Salt and Trace Minerals for Livestock, Poultry and Other Animals

TRACE MINERALS

There are seven trace minerals that have been shown to be needed in supplementing animal diets. They are iron, copper, zinc, manganese, cobalt, iodine and selenium. They are needed in very small amounts, or traces, in the diet, and hence their name, "trace minerals."   Many times their requirement is expressed as parts per million (ppm) or as milligrams per kilogram (mg/kg) of diet dry matter.  One ppm concentration is the same as one penny in $10,000.  With some minerals, such as cobalt the concentration in the diets may be expressed on a parts per billion basis (ppb). If there are approximately 300 million people in the United States, then each person is 3.33 ppb of the population.  Even though the concentrations are very low, they are still significant nutrients.  

Subclinical trace mineral deficiencies occur more frequently than recognized by most livestock producers. Currently this is a bigger problem than clinical mineral deficiencies, because the farmer does not see specific symptoms that are characteristic of a subclinical trace mineral deficiency. Instead, as shown in Figure 1, the immune system is depressed, the animal begins to grow more slowly, and fertility is impaired (306). The end result is inefficient production and lower profitability. Therefore, a profitable and efficient farm operation must provide the supplemental trace mineral elements. In highly competitive animal enterprises, it can be the difference between profit and loss.

Figure 1. Schematic depiction of the relationship between nutrient status and presence of subclinical or clinical disease manifestations. (Redrawn from S. Wikse, 1992, Texas A&M University Beef Cattle Short Course).

Deficient Area Problem

There are several examples where an area of the country was not recognized to be trace mineral deficient in the past but now has been shown to require supplementation. For example, a selenium deficiency was not considered a problem in the United States until relatively recently. Now at least 44 states have been shown to contain low-selenium areas. In only a few states have the classical selenium deficiency symptoms been observed, but performance responses have demonstrated the need for selenium supplementation. When a cobalt deficiency was first found in Western Australia, the problem was believed to be confined to about 5,000 acres. Further studies showed that at least 25 million acres are cobalt-deficient in Western Australia.

Another factor to consider is the shipment of feeds from one region to others. This alone makes it almost impossible to isolate areas of specific trace mineral deficiencies. There is nothing to prevent feed grown in a trace mineral deficient area from being shipped to another area where the feed grown is supposedly adequate in that mineral. For example, corn and soybeans grown in Midwestern states with areas deficient in selenium, iodine and other trace minerals are shipped to and fed in many other areas of the United States and the world. For example, selenium deficiencies have been observed in pigs fed U.S.-produced corn and soybean meal in Taiwan in 1978. Locally grown feeds in Taiwan are usually adequate in selenium.

The only precaution is to make sure the trace mineral is not fed in areas where selenium may be in excess. This can be accomplished by working with the local feed manufacturer or consulting nutritionist to determine if there are any potentially toxic minerals in that area. For example, sheep producers should be aware of the copper levels of locally grown feeds. If the feeds are high in copper and low in molybdenum, then feeding a trace mineralized salt with little or no copper is recommended. Most feed manufacturers will be aware of situations where near toxic levels of one of the trace minerals are present in feeds and will provide trace minerals mixes formulated accordingly.

TRACE ELEMENT CONCENTRATIONS OF FORAGES

Forages, either harvested mechanically or by grazing, are the basal dietary ingredients for beef cattle, dairy cattle, sheep and horses.  Based on the National Animal Health Monitoring System’s Beef Study (1997) only 9% of the cow-calf producers analyzed the nutrient content of their feeds.  This number is probably higher for dairy producers, but lower for sheep and horse producers.  With this in mind, Mortimer et al. (264) summarized feed analysis data from 709 forage samples, collected from 678 producers in 23 states.  The number of feed samples per state ranged from 98 for Texas to 4 for Florida and New Mexico.  The states were primarily those with major beef cow populations.  All sections of the U.S. were represented except for the Northeast.  Wet chemistry was used for the analysis of aluminum, copper, iron, manganese, molybdenum, sulfur, selenium and zinc.   

For purposes of analysis, data were combined into 11 different forage categories.  The copper, manganese, zinc and selenium concentrations were classified as deficient, marginally deficient, or adequate (Table 9).  The Maximum Tolerable Concentration (MTC) for each element is defined as the dietary level, when fed for a limited period of time, will not impair animal performance and should not produce unsafe residues in human food derived from the animals.

Table 9. Classification of Trace Elements in Forage Relative to Their Abilities to Meet Either Dietary Requirements or Cause an Antagonistic Problem with Copper

For trace minerals the concentration of one can have a great effect on the requirement of another.  For example, copper requirements are greatly affected by the concentrations of antagonists such as molybdenum, sulfur, iron, and zinc.  If molybdenum concentrations are below 1.0 ppm it will have no effect on copper requirement, but if molybdenum is above 3 ppm it can be very antagonistic to copper absorption.  Thus the relationship between iron, molybdenum and sulfur relative to copper is described as ideal, marginally antagonistic, or highly antagonistic.  These relationships apply to beef and dairy cattle, but may not be relevant to sheep.

Table 10. Alfalfa/Alfalfa Mix

The alfalfa and alfalfa mix was the largest forage category with 196 samples (Table 10).  Each trace mineral has a mean and standard error (S.E.) value.  For example the copper concentration averaged 10.5 ppm with a standard error of 0.32.  The 1996 NRC for beef cattle gives the copper requirement at 10 ppm.  On average it would appear that alfalfa based diets would meet this requirement, but when the individual samples are compared a different picture develops.  Only 41.84% of the samples were adequate with 57.65% of the samples being marginal and 0.51% deficient.  However, the copper:molybdenum ratios were such that 54.08% were marginal and 16.33% were highly antagonistic and could produce a copper deficiency.  In addition, the iron and sulfur concentrations were such that 26.02 and 48. 47% were marginally antagonistic and 8.67 and 18.88% of the samples were highly antagonistic, respectively.  These data illustrate the potential problems that can occur when just looking at mean book values to decide whether trace mineral supplementation is required.  While the mean value suggest that alfalfa is an adequate source of copper, in reality over 60% of the samples required copper supplementation.   Similar data were reported by Adams (284) from alfalfa and alfalfa-mixed forages grown in Pennsylvania from 1969 to 1973.  In that data set the standard deviation for copper concentration was over 5.7 ppm.  With that sort of variation, book values are relatively meaningless.

Manganese was marginal in approximately 24% of the samples with 1.53% being deficient.  In contrast, only 13.78% of the samples contained adequate zinc with approximately one-third being deficient (Table 19).  With selenium, 54.59% was adequate, 20.41% marginal, and 23.98% deficient.  The MTC was exceeded by 5.61% of the sample for molybdenum.     Even though alfalfa is often considered an excellent source of trace minerals, these data show that copper, zinc and/or selenium are often deficient are require supplementation to optimize cattle performance.

Brome is a common forage fed to cattle, sheep and horses in the Midwest (Table 11).  Copper was marginal in 75% and manganese was adequate in 85% of the samples tested.   Only 5% of the samples were adequate in zinc and 80% were deficient.  Selenium was marginally deficient in 25% and deficient in 45% of the brome samples.  These brome samples did not contain highly antagonistic concentrations of iron, molybdenum or sulfur.  With only 20 samples in this data set, these results are less conclusive than with some of the other forages.

Table 11. Brome

There were 120 Bermuda grass samples analyzed and they came primarily from the Southern tear of states in the U.S (Table 12).  Although there were no samples that were definitely deficient in copper, approximately two-thirds were marginal.  Nearly half the samples were marginally deficient in zinc, and over half were deficient in selenium.  Sulfur levels were reasonably high (0.27%) such that approximately 34 and 45% were highly or marginally antagonistic to copper. Over 8% of the samples exceed the MTC of sulfur.    If animal diets were based on Bermuda grass, manganese is the only trace mineral not required as part of a routine supplementation package, according to this data set.

Table 12. Bermuda