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Estimated
Acid Detergent Fiber (ADF), Neutral Detergent Fiber (NDF), and Relative
Feed Value (RFV) in Preharvest Alfalfa
Earliest Alfalfa Harvest in Years: How to Know When to Harvest
North Dakota's first alfalfa harvest will be earlier this year than
it's been in more than a quarter century--in some places as early
as May 26, according Dwain Meyer, forage management specialist at
North Dakota State University. That's why producers need to start
watching their fields right now to be sure they catch their crop
at the optimum harvest stage.
"People
who normally harvest about the 10th to the 15th of June will probably
be harvesting alfalfa in the last days of May to the first days
of June this year," says Meyer. "That's the time the crop
will reach the mid-to late-bud stage, the stage many dairy producers
like to shoot for."
The unusually
early crop is the result of three weeks of unusually warm April
temperatures combined with the right moisture.
How to evaluate
the optimum stage for harvesting alfalfa? "In the past the
recommended stage was the late-bud to the 10-percent-blooming stage,"
says Meyer. "But today there's a much more accurate system
available to measure the feed quality of alfalfa standing in the
field. It's called the PEAQ system--which stands for Prediction
Equation for Alfalfa Quality."
Here's how
to use it.
In a 30-acre
alfalfa field go into five different areas. Select the most mature
stem in each area and determine its maturity index using the following:
* Maturity index 2, the late vegetative stage--Stem is more than
12 inches tall, no visible buds or flowers.
* Maturity index 3, early bud stage--1 to 2 nodes with visible buds,
no open flowers.
* Maturity index 4, late bud stage--More than 2 nodes with visible
buds, no open flowers.
* Maturity index 5, early flower stage--1 node with at least one
open flower
* Maturity index 6, late flower stage--2 or more nodes with open
flowers.
In each of
the areas also measure the height of the tallest stem, from the
soil surface to the tip of the stem--not to the tip of the highest
leaf. You can pick the same stem that you've already selected as
the most mature, or a different one. Just so it's the tallest.
Next, take
the two numbers you've determined in one of the areas--the maturity
index number and the height of the tallest stem in inches--and use
them to solve the following two equations (the first equation gives
the alfalfa's NDF, or neutral detergent fiber, while the second
gives the ADF, or acid detergent fiber):
NDF=15.86 + (0.69 x height) + (0.81 x maturity index)
ADF=10.78 + (0.53 x height) + (0.79 x maturity index)
Do this same
thing for the other four representative areas you've selected, and
then average the five NDF numbers and the five ADF numbers so you
end up with an average NDF and an average ADF for the entire field.
Now convert
these two averages to the average relative feed value for the field
of alfalfa. To do this you'll first need to find the percentage
of digestible dry matter (DDM), then the percentage of dry matter
intake (DMI). Using these two numbers you can find the relative
feed value, or RFV.
Like this:
DDM= 88.9-(0.779 x ADF)
DMI= 120/NDF
RFV= (DDM x DMI)/1.29
"Producers
should shoot for a relative feed value of 150 to 160 in the bale,"
says Meyer. "But whatever the relative feed value of alfalfa
standing in the field, it will be less in the bale since there will
be harvesting losses. The average producer will lose about 25 units
of RFV in the harvesting procedure. That means if you want to harvest
prime hay at 151, you've got to start harvesting when the RFV of
alfalfa standing in the field is 175 or greater."
Many people
will find it easier to determine NDF, ADF and RFV by using tables
rather than equations. The tables below are available from any county
office of the North Dakota State University Extension Service.
Meyer notes
that recent rains have caused some lodging in alfalfa fields. He
warns that if alfalfa is lodged extensively, producers should consider
harvesting it immediately since at that point the alfalfa won't
improve. "You'll start to lose dry matter on the lower leaves
of the canopy, and overall dry matter may actually start to decrease
rather than increase in the field," he says.
Table
1: Estimated Acid Detergent Fiber (ADF) in Preharvest Alfalfa
Table
2: Estimated Neutral Detergent Fiber (NDF) in Preharvest Alfalfa
Table
3: Relative Feed Value (RFV) in Preharvest Alfalfa
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