As a veterinary pathologist, I've clinically studied and performed necropsies on numerous cattle with grass tetany for more than 30 years. In this time, I've observed a relationship between grass tetany and reproductive losses in cattle and horses grazing the same pastures, especially lush spring pastures after drastic changes in climatic conditions.
High potassium, particularly spikes in potassium during sudden climatic changes, is now known to be associated with grass tetany. I believe high potassium may also induce other disease conditions including reproductive losses in cattle, horses and other herbivores.
The old theory held that grass tetany in cattle was caused primarily by magnesium deficiency in soils and grass. We now know grass tetany (see “Watch for grass tetany” p. 14, April 2003 BEEF), milk fever and, likely, downer cow syndrome are primarily induced by high-potassium forages.
During cool, wet conditions — including situations that frost and freeze cool-season grasses — sodium, calcium and magnesium decrease in forages, while potassium increases.
Such sudden shifts in cool-season grasses can be catastrophic during drastic weather changes. Not only are soils affected, but the grasses grown on them. The most dramatic shift seemingly occurs in prolonged droughts, followed by sudden changes after abundant rainfall, along with frosts and freezes to forages, then sudden heat elevations that cause a rapid grass growth in frost-damaged plants.
A Sudden Shift To Discovery
Such a shift occurred in the Midwest in spring 2001 when reproductive losses occurred in several thousand horses, cattle, sheep and goats. During the same period, numerous cattle were found dead in pastures a few hours after frosts and freezes to lush, rapidly growing grasses.
Horses and cattle examined postmortem had very elevated levels of potassium in their eye fluid. Meanwhile, cattle were affected in some pastures but not others. We also discovered that affected pasture forages had very elevated potassium levels and wide potassium-to-calcium ratios.
Meanwhile, cattle in adjacent pastures on similar forage types with access to loose sodium chloride were less affected. This is consistent with my observations over the past seven years that producers feeding loose sodium chloride salt rarely have grass tetany in their cattle. Conversely, cattlemen feeding high levels of magnesium and other macronutrients (with a low level of sodium chloride salt) in mineral mixes frequently have grass tetany and downer cow syndrome in their herds.
Excessive potassium in forages not only causes metabolic disorders like grass tetany and milk fever, but seems to be associated with immune suppression followed by a host of opportunistic and infectious diseases of livestock. Excessive potassium and deficiencies of calcium and sodium may induce imbalances of macrominerals as well as essential micronutrients like copper and selenium that are essential for the immune system.
Potassium promotes the overgrowth of saprotrophic (microorganisms that normally grow on dead matter), commensal (organisms that live together but don't harm each other) and pathogenic (microbes that cause disease) microorganisms in plants, especially plants damaged by droughts, frosts and freezes. Thus, such forages become the source of many opportunistic, potentially pathogenic bacteria and fungi.
After ingesting them, livestock face an overgrowth of opportunistic, pathogenic organisms in the gut. The organisms rapidly proliferate to produce toxic by-products, like excessive ammonia, which is acutely toxic to fetuses and the immune system.
These pathogens infect not only the foraging animals but their fetuses. Early and mid-term fetuses may abort, while near-term fetuses may suffer premature birth, and/or septic weak neonatal birth.
Similarly, it's felt that high-potassium forages encourage excessive growth of endophytic and other pathogenic fungi, especially in fescue and rye grasses. The toxins these fungi produce add to the reproductive problems in cattle and horses.
Feeding readily available sodium appears to counteract excessive forage potassium. It also discourages overgrowth of potentially pathogenic opportunistic organisms in the gut, while potassium encourages their overgrowth in the gut.
One solution to high-potassium forage is to apply potassium and nitrogen to pastures in optimum amounts for the desired forage growth. Fertilizing with high-calcium lime will decrease forages' potassium-to-calcium ratio, and likely discourage the growth of endophytic and other pathogenic fungi in pasture forages. Such practices must be ongoing, as years will likely be needed to correct severely imbalanced soils.
Since chloride drastically increases in forages when potassium levels spike, it's theorized that sodium bicarbonate may neutralize the effect of excessive chloride as well as potassium. In fact, many dairymen force-feed sodium bicarbonate, as well as sodium chloride, to neutralize excessive potassium while reducing many metabolic disorders in high-producing dairy cows.
Consider Supplementing Forage
Most forages don't contain enough sodium to meet herbivores' essential requirements for proper electrolyte and mineral balances, so it must be supplemented. A paradox is that cattle and horses appear to consume more sodium chloride salt when grazing more mature forages or forages that are dormant during the winter, which contain less potassium. Thus, metabolic disorders like grass tetany are less likely to occur when cattle graze mature forages.
However, in early spring and early fall during climatic conditions that favor increased potassium in forages, herbivores consume less sodium chloride. It's theorized that this induces acute mineral imbalances.
Since potassium and sodium are similar and may substitute for each other, herbivores may not be able to detect a critical sodium deficiency, as their desire for salt seems to lessen when they graze high-potassium forages. The desire for sodium is apparently being fulfilled with the cation potassium, which is excessive in the ration.
Consequently, they don't consume enough sodium for essential electrolyte and mineral balances. Acute metabolic disorders, and spikes in abortions in horses and cattle may be associated with an acute sodium deficiency due to these potassium spikes, especially after frosts to forages in the spring and fall.
Because of high potassium in forages, force-feeding sodium chloride and sodium bicarbonate is essential. Ironically, even when sodium chloride is force-fed at levels recommended on a dry weight basis, many animals will consume additional sodium chloride if available free-choice, which I recommend, in addition to force feeding.
Observations in horses, primarily pregnant mares, show similar benefits to force-feeding salt in the feed, as mares won't consume enough free-choice sodium to neutralize the excessive forage potassium. Placental edema and secondary bacterial infections are less likely if sodium is force-fed to horses and cattle.
When the only supplemental source of sodium is free-choice, cattle and horses can't consume enough sodium chloride from hard salt blocks or hard trace-mineralized salt blocks in periods of acute need. If hard salt blocks are used, additional loose salt must be readily available to all animals during periods of acute needs.
Also, sodium chloride shouldn't be limited in mineral mixes. If large amounts of macro-minerals are fed free-choice, feed them separately, in addition to the sodium chloride salt fortified with trace amounts of macro- and micro-nutrients. Naturally balanced mineralized sea salts are high in sodium chloride and are excellent sources of many essential macro- and micro-minerals.
Be sure to locate salt and mineral mixes near water sources. Readily available, fresh water is essential for animals force-fed sodium, or sodium toxicity can occur.
In addition, be sure to balance rations for all macro- and micro-nutrients. If animals have adequate, readily available sodium and calcium, all macro- and micro-nutrients will more likely remain balanced and induced deficiencies are less likely to occur.
Soils and forages constantly change. Sudden changes in environmental and climatic conditions can, and frequently do, induce mineral imbalances in forages. Farm managers, veterinary clinicians and pathologists, soil and forage specialists, and nutritionists must recognize these induced mineral imbalances in forages. All need to work together to solve the complicated, multi-factor and emerging livestock diseases associated with high-potassium forages.
T.W. Swerczek, DVM, PhD, is a professor in the University of Kentucky's Department of Veterinary Science. He can be contacted at [email protected].