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Analyze forage minerals to avoid animal health issues

Rebecca Kern for Progressive Forage Published on 01 March 2020

Each year I consult with several different producers regarding mineral issues in their feed. Some common things I hear are: “They just aren’t thrifty,” “They aren’t gaining like they should,” or, “We had more open cows than I was expecting.”

In some cases, producers are also working with a veterinarian because animal health has been severely affected. Many livestock producers use forage reports to ensure proper protein and energy supplementation but pay little attention to the micronutrients provided by forage. Knowing the mineral content of available forages can help producers choose the best mineral supplementation program for their herd.

Mineral content of forages is variable. Data was compiled from Nebraska producer samples submitted to our laboratories for mineral analysis from 2012 to 2018 (3,522 samples). Not all minerals were measured on all samples. The samples were grouped into 14 categories of forages: alfalfa haylage, brassicas, corn silage, cornstalks, cover crop mixes, earlage, grass hay, legume hay, mixed hay, range, small-grain forage, small-grain silage, sorghum species and straw. These samples represent forages available to producers in Nebraska; however, the concepts extend beyond our state.

Calcium and phosphorous

These two minerals play critical roles in bone and muscle functions. Calcium (Ca) deficiency, excess phosphorous (P) and phosphorous deficiency can all result in diseases like rickets and osteomalacia. The optimal Ca-P ratio for beef cattle is 1.5-to-1 with an acceptable range of 1-to-1 to 4-to-1. The Ca-P ratio for most forage samples fell within the acceptable range. However, samples from all forage types had some that exceeded 4-to-1. It is also noteworthy that no sample was above the 1% maximum tolerable level of phosphorous. This indicates, in general, producers may be oversupplementing calcium, based on common mineral tag formulations for beef cattle.


Across all forage types, magnesium was variable with samples below cattle requirements (0.12% to 0.20%) and some samples above the maximum tolerable level (0.40%). Magnesium is of interest due to the deficiencies resulting in grass tetany. However, this data suggests producers should exercise caution when feeding a high-magnesium mineral supplement without knowing the level of magnesium in the forage. Supplementing magnesium to some forages could push the levels of the total diet to toxicity (Figure 1).

Magnesium dry basis

Toxic levels of magnesium will result in “unthrifty” animals.


High levels of potassium can interfere with magnesium absorption, also resulting in grass tetany. Potassium deficiency is rare but can result in poor animal performance and reduced body condition. Most samples indicated potassium concentrations were between beef cattle required levels and toxic levels (0.70% to 3%). However, this mineral also varied with concentrations in the analyzed forages ranging from 0.12% to 8.12%. The implications of this range affect supplementation levels needed for potassium and magnesium.


There have been many articles pointing to copper as the most prevalent deficiency in the U.S. beef herd. The forages analyzed support this observation. Across all forage types, most samples did not meet the livestock requirements of approximately 10 parts per million (ppm). Outlying samples that did meet requirements were far from the beef cattle toxic level (100 ppm). Copper in the forages we tested ranged from 0.60 to 61.5 ppm; however, the interquartile range (middle 50%, a measure of statistical dispersion) was between 4.30 and 7.5 ppm, meaning most samples did not meet livestock requirements. It should be noted that sheep and goats are especially sensitive to copper with a maximum tolerable level of only 15 ppm and 40 ppm, respectively. Some of these forages could be detrimental to small ruminant health, while copper could be supplemented to cattle without worries.


Cattle, swine and horses have no requirements for molybdenum. Small ruminants, however, do require between 0.1 and 100 ppm. Molybdenum binds copper, making it unavailable to the animal. Beef cattle have a maximum tolerable level of 5 ppm. Only grass hay, legume hay, range and sorghum had a few samples with molybdenum levels greater than 5 ppm. Across all forage types, the interquartile range was 0.54 to 1.56 ppm. Therefore, most forages may not have the issues with molybdenum tying up copper as is often discussed.


Iron deficiency is rare; all forages analyzed met or exceeded the beef cattle requirement (50 ppm). Iron did have a range in all analyzed samples from 31 to 7,316 ppm. It should be noted that bioavailability plays a key role in iron toxicity, and often samples with high iron content in the lab have high ash content as well, indicating soil contamination. Iron in the soil is not typically available to animals, and therefore does not contribute to the toxic level. Unfortunately, bioavailability isn’t something commercial testing labs can analyze. The interquartile range for iron was 148 to 354 ppm; therefore, most samples were well below the toxic level of iron for livestock species (600 to 1,000 ppm).


No samples were above the maximum tolerable levels of livestock species (500 to 2,000 ppm). Most samples were relatively close to animal requirements (40 ppm). The interquartile range was 33 to 73 ppm, meaning some forages meet animal requirements while others fall short. Analysis is the only way to know if supplementation is required for manganese.


Like magnesium, sulfur concentrations are highly variable. Sulfur toxicity has major animal health repercussions. While deficiency of sulfur results in poor performance, toxicity results in polioencephalomalacia (PEM). Cattle with this disorder are often referred to as “brainers.” Symptoms include blindness, “star gazing,” head pressing, excessive salivation and labored breathing. Supplementation of sulfur without knowing the baseline level provided by forages can contribute to the development of PEM. Sulfur levels in the forages analyzed ranged from 0.05% to 0.82% (Figure 2).

Sulfur dry basis

The toxic level of sulfur is 0.40% for ruminant species. Additional sulfur may be necessary to meet animal requirements, or it may result in toxic levels. It is important to know how much sulfur the forage is contributing to the diet. Brassicas are particularly known for having high levels of sulfur.


None of the forages analyzed were near the toxic level for zinc. All forages were very near the beef and small ruminant requirement for zinc (30 ppm). Samples analyzed ranged from 4.70 to 629 ppm with the interquartile range being 19.2 and 30.3. Consequently, most forages do not meet animal requirements and supplementation of zinc is generally required.

Producers who contact me to help them solve mineral supplementation issues have already lost income through poor animal production, the cost of improperly balanced feedstuffs and often vet bills. Knowing the mineral concentrations in forages and other feed supplements can avoid production losses and animal health issues. These diseases and disorders are easily preventable by proactively analyzing minerals in feedstuff. Producers accept the variability of protein and energy content in forages, and it is time to dig deeper and develop more precise mineral supplementation strategies through mineral analysis of forages.  end mark

All requirements and maximum tolerable level (toxicity) are based on the National Research Council references.

Rebecca Kern
  • Rebecca Kern

  • Animal Scientist
  • Ward Laboratories Inc.
  • Email Rebecca Kern