Editor’s note: This is the first part of a series about foraging through the weeds of climate change. Read part 2 "The biological risk of drought."

Bravo melissa
Livestock Herd Health Extension Agent / Rutgers Cooperative Extension

The first extreme precipitation event of 2021 hit Tennessee the end of March with a 12-hour deluge delivering 4 to 8 inches of rain in a swath tracking northeasterly across the eastern side of the Ohio River Basin.

At the same time, a two-decade-long mega-drought around the Colorado River Basin shows no signs of relenting. In contrast, parts of the Southeast have already received 10 to 35 inches of rainfall as of April 20, 2021. (See NOAA 90-Day Observed Precipitation Map.)

These year-after-grazing-year opposing precipitation extremes have both a short-term and long-term effect on availability of plant-translocated soil nutrients necessary to meet nutritional needs of grazing livestock. But they also can cause toxicities resulting in animal mortalities.

In an ideal world, forages, water and mineral supplements should supply livestock with adequate levels of seven essential macronutrients (Ca, P, K, Mg, Na, S, Cl) and 10 essential micronutrients (Fe, Mn, Cu, Zn, Se, Co, I, Cr, Mo, Ni).


When it pours and soils become saturated, mobile nutrients leach. Consecutive days (weeks) of overcast skies result in less oxygen produced (photosynthesis process) and more carbon dioxide exhaled (cellular respiration). Waterlogged roots become hypoxic, starving for oxygen. Waterlogged crops may have already lost nutritional quality beyond the smell and sight of mold, wilt and rot. What you can’t see or smell is also of concern.

  • How can we anticipate deficiencies or toxicities? Along with a current soil test that captures macro- and microelements, knowing soil hydrologic characteristics and how each soil type drainage is classified can give an indication what nutrients are likely to be deficit or excessive during and after too much precipitation. In particular, if you have “poorly drained” soil types, waterlogged for an extended period of time, take forage tests and monitor cattle weights and changes in cattle grazing behavior.
  • Before interpreting forage results: Understand the importance of knowing the dry matter intake (DMI) of forages in their current waterlogged state as a percentage of cows’ bodyweight. A cow consuming lush grass can easily consume 2.5% of her bodyweight in forage intake but may only be able to consume 1.8% of her bodyweight from low-quality forage with greater than 50% undigestible content.
  • Crude protein: A pregnant cow needs to intake forages with a minimum of 7% crude protein (CP) on a dry matter (DM) basis in the first trimester and 8% in the last. After gestation, mature cow CP need goes up relative to milk output and bodyweight (8.5% to 10.6%). First-calf heifer CP needs increase after calving (9% to 13.5%) as she continues to grow (“Nutrient Requirements of Beef, Oklahoma State University”).
  • Carbohydrates: Soluble carbohydrates can leach from waterlogged plants, especially after days of overcast skies. This is true of very mature hay, i.e., more structural carbohydrates remain. But stressed lush plants can also accumulate too many rapidly digestible carbohydrates (fructan, starch, sugars), leading to instances of laminitis. This is also true in overgrazed areas, where stems have concentrated carbs. In mature swards, carbohydrates naturally accumulate in seeds.
  • Calcium: Forage calcium deficiency can occur in waterlogged soils that have a high pH. At higher PH, more calcium becomes mobile. Older cows have higher demand for readily available calcium at parturition. Dietary calcium for mature cows ranges from 0.17% to 0.31% of DMI depending on age, weight and lactation stage. Heifers, on the other hand, are still growing and need diets containing 0.36% to 0.53% calcium in their diet (“Nutrient Requirements of Beef,” OSU).
  • Phosphorus: Root architecture has a lot to do with adequate phosphorus uptake. Expect phosphorus levels to be adequate in fibrous rooted grasses if soil levels are in optimum range and lower in waterlogged taproots and rhizomes. Heifer and cow daily phosphorus needs range from 0.14% to 0.26% of DMI depending on age, weight and lactation stage (“Nutrient Requirements of Beef,” OSU).
  • Potassium-to-magnesium ratio: Magnesium tetany occurs when magnesium levels are too low compared to potassium. Fertilize pastures with the lowest rate of potassium during the non-grazing season. Gradually bring up soil levels as opposed to applying the recommendation all at once as we do high-Mg lime. The worst scenario risk for grass tetany occurring is older lactating cows grazing lush forages on waterlogged soils with high pH, too high K and soil Mg levels less than 12% of the cation exchange.
  • Sodium: In addition to preventing plants from taking up adequate water, soils with excessively high sodium levels limit calcium uptake. Overwatered plants taking up too much sodium wilt. Immediately after a wet period, nitrate levels tend to be at their highest in immature wilting vegetation, especially if recently manured or fertilized with N.
  • Sulfur: Animals are likely to be deleteriously ingesting too much sulfur as opposed to too little. Flooding and atmospheric deposition only adds to this worry. Blind staggers can be a result of sulfur toxicity. Maximum dietary intake is 0.4% on a DM basis.
  • Manganese: This reproductive fitness element is more likely toxic in plants on waterlogged, low pH, poorly drained soils. Dietary intake in cattle should not exceed 1,000 milligrams per kilogram. Weather factors, aluminum and iron uptake occurring at the same time influences manganese and iron availability. Manganese deficiencies have a negative impact on reproductive success. Too little iron in animal diets leads to anemia. Iron toxicity is rare but has occurred from ingestion of processed feed containing excessive levels of iron.
  • Copper: When levels of copper in the diet exceed 100 parts per million (ppm) in cattle, toxicity occurs. Even 40 to 50 ppm could have toxic effects. Pay particular attention to flood-deposited sediments containing copper on plants already high in copper. Conversely, marginal copper levels in diet negatively impact cattle fertility, and copper is less available for plant uptake in continuously waterlogged soils.
  • Selenium: As oxygen levels return to flooded soils, selenium becomes more mobile. In situations involving repeated flooding and excessive rainfall, test soil and forages for changes in selenium, every year, before the third trimester. It’s not uncommon for soils testing very low in selenium (less than 0.2 milligram of Se per kilogram) to cause symptoms of white muscle disease and cardiac problems in newborns.

Too much selenium is lethal. This is why the FDA regulates selenium addition to feed and minerals. Selenium is not only atmospherically deposited, it is a natural element of phosphate fertilizers, used in the production of electronics and glass manufacturing. Toxic selenium concentration in forage analyses is 2 to 5 milligrams per kilogram of DM. Cattle total dietary intake from all sources should not exceed 3 mg Se per head.

  • Cobalt: Cobalt is essential for vitamin B12 structure. Cobalt levels toxic to plants and of concern in cattle diets is more likely to occur on acidic soils. Total dietary intake of cobalt levels should not exceed 0.1 ppm, including mineral supplements. Remember, sheep are much more susceptible to higher cobalt levels. Due to the potential for flood-related contamination from industrial sites upstream (at any time in history), soil cobalt greater than 40 ppm indicate need to test water sources in addition to forages.

Conversely, if soils test below 2 ppm of cobalt, deficiencies in the diet may arise. Cobalt is more mobile in flooded soils. Cobalt-deficient, moderately deficient and severely deficient areas, and other nutrients like selenium are found in “USGS, 2014, Geochemical and mineralogical maps for soils of the conterminous [contiguous] United States.”

Zinc, iodine and nickel are most likely to be added to cattle diets as supplements without knowing if cattle diets are deficient. If flooding has occurred, take a close look at all sources of dietary intake. Know river and upstream heavy metal sediment content and test for the entire spectrum of industrial pollutants. Atmospheric deposition only increases the likelihood of excessive levels.

Don’t forget to test livestock water sources. If soil tests, forage tests or floodwater-deposited sediments test high for trace minerals, test all sources of livestock water for the full panel of elements (30-plus), not just the 10 essential micronutrients.