State University and Co-Chair of the NASEM Dairy Requirements Committee, addressed the changes to the trace minerals, macro minerals and vitamins sections of the new NASEM guidelines.

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