By: Adele Harty, Cow/Calf Field Specialist, SDSU Extension
Potassium may often be an underappreciated mineral in the world of ruminant nutrition, but it plays key roles in the body. Potassium is involved in acid-base regulation, osmotic pressure maintenance, nerve impulse transmission, muscle contraction, and carbon dioxide and oxygen transport. Potassium works with sodium and chlorine to maintain the acid-base balance. We often hear of an animal’s salt requirement, but why don’t we hear about potassium? The concentration of potassium inside cell walls is nearly equal to that of sodium in the extra-cellular fluid, therefore the diet requirements are significant.
The answer is that potassium is more abundant in forages. Recently balanced rations for backgrounded calves have indicated excessive or toxic levels of potassium. Upon review of the feed analysis, potassium levels are often in excess of 2.5 percent on a dry matter basis and readily available for absorption. Requirements are approximately 0.65 percent for 550 lb steers, therefore forage based diets have excess potassium. What result do these excess levels have on cattle over an extended period of time? Some minerals have very detrimental effects when they are fed in excess or at toxic levels. One example is when excessive or toxic levels of sulfur are supplied, cattle can develop polioencephalomalacia.
So what happens with toxic levels of potassium? Unfortunately the answer is not clear cut. Dr. Terry Engle, a Professor at Colorado State University has a research focus on trace minerals. I asked him some questions regarding potassium and at what point do we need to be concerned about toxicity and what potassium toxicity looks like. Dr. Engle referred to the "Mineral Tolerances of Animals" Second Revised Edition 2005. This book discusses the chemistry of minerals and how they work in the body. Potassium is key to body function, but deficiency is outlined better than toxicity. In most cases toxicity is rare as excess potassium is excreted in urine, however this document also states 3 percent potassium as the maximum tolerable level. As the levels of potassium increases, magnesium absorption in the gastrotintestinal tract will decrease, which could have a resulting effect of milk fever or grass tetany depending on the forage situation. Aside from the issues with magnesium absorption, levels up to 6 percent potassium have been added to the diet of non-lactating animals with no toxicity symptoms being evident.
Why have potassium levels increased in forages? There have been some recent trials from the University of Illinois evaluating soil potassium levels and how they increase in the soil. They did a four year trial with bi-weekly testing of potassium levels to see how the levels change over time. The study showed that in order to produce a 200 bushel corn crop, 46 lbs of potassium is removed with the grain, while the residue puts 180 lbs of potassium back into the soil. This is three times more than the next crop needs and is all readily available. So as the plants go through their life cycle, a small portion of potassium is taken into the seed, with a larger portion going back into the soil from the residue.
There are a few take home messages with potassium:
- Potassium is crucial for many metabolic systems in the body, but excess potassium is readily excreted in the urine.
- The excess levels will have a more negative effect on lactating females than on non-lactating cattle.
- Potassium fertilization of grains is not necessary in many situations as the grain takes a smaller portion of potassium out of the soil than the residue puts back in.
- Most potassium sources are readily available, therefore when there are higher levels in forages, that potassium will be nearly 100 percent available.