Balancing Minerals and Vitamins for Production, Reproduction and Health

Tim Brown

Tarleton State University, Department of Animal Sciences, Box 0070, Stephenville, TX, 76402 U.S.A.

# Take Home Messages

# Introduction

The objectives of this paper are: (1) To discuss the differences between the concept of Abalancing@ rations for minerals and vitamins and that of Afortifying@ rations with these nutrients, (2) To briefly describe some of the mineral and vitamin requirements of dairy cattle, and to provide reference sources for more detailed information on mineral and vitamin requirements and metabolism. (3) To provide examples of how to determine the adequacy of dietary minerals based on laboratory analyses, and how to calculate the amount of mineral supplement that is required. (4) To provide some practical guidelines for fortifying dairy rations with minerals and vitamins to prevent a few of the more common production, reproduction, and health problems in today=s modern dairies.

The nutritional concept of Abalancing@ minerals and vitamins involves our attempts to provide, in the diet, appropriate amounts of available nutrients to meet the animal=s requirements (maintenance, growth, production, reproduction, and health) while at the same time, preventing interactions among the nutrients that will limit the animal=s ability to use them. When we formulate diets for livestock, we frequently consider that we are Abalancing A all of the minerals and vitamins in the ration. However, for some of these, we often are merely providing supplementation or fortification to ensure that the nutrient supply to the animal falls into a rather broad range, somewhere between levels that are so low that they cause losses of production efficiency, and so high that they cause imbalances, toxicities, or exorbitant costs. Often, our ability to truly balance for certain minerals and vitamins depends on the information that is available to us regarding the content of those nutrients in our feedstuffs, and the absolute availability or activity of those nutrients for metabolism by the animal. We generally have more concrete information to work with when we evaluate feedstuffs for mineral content than we do when we try to gather information on the vitamin content or vitamin activity of feedstuffs. The mineral concentrations in our feedstuffs, particularly forages, are variable from load to load, and are affected by a number of factors, necessitating a laboratory analysis to tell us what those values are. However, minerals within the feeds are quite stable, and once we have analyzed those feeds, the mineral content is unlikely to change appreciably. On the other hand, the vitamin content of our feedstuffs will change over time, so we can never have a great deal of confidence in how much vitamins our cattle are getting from the feeds. Thus, for minerals, we are able to come closer to actually matching dietary supply to metabolic needs, whereas for vitamins, we depend more on Afortification@ of the diet to remain within Atolerable@ levels. Some of the factors that affect our ability to balance minerals and vitamins in dairy rations include:

Numerous excellent discussions of the above-mentioned factors have been published (1, 4, 11, 12, 14, 17). Our concern in today=s dairy nutrition programs is not preventing the development of the classical deficiency symptoms that research has characterized for each mineral and vitamin, but is avoiding losses in production efficiency that result from sub-optimal levels or improper ratios of these nutrients in the diet.

 # The Mineral and Vitamin Requirements

There are so many variables that affect the manner in which dietary minerals and vitamins are used by animals that science has not been able to definitively determine the absolute requirement for each of these nutrients under every condition. For the purpose of providing practical guidelines for mineral requirements, certain assumptions have had to be made. Depending on which research data are evaluated, differing conclusions can be drawn regarding the absolute mineral and vitamin requirements of the animal, and different recommendations can be made regarding the optimum concentrations of these in the diet. One rich source of information regarding dietary mineral requirements is the National Research Council=s (NRC) Nutrient Requirements of Dairy Cattle (14). This publication represents a survey and critical review of the published scientific literature on dairy nutrition. Despite the fact that we are still refining our knowledge of mineral and vitamin nutrition and metabolism, we have come a long way, and the classic mineral and vitamin deficiency symptoms of yesteryear are rarely seen in today=s well managed dairy herds. Some ranges in the NRC recommended mineral content of diets for lactating cows and the recommended maximum tolerable levels for certain minerals and vitamins are listed in Table 1.

More detailed descriptions of mineral and vitamin requirements for all classes of dairy cattle are included in the NRC publication, available from National Academy Press, 2101 Constitution Avenue, NW, Lockbox 285, Washington, DC 20055. Cost is $19.95 + $4.00 shipping.

Several universities, feed manufacturers, and nutrition consultants will recommend dietary mineral and vitamin levels that differ from those in the NRC. Many of these deviations from NRC have merit based on recent research and field observations. However, dairymen are cautioned against indiscriminate overfeeding of any mineral or vitamin with the attitude that Aif my cow needs just a little of this, more will be better@. Mineral imbalances created by indiscriminate over supplementation can be just as detrimental as simple deficiencies. A few of the more common trace mineral recommendations that differ from those of the NRC requirements are listed in Table 2. Some recommendations for feeding supplemental vitamins in excess of NRC recommendations are given in Table 3. Some of these recommendations likely will change as we learn more about specific involvement of minerals and vitamins in dairy cattle production, reproduction and health.

Notice that for several minerals in the NRC table, the dietary requirement varies with the size of the cow and her milk production level. That is because these minerals contribute to milk synthesis, and higher producing cows need more of them. Dietary requirements for some of the other minerals, however, are set at constant concentrations in the diet dry matter for all classes of dairy cattle.

Table 1. Selected ranges of NRC recommended mineral and vitamin content of diets for Lactating Dairy Cows.1



 Range of Nutrient Density in the Diet Dry Matter

Smaller Cows

Lower Production

Larger Cows

Higher Production

Maximum Tolerable2


Major Minerals

Calcium, %




Phosphorous, %




Magnesium, %




Potassium, %




Sodium, %




Chlorine, %




Sulfur, %




Minor Minerals

Iron, ppm

50.0 ppm for all production levels


Cobalt, ppm

0.1 ppm for all production levels


Copper, ppm

10.0 ppm for all production levels


Manganese, ppm

40.0 ppm for all production levels


Zinc, ppm

40.0 ppm for all production levels


Iodine, ppm

0.6 ppm for all production levels


Selenium, ppm

0.3 ppm for all production levels



A, IU/kg diet dry matter

3,200 for all production levels


D, IU/kg diet dry matter

1,000 for all production levels


E, IU/kg diet dry matter

15 for all production levels


1 Taken from NRC (14), 1989, page 87.

2 Many factors affect the maximum safe levels of dietary mineral elements. More information is available in Mineral Tolerances of Domestic Animals (13).

Table 2. Some recommended trace mineral allowances in excess of NRC recommendations1.








parts per milliion (mg/kg) of diet dry matter

University of Illinois





University of Florida





NC State University





1 Recommendations taken from Miller et al. (12).

Table 3.Recommended feeding amounts for supplemental vitamins in dairy cattle diets.1,2


Dry cows

Lactating cows

Heifers (12-24 mos.)

Calves (6-12 mos.)

Vitamin A (IU/hd/day)





Vitamin D (IU/hd/day)





Vitamin E (IU/hd/day)






6 grams/day

12 grams/d



 (2 weeks pre-calving)

(freshening to about 100 days in milk)

1 Recommendations for Vitamins A, D, and E from Erdman (5).

2 Recommendations for Niacin from Hutjens (9).

Evaluating Rations For Mineral Content

Fortunately, for many of the minerals of nutritional interest, reasonably priced laboratory analyses are generally available. The important questions are Afor which minerals should I analyze, how many of my feedstuffs should I analyze for minerals, and how often should I do it?@. The minerals that are most commonly analyzed in feeds are calcium, phosphorous, magnesium, sodium, potassium, copper, zinc, manganese, and iron. Although there are other minerals for which we also need to be concerned (chlorine, sulfur, cobalt, iodine, selenium), these generally are more difficult and expensive to analyze in the laboratory, and thus, we tend to rely more on Abook@ values for these elements. We must remember, however, that the more Aguesses@, or book values, that we use, the less confidence we will have in our final analysis of mineral adequacy in our rations. Generally, the mineral content of forages varies more than that of grains and by-product feeds. As such, if you want to save some analytical costs, just analyze your forages for mineral content. This will reduce your confidence in the mineral content of those grains and by-products, but Abook values@ for these will be closer to correct than will be book values for the mineral content of the forages. An analysis should be obtained for each newly purchased lot of forage, and for each home-grown forage for which the production conditions suggest that there may be differences in nutrient composition (different fields, harvest dates, fertilization rates, weathering conditions, etc.). When one considers the time and effort it takes to sample feeds, and the expense of analyzing the feeds, the concept of the Total Mixed Ration (TMR) greatly simplifies our efforts. With a TMR, where all feeds and supplemental ingredients are incorporated into the ration, we can simply send the complete mixture to the laboratory and compare the results directly with the animal=s dietary requirements. However, those who do not feed a TMR (component feeding) will need a mineral analysis (or book value) for each feed, and make a few calculations (example provided below) from the results. Of course, we have available to us many user-friendly ration balancing and ration evaluation computer programs that provide great utility for assessing nutrient adequacy in cattle diets.

 # Calculating Dietary Mineral Concentration from Analysis

of Feeds, and Calculating the Amount of Mineral

Supplement Needed

In component feeding situations, you will need some estimate of how much of each component the animal eats each day, and you will need an analysis of each component. By dividing total mass of mineral contributed by all feeds by the total daily feed intake and multiplying by 100 (to convert to percentage), you will calculate nutrient density in the total ration, which you can compare to the NRC table.


Amount of feed DM/day


 Laboratory or book values


 Mass of mineral from feed





6.8 kg alfalfa DM/day


1.40% calcium in alfalfa


.095 kg calcium from alfalfa

4.8 kg corn silage DM/day


0.10% calcium in corn silage


.005 kg calcium from silage

10.9 kg grain mix DM/day


0.60% calcium in grain mix


.065 kg calcium from silage

21.8 kg total DM intake/day




 .165 kg total calcium

 .165 kg total calcium ) 21.8 kg total DM intake x 100 = .76% calcium in the total diet dry matter. You can then compare this number to the figures in the nutrient recommendation tables for your particular size of cow and her production level. You will see that .76% calcium in the diet falls in the upper range of NRC recommended calcium levels.

For another example, suppose you either analyzed your TMR for copper, or calculated the copper content of your component fed ration and came up with a copper concentration of 5 ppm in the total diet dry matter. Compared to the NRC table, this is an inadequate concentration. Here is how you would figure the amount of a copper supplement to feed to bring your copper level up to the 10 ppm NRC recommended requirement.

Hint: remember that 5 ppm copper in the diet dry matter is the same as 5 mg copper/kg diet dry matter.

The amount (mass) of copper supplied per day from this diet (at 5 ppm) is:

5 mg Cu/kg dry matter x 21.8 kg diet dry matter = 108 mg copper supplied per day.

The amount (mass) of copper actually needed from this diet (at 10 ppm) is:

10 mg Cu/kg dry matter x 21.8 kg diet dry matter = 218 mg copper needed per day.

218 B 108 = 110 mg supplemental copper needed per day.

The next step is to find a source of supplemental copper and figure out how much to feed. Suppose you have available to you a trace mineral supplement with the following specifications:




Hint: To convert from percentage (%) to ppm, move the decimal point 4 places to the right. For this supplement, 0.88% copper is the same as 8,800 ppm, or 8,800 mg copper per kg supplement.

Divide the amount of supplemental copper you need by the copper concentration in the supplement (110 mg copper needed ) 8,800 mg copper/kg supplement) = .0125 kg supplement, or 12.5 grams of this supplement per cow per day. Remember to take into account the contributions that this much supplement will make to the zinc and manganese levels in the ration.

 # Bioavailability of Minerals from Different Supplements

Bioavailability is the term used to describe the degree to which minerals from different sources can be used by the animal. Simple solubility of the mineral elements seems to be one of the major factors affecting bioavailability, thus, mineral supplements from which the elements are readily soluble in water will be the most bioavailable. A laboratory analysis of the mineral content of feedstuffs will not provide an estimate of their bioavailability. However, the bioavailability of minerals from feedstuffs has been estimated and those estimates have been taken into account in calculating the recommended mineral levels in dairy diets (14). All dairy rations will require some supplemental minerals. The objective of supplementing the ration is to provide elements that the animal can use. Unfortunately, many sources of supplemental minerals have limited ability to provide Auseable@ minerals to the animal. Dairymen are encouraged to avoid the mineral supplements known to be low in bioavailability. Table 4 lists a few common supplements and their relative bioavailability.

Table 4. Relative bioavailability of some supplemental mineral sources.1,2



Relative Bioavailability





Steamed bonemeal

Dicalcium phosphate

Monocalcium phosphate

Defluorinated rock phosphate

Calcium carbonate

Ground limestone

Dolomitic limestone

Soft phosphate


Cupric sulfate

Cupric chloride

Commercial chelates, proteinates, and complexes

Cupric carbonate

Cupric nitrate

Cupric oxide


Ferrous sulfate


 Ferrous carbonate

Red iron oxide (unavailable)


Magnesium carbonate

Magnesium chloride

Magnesium oxide

Magnesium sulfate






Manganous sulfate

Manganous oxide

Commercial chelates,

proteinates, and complexes






Calcium phosphate

Phosphoric acid

Sodium phosphate

Steamed bonemeal

Defluorinated rock phosphate

Dicalcium phosphate

Soft phosphate


Zinc carbonate

Zinc sulfate

Zinc oxide

Commercial chelates,

proteinates, and complexes

Zinc chloride


 1 Taken from McDowell et al. (11), with the exception of chelates, proteinates, and complexes.

2 Organic trace minerals (chelates, proteinates, and complexes) have been added to the Ahigh bioavailability@ based on recent research (16).

The cost of mineral supplements also needs to be considered when deciding which source to use. The marketing of mineral supplements is highly competitive, and marketing claims abound regarding superiority of one source over another. Perhaps one area in which marketing is most aggressive today is in the promotion of Aorganic@ trace minerals. These come in the form of chelates, proteinates, and complexes in which the mineral element is chemically attached to an organic molecule(s), usually amino acids or pieces of protein. Peer review research has shown that in some instances organic mineral supplements are better at improving immune system function, reducing somatic cell counts, improving reproductive performance or improving hoof health. However, in most of these cases, the difference in performance between the organic source and highly bioavailable inorganic sources has been slight. This research indicates that the organic mineral sources are themselves highly bioavailable, but, from a practical standpoint, one should only pay slightly more per unit of element supplied for the slight advantage over a good inorganic source.

 # Practical Considerations in Balancing and Fortifying

Dairy Diets for Minerals and Vitamins

By adhering closely to the recommended dietary mineral levels (either NRC or one of the slight variations) production and health problems associated with minerals can usually be avoided. Oftentimes, excesses or imbalances of certain minerals can cause production losses just as deficiencies would. The mineral elements play important roles in milk synthesis, energy metabolism, growth, reproduction, and health. Thus, the importance of maintaining a balance between dietary supply and what we currently understand to be the animal requirements for all mineral elements is essential if we hope to have healthy cattle. Preventing simple deficiencies should be the goal in evaluation of dietary mineral content and supplementation programs. In situations where dietary excess of certain minerals is unavoidable, fortification of the diet with the elements for which utilization is known to be impaired may be the desired management step. Identifying sources of undesirably high excesses of certain minerals, whether in the feed or water, and eliminating those excesses, is another management approach to balancing the minerals in the diet with the needs of the animal. The key to practical management of mineral nutrition is knowing how much of each mineral the animal eats. The feeding of free choice minerals adds a large degree of variability to our estimates of mineral intake. Dairymen who are serious about proper mineral nutrition should incorporate all necessary minerals into the feeds that the cows are eating, and avoid the feeding of minerals free choice.

Calcium and Phosphorous levels and ratio. As long as the minimum requirement for phosphorous is met, providing one and one half to two times more calcium will help ensure both adequate intake and adequate utilization of both elements. Although some dairymen intentionally feed phosphorous in excess of NRC recommendations in hopes of avoiding reproductive problems, there is little scientific evidence to show that this practice will improve display of estrus or otherwise improve reproductive efficiency. Most phosphorous fed in excess of animal requirements will be excreted into the environment, and phosphorous pollution from dairy operations is becoming the number one environmental issue in some areas of intensive dairy concentration.

Copper to Molybdenum ratio. Since molybdenum is known to form insoluble complexes with copper in the rumen of the cow, we must strive to maintain a copper:molybdenum ratio of 4:1. Since analyses for molybdenum are not usually included in most feedstuff mineral analyses, special requests for this analysis may be needed if copper deficiency is suspected. One of the most obvious signs of copper deficiency is a reddish tinge to the black hair of Holsteins. Copper deficiency of sufficient magnitude to cause altered hair color will almost surely result in production and health losses due to impaired immune system function.

Excessive intake of iron or manganese. Several feedstuffs and mineral supplements have extremely high iron and(or) manganese content. Additionally, some water is high in iron and(or) manganese. We should monitor our ingredients to make sure we do not exceed maximum tolerable levels of these elements.

Dietary cation (sodium and potassium) and anion (chlorine and sulfur) levels. Recent research has enlightened us greatly to the effects of these elements on dairy cow performance, particularly during the transition from dry cow to milking cow. This specific area of mineral nutrition should be covered thoroughly by Dr. Beede, Dr. Stallings, and possibly others in presentations during this seminar. Dependable feedstuff analyses for these four elements are essential for successful manipulation of their levels in the diet, particularly because they vary so much in forages.

Niacin fortification of high fat and early lactation diets. Results of controlled experiments on the effects of supplemental niacin on milk production and composition have proven niacin to be a cost effective additive in certain dairy rations. There is a rather large body of evidence indicating that the feeding of niacin, either as nicotinic acid or as nicotinamide, can increase milk production for certain cows, and increase milk protein content and yield when high fat diets are fed (3, 7, 8, 10,). Furthermore, cows fed niacin during the transition from the dry period through peak lactation may eat more feed and be less likely to develop ketosis.

Generally, the cows that respond favorably to feeding of niacin are high producing cows in early lactation, and cows being fed supplemental dietary fat. The most commonly recommended amount of niacin to feed is 6 to 12 grams per day. The key to deciding whether or not to feed niacin is knowing the conditions under which cows will respond economically. Other contributors to this seminar will cover that topic in detail as they discuss transition cow management, early lactation performance, and the cost effectiveness of feed additives.

Vitamin A and D fortification of lactating cow diets. Although most feedstuffs will at some time contain some vitamin A and vitamin D, these nutrients lose their effectiveness over time, and as such, their levels in the feedstuffs can not be depended on. As cheap insurance, it is often recommended (4, 5) to supplement in the feed of dairy cows an amount of vitamins A and D equal to the minimum daily allowance of each (around 100,000 IU of A and 10,000 IU of D)

Vitamin E fortification of dry cow diets. The feeding of vitamin E at supplemental rates of 1,000 IU/day pre-partum and 500 IU/day during lactation is now commonly recommended as one means to reduce the incidence of mastitis. Recent research (18) has shown that fortification of dry cow diets with Vitamin E at even higher levels (4,000 IU/day pre-partum and 2,000 IU/day during lactation) may reduce mastitis even further, however, more research with those levels is needed.

Raising dietary calcium and magnesium levels when unprotected fats are fed. It is commonly recommended that dietary calcium and magnesium levels be increased to 1.0% and .45% in the diet dry matter, respectively, when ruminally unprotected fats (oilseeds, tallow) are fed. This doesn=t mean that the animal=s metabolic requirement for these elements has been increased by the extra fat, it just means that providing these elements at slightly higher levels in the diet is a cheap, easy, and effective way of overcoming some of the negative effects (depressed fiber digestion, reduced microbial protein synthesis, etc.) of feeding unprotected fats and oils.

Effects of selenium on the incidence of retained placenta. The retention of fetal membranes (retained placenta) following calving is one of the classic symptoms of a selenium deficiency. An entire section of our seminar this year will cover the effects of selenium and vitamin E on the health and performance of cows. The allowance for selenium in dairy diets in the US was increased in 1987 by the US Food and Drug Administration to .3ppm. Most dairymen who increased the selenium levels in their dairy diets realized a marked reduction of retained placenta problems. Dairymen should be diligent in monitoring the selenium intake of their cows, both from the standpoint of preventing deficiency, and out of concern for possible toxicity from over feeding this element.

# Possible Mineral and Vitamin Applications of the Near Future

Ongoing research into the mineral and vitamin requirements of cattle will continually refine our understanding of existing requirements, and possibly add some mineral and vitamin requirements to the above list. Although metabolic roles for the element chromium have been known for years, no recommendation regarding its level in the diets of cattle has been advanced. Burton (2) reported that supplemental chromium enhanced the immune system function of periparturient and early lactation cows. Further research with this element will determine if supplementation of dairy diets is efficacious.

Additionally, we have known the metabolic roles for the vitamin choline for a long time, but fortification of dairy diets with choline has not proven beneficial due to the fact that choline is extensively degraded in the rumen. However, in experiments where choline was either infused post-ruminally or fed in a form that resisted degradation in the rumen, production of milk and milk components increased (5, 6). Further research evaluating the benefits of feeding ruminally protected choline may bring about recommendations regarding it=s supplementation in dairy diets. There is currently no consensus among the scientific community regarding whether or not these early indications will prove to be practically beneficial in dairy nutrition. If supplementing choline is beneficial for dairy cattle, further research must define the appropriate amounts and forms of supplementation of chromium and choline that will be most acceptable. Again, supplementation of minerals and vitamins in this fashion is not actually a balancing procedure, but a fortification to achieve improved animal production, reproduction, or health.

 # Conclusion

One must keep in mind that even though the micronutrients in the diet constitute only a small proportion of the mass of the total diet, they are essential for normal health and production. Diligence in evaluating feedstuffs for mineral content is essential for managing mineral nutrition. Careful balancing and fortifying of the diet with minerals and vitamins will help promote high production efficiency and avoid health problems related to minerals and vitamins. Minor errors in our attempts to fortify diets or Abalance@ the micronutrients may cause minor problems in our herds. These minor problems are often unnoticeable, but they can still negatively impact our production efficiency. However, we must not mislead ourselves into thinking that major health or production problems are the result of minor deficiencies or imbalances in the micronutrients. Too often, producers and consultants will discuss the possibility that a difference in a few parts per million of a certain trace element, or a few I.U. of some vitamin in the diet has caused an outbreak of illness or the development of severe reproductive inefficiency or a severe reduction in milk production. In many of these cases, other nutritive, herd health, or management factors have precipitated the observed problems, and the producer should focus his attention first on solving the major problems.

 # References

  1. Beede, D. K, W. K. Sanchez and C. Wang. 1992. Macrominerals. Page 272 in Large Dairy Herd Management. Van Horn and Wilcox, eds., American Dairy Science Association, Champaign, IL.
  2. Burton, J. L., B. A. Mallard and D. N. Mowat. 1993. Effects of supplemental chromium on immune response of periparturient and early lactation dairy cows. J. Anim. Sci. 71:1532.
  3. Cervantes, A., T. R. Smith, and J. W. Young. 1996. Effects of nicotinamide on milk composition and production in dairy cows fed supplemental fat. J. Dairy Sci. 78:105
  4. Church, D. C., ed., 1988. The Ruminant Animal. Digestive Physiology and Nutrition. Waveland Press, Prospect Heights, IL.
  5. Erdman, R. A. 1992. Vitamins. Page 297 in Large Dairy Herd Management. Van Horn and Wilcox, eds., American Dairy Science Association, Champaign, IL.
  6. Erdman and Sharma. 1991. Effect of dietary rumen-protected choline in lactating dairy cows. J. Dairy Sci. 74:1641.
  7. Erickson, P. S. M. R. Murphy and J. H. Clark. 1992. Supplementation of dairy cow diets with calcium salts of long-chain fatty acids and nicotinic acid in early lactation. J. Dairy Sci. 75:1078.
  8. Horner, J. L., C. E. Coppock, G. T Schelling, J. M. Labore, and D. H. Nave. 1986. Influence of niacin and whole cottonseed on intake, milk yield and composition , and systemic responses of dairy cows. J. Dairy Sci. 69:3087.
  9. Hutjens, M. F. 1992. Selecting Feed Additives. Page 309 in Large Dairy Herd Management. Van Horn and Wilcox, eds., American Dairy Science Association, Champaign, IL.
  10. Jaster, E. H., G. F. Hartnell, and M. F. Hutjens. 1983. Feeding supplemental niacin for milk production in six dairy herds. J. Dairy Sci. 66:1046
  11. McDowell, L. R. , J. H. Conrad, G. L. Ellis, and J. K. Loosli. 1983. Minerals for Grazing Ruminants in Tropical Regions. Department of Animal Science, Center for Tropical Agriculture, University of Florida, Gainsville.
  12. Miller, J. K., and F. C. Madsen. 1992. Trace Minerals. Page 287 in Large Dairy Herd Management. Van Horn and Wilcox, eds., American Dairy Science Association, Champaign, IL.
  13. National Research Council. 1980. Mineral Tolerances of Domestic Animals. Natl. Acad. Sci. Washington, DC.
  14. National Research Council. 1989. Nutrient Requirements of Dairy Cattle. 6th rev. ed. Natl. Acad. Sci. Washington, DC.
  15. Sharma and Erdman. 1989. Effect of dietary and abomasally infused choline on milk production responses of lactating dairy cows. J. Nutr. 119:248.
  16. Spears, J. W., E. B. Kegley and J. D. Ward. 1991. Bioavailability of organic, inorganic trace minerals explored. Feedstuffs 63(45):12.
  17. Underwood, E. J. 1977. Trace Elements in Human and Animal Nutrition. 4th ed. Academic Press, New York.
  18. Weiss, W. P., J. S. Hogan, D. A. Todhunter, and K. L. Smith. 1997. Effect of Vitamin E supplementation in diets with a low concentration of selenium on mammary gland health of dairy cows. J. Dairy Sci. 80:1728.