The term mid-lactation milk fever (MLMF) has been used for years to reference lactating animals that abruptly go down in the pen and are unable to rise, much like a traditional fresh cow milk fever (hypocalcemia) due to low blood calcium levels.

Michael neil
Nutritional Consultant / Progressive Dairy Solutions
Michael received both DVM and MBA degrees from Purdue University and has over 30 years of industr...
Martin rod
Dairy Nutritionist / Protekta-USA

MLMF animals are commonly multiparous higher-producing animals in the pen and were “fine” the previous milking. These animals usually respond to calcium (Ca) and magnesium (Mg) treatment, unless recumbency extends beyond 12 hours or injury occurs – but the causes are somewhat different than calcium milk fever in fresh cows, which makes the term MLMF somewhat misleading.

Let’s review MLMF further to understand causes and solutions to reduce the risk of MLMF in your herd.

Nutritional factors

Unlike with a traditional calcium milk fever, MLMF is often due to low magnesium. Magnesium is not stored in the body and can’t be resorbed from bone, making constant absorption from the diet necessary. Multiple nutritional factors can lead to hypomagnesemia, including soil types, plant mineral content and magnesium antagonists that can inhibit absorption. Hypomagnesemia, often referred to as tetany, can occur in any feeding situation and presents itself very similarly to calcium milk fever with some subtle differences. Most often high-producing older cows in mid-lactation are affected. Recumbent cows often appear more alert than a traditional calcium milk fever, with paralysis occurring more so in the hind legs.

Regarding diet management, of particular concern are feeding forages high in potassium (K) because excessive levels of potassium in the diet can interfere with magnesium absorption. In addition, soils in which forages are grown can be low in pH, potentially yielding forages that are lower in magnesium. Potassium and magnesium imbalances can occur in any forage, including corn silages.


Feeding high levels of ensiled, baled or grazed grasses can also alter magnesium utilization and lead to hypomagnesemia.

Grasses, which includes corn silage, grown with specific soil conditions may accumulate a compound called trans-aconitic acid, which can tie up dietary magnesium and contribute to low blood magnesium conditions.

As a solution, nutritionists should test forages for major mineral content using wet chemistry. Nutritionists may also consider increasing magnesium levels in the diet to 0.4 to 0.45 to reduce incidence of low magnesium. Lastly, the source of magnesium in the diet should be evaluated since some sources are less available for absorption by the animal.

Inflammation and immune function

The gut is a major contributor to inflammatory response containing 75% of the animal’s immune cells and providing a barrier function to prevent absorption of toxins, pathogens and inflammatory compounds within diet ingredients. Recently, researchers at Iowa State documented that inflammatory responses in the gut (termed leaky gut) disrupt absorption of major minerals (like calcium and magnesium) and nutrients from the gut lining. Their research also has shown that inflammatory activation consumes glucose needed for milk production and reduces calcium levels to produce weakness or down cow syndrome (hypocalcemia). It is also known that the udder and mastitis can be a significant source of inflammatory events that result in reduced blood calcium levels and down cows in mid-lactation.

Finally, gut immune response releases inflammatory products into the bloodstream, leading to secondary disorders like pneumonia, laminitis and weight loss.

Common causes of inflammatory events

Traditionally, gut inflammation has been attributed to “infectious” bacterial sources like salmonella, clostridia or E. coli. Field experience shows many bacterial cases are opportunistic due to overgrowth of bacteria during altered gut conditions, especially common with high starch and/or lowered particle size diets.

Field experience and research has shown that multiple types of on-farm stressors alter rumen function and increase rate of passage. As a result, increased starch and undigested nutrients escape to the lower gut, breaking the protective barrier and leading to an immune response.

Common on-farm stressors include suboptimal bunk management, pen overcrowding and heat stress that all contribute to large meals being consumed (slug feeding), thereby altering rumen function and rate of passages. Producers should ensure ample non-sortable total mixed ration (TMR) in the bunk at all times with frequent pushups and provide comfortable cow environments to minimize the risk of these gut inflammatory events.

Finding the source of mid-lactation issues

In summary, both nutritional and inflammatory challenges may lead to mid-lactation mishaps – the following is a partial list to help dairy producers and their nutritionists determine the causes and necessary solutions:

  1. Forage sampling including major minerals using wet chemistry.
  2. Check bioavailability of magnesium sources in diet.
  3. Ensure adequate magnesium inclusion in high-production lactating diets.
  4. Test ration ingredients for possible toxins and pathogens.
  5. Evaluate bunk management and feeding behavior at the bunk using time-lapse cameras over several days.
  6. Feed research-proven compounds to stabilize the gut barrier and gut inflammatory responses. Contact your nutritionist for research-proven products that have been shown to reduce inflammation and maintain gut barrier function.

As described above, there are many factors that can lead to scenarios labeled mid-lactation milk fevers. Regardless of what name we give to the scenario, finding the causative reasons is critical. Producers are encouraged to work with their management team to identify risk factors that may lead to this mid-lactation down cow syndrome.