Agri News, MN
02-13-07
Thoreson grows concerned about high-potassium soils
By Jean Caspers-Simmet
Agri News staff writer
WAVERLY, Iowa -- Iowa State University Extension dairy specialist Dale
Thoreson is concerned about high potassium soils and the problems they can
cause for dairy cows that eat forage grown in them.
High potassium blocks magnesium absorption causing a sometimes fatal
condition called hypomagnesaemia.
"I hope this is never a problem for any of you, but I'm afraid we are getting set up
to have that problem," Thoreson said at last week's ISU Extension Dairy Day at
the Bremer County Fairgrounds in Waverly.
Central Wisconsin had big problems with hypomagnesaemia in 1997 with 180
cows from seven herds getting sick, Thoreson said. Two herds in northeast Iowa
had problems in the past several years. In Iowa at least six cows died.
Hypomagnesaemia occurs when the blood plasma level of magnesium falls
below 1.5 milligrams per deciliter. Typical symptoms of hypomagnesaemia
include downed cows and stall trauma.
The problem has shown up in total mixed ration-fed herds, Thoreson said. It's
been more prevalent in older, high-producing cows. Affected cows have a major
drop in milk production and muscle twitching can occur. Milk fever symptoms are
common, but when treated for milk fever, cows don't improve.
Magnesium is required to have normal muscle, nerve and bone functions,
Thoreson said. Normal levels of magnesium in blood plasma are 1.8 to 2.4
milligrams per deciliter. The major source of the mineral is in the cow's diet.
Thoreson said National Animal Disease Laboratory research shows that a cow's
requirement for magnesium is about 0.35 to 0.4 percent, higher than current
National Research Council requirements.
Today's forages contain more potassium, Thoreson said. As livestock becomes
more concentrated on farms, the land available for manure becomes limited.
"Plants are luxury consumers of potassium in that they will continue to
accumulate potassium as long as there is an available supply in the soil,"
Thoreson said.
Research at ISU's Nashua Research and Demonstration Farm on four forage
species shows this cumulative effect. Researchers were able to decrease
potassium content by cutting potassium fertilizer.
Pennsylvania research shows the cumulative affect of manure on potassium
concentration, but also shows the decline in magnesium in forages, Thoreson
said. As potassium increased, magnesium decreased. High levels of potassium
tie up or prevent the absorption of magnesium from the blood.
The sick Wisconsin cows showed a good response to intravenous magnesium
sulfate and magnesium oxide bolus. Farmers need to increase dietary
magnesium in rations to 0.35 to 0.4 percent of dry matter, Thoreson said.
Farmers need to reduce potassium in their feeds through proper fertilization or by
purchasing pre-calving hay that has a low potassium level as possible. Farmers
who raise low-potassium hay can get paid a premium for it. Forage mineral levels
must be measured with wet chemistry because near infrared spectroscopy isn't
accurate for mineral concentrations.
"We need to watch where and how much manure is applied to our fields,"
Thoreson said. "Manure contains significant potassium."
Farmers should soil test fields every three to four years and aim to keep soils in
the moderate range.