The potassium (K) content of harvested alfalfa has become an increasingly important issue in recent years. As most dairy producers already know, high levels of potassium in forages has been identified as the causative factor for milk fever in dry and transition cow diets. This has focused the attention of some hay producers and buyers on forage tissue K content on a near level playing field with the more common qualitative factors of RFV, CP, ADF, and NDF. The following are some thoughts that may help to keep tissue K levels at manageable levels. Use wet chemistry
to evaluate K concentrations
The type of analysis used doesn’t impact the actual tissue K level, but
it does have a bearing on the accuracy of the analysis.
Although NIR has its place, determining mineral levels of forages
is not one of them. Do not buy, sell, or feed forage based on an NIR
analysis for K concentration. If
this is important, spend the extra money to have the determination done
with wet chemistry techniques. Strive for
reasonable soil test K levels
It’s well documented that alfalfa will take-up K beyond its needs
if high levels of soil or applied K are available.
This is referred to as “luxury consumption”.
Typically, alfalfa yields plateau at about 120 ppm soil test K. At this level, approximately 200 lbs. of K2O need
to be applied annually as topdress to maintain soil test levels.
Little to no yield response to topdress K is experienced when soil
test K exceeds 150 ppm. If
soil tests are between 120 and 150 ppm, never topdress more than 160 lbs.
of K2O. Applying
fertilizer K beyond these recommended levels provides a double whammy of
no increase in yield and elevated tissue K levels. Cut
alfalfa as low as possible
Potassium concentrations in the alfalfa plant tend to increase with
height up the stem. Additionally,
more K tends to be found in the stem compared to leaves.
Cutting height research in Wisconsin and North Dakota already
points to the yield benefits of cutting alfalfa as low to the soil surface
as possible (given machinery and stone limitations).
Here is yet another reason not to leave a high stubble. If possible, separate late-cut alfalfa for dry transition and dry cows
The concentration of K drops dramatically with advancing maturity.
In most cases, this is not a reason to delay alfalfa harvest, but
for those where cutting is delayed by weather or for other reasons, expect
this to be your lowest K forage when all other factors are equal. Feed
some grass???
Conventional feeding theory sometimes
throws this recommendation into the mix.
However, grasses tend to be more efficient than legumes in their
ability to extract K from the soil. For
this reason, grasses will often be at or above tissue K levels reported
for alfalfa. The reason some
grass hays are lower in K than alfalfa is because they are more often
grown on fields or soils with inherently less fertility.
Adding a grass to your forage establishment mix will not
necessarily result in lower tissue K levels of the harvested forage. Timing
of fertilizer topdress application
This one is saved for last
because at this point it is more Rankin theory rather than proven
scientific fact. My assertion
is that topdress fertilizer K be applied after the final harvest of the
year rather than after the first (or split between first and third) in
situations where total actual fertilizer applications are less than 400 to
500 lbs. per acre. I base this assumption on the fact that alfalfa will usually
take-up more K than it needs following a topdress fertilizer application
(luxury consumption). If
fertilizer is applied after first cut, it is likely there will be a spike
in tissue K concentration of the second cutting.
To avoid this, why not put the fertilizer down following the last
cutting? I can potentially
see several advantages:
It’s
merely a theory at this point but perhaps one worth considering if forage
tissue K is an issue.
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