It's always difficult to single out a mineral or vitamin that we can consider as being "the most important" when it comes to milk production or overall cow health. Potassium is one of those minerals that cows need lots of when milking, but they hardly need any as dry cows and when getting ready to calve. Because most of the potassium consumed in dairy cow diets comes from forages, it's rarely in short supply. However, because forages are high in potassium, overfeeding of potassium can be problematic at the time of calving.
Potassium is absolutely necessary for many biological processes in mammalian bodies. Because of its cationic characteristics, it's a major player in the cationic-anionic exchange function for cellular metabolism. It's also essential for cardiac, skeletal and smooth muscle function, as well as the maintenance of normal kidney function. Milk actually contains more potassium than it does calcium.
Even though potassium nutrition tends to be straightforward, there are three areas of dairy cow management in which potassium nutrition must be addressed:
- Heat stress and sweating can affect potassium balance.
- Potassium can interfere with magnesium absorption.
- Potassium can interfere with calcium metabolism at the time of calving.
Having a potassium deficit in milk cow diets is seldom an issue, due to the fact that forages contain high levels of potassium. Grasses and legumes take up more potassium than they need and tend to average about 2.5 percent potassium. However, on fields or pastures that are fertilized heavily with cow manure, potassium levels can be much higher. I've seen grass samples come back with over 5 percent.
When balancing feed rations for milk cows, the recommended potassium level is 1 to 1.5 percent of total feed dry matter intake. However, when cows are sweating and panting due to extremely hot and humid weather conditions, the potassium level should be raised to 2 percent of dry matter intake. Since potassium plays a critical role in the osmotic and acid-base balance of the body, cows can quickly become ill from an electrolyte imbalance.
The challenge with maintaining a proper potassium level in heat-stressed cows is keeping them eating. Even though diets may be formulated with enough potassium, cows' appetites often diminish while they are subjected to excessive heat and humidity. Water, while absolutely necessary for survival in hot environments, will not supply minerals and vitamins like our sports drinks do. They also need electrolyte replenishment when sweating copious amounts.
Because of similarities in the atomic structure of many elements, they can interfere with each other's functions. One of the most well-known examples is that of copper deficiencies caused by excessive molybdenum or sulfur in a diet. The enzymes that need the copper are "fooled" due to the similarities of molybdenum or sulfur, and even though there may be adequate copper in the diet, the copper is not properly metabolized. A similar situation occurs with potassium and magnesium. Too much potassium in a ration will hinder the absorption of magnesium.
This is the cause of grass tetany, a magnesium deficiency more often found in beef cows, but found in pastured dairy cows as well. In most dairy cow rations, magnesium levels are set at around 0.35 to 0.38 percent of feed dry matter intake. Another rule of thumb is to maintain a potassium-to-magnesium ratio of less than 5-to-1 in the ration. In other words, never let the potassium level exceed five times the magnesium level in the diet. If potassium is at 2 percent due to an elevated heat stress situation, for example, then magnesium should be at least 0.4 percent of dry matter.
The third issue concerning potassium in dairy cow diets occurs during the weeks prior to calving. The excessive presence of potassium in the diet will interfere with calcium absorption, which is critical for a cow's muscle function and milk production. Potassium levels in pre-fresh diets should be low and monitored to avoid metabolic problems around the time of calving. Milk fever (hypocalcemia) is the primary concern with high-potassium diets at the time of calving.
Even though anionic diets with elevated sulfate and chloride levels are well-researched and proven to counteract milk fever problems in fresh cows, focusing on forages low in potassium will reduce the need for anionic diets. When grass silage (haylage) greater than 4 percent potassium makes up a major portion of the diet, potassium intake generally exceeds the recommended level and will often result in hypocalcemia, which can then exacerbate other metabolic diseases, such as retained placentas, twisted stomachs and ketosis. Oftentimes, fields receiving only minimal amounts of manure will produce forages with potassium levels lower than 2 percent. Nitrogen can still be applied.
Nutrient management plans for dairy farms should include the application of nitrogen and the spreading of manure on selected fields, which will enable dairy farmers to grow forages that will limit potassium intakes in pre-fresh cows and maximize potassium in forages fed to lactating cows that will benefit most from potassium. As legumes and grasses mature, their potassium content declines - a good choice for dry cow forage. Grasses at flowering may have half the potassium concentration of immature forages earlier in the season. This is just one more reason why forages should be harvested in the early vegetative stage for lactating cow diets, since both potassium and protein will be higher.
Feeding forages with low potassium levels is advised for close-up dairy cow diets. Forages high in potassium should be discontinued to close-up cows at least three weeks prior to calving. Forages must be analyzed by a reputable forage testing laboratory.
Potassium has been shown to increase milk production through its natural buffering ability, allowing cows to increase feed intakes. Once cows have calved, the potassium level in the lactation ration should be increased immediately. The most efficient and cost-effective source of potassium in lactating diets is through high-quality forages.
The author is a dairy nutritional consultant and works for Central Connecticut Cooperative Farmers Association in Manchester, Conn.