Timing Spring Alfalfa Harvest - The Final Word?
Current Weather · Forecast Products
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Timing Spring Alfalfa Harvest - The Final Word?
Mike Allen, Rich Leep, and Jeff Andresen Departments of Animal Science, Crop and Soil Science, and Geography
There is no doubt in most dairy farmers' minds that forage quality is an
important determinant of farm profitability. Poor quality forages increase
feed costs and limit milk production. The most important factor affecting
quality of alfalfa is maturity at harvest; as alfalfa matures protein
content and digestibility of dry matter and fiber decrease and fiber
content increases. However, harvesting too early reduces yield and results
in alfalfa that is difficult to feed because its quality is too high.
Although it is impossible to always harvest alfalfa at the optimum maturity
because Mother Nature doesn't always cooperate, there are new methods to
help you come closer to your goal.
What is your goal? It is important to have a specific quality goal in mind
for alfalfa harvest. Alfalfa to be fed to lactating dairy cows normally
should be harvested at 40% neutral detergent fiber (NDF). Why use NDF as a
criterion? Because NDF is an important factor for formulating diets; diets
with excess NDF content are more filling and limit intake and diets with
inadequate NDF content result in increased health problems such as
acidosis, displaced abomasums, and laminitis. Alfalfa should be harvested
at the lowest fiber content that will provide adequate levels of fiber in
the diet and allow a reasonable amount of grain to be added to maximize
energy intake and milk production. When diets are formulated with an
overly mature alfalfa (high NDF content), the NDF content of the forage
must be diluted with high concentration of grain to increase energy density
and reduce the filling effect of the diet. Because protein content is
usually lower for mature alfalfa, more protein supplement is required as
well. Besides being costly to supplement, lower NDF digestibility of diets
formulated using overly mature alfalfa might limit feed intake and milk
production, particularly for the highest producing cows. When diets are
formulated with an immature alfalfa, little grain can be included because
the NDF content of the alfalfa is close to the NDF required in the diet.
This will result in lower energy density and might result in lower energy
intake. Diets containing immature alfalfa also tend to have excess protein
content because the protein content of immature alfalfa tends to be very
high. Not only is feeding excess protein wasteful, but it costs energy for
the animal to excrete, might be an environmental contaminant, and might
reduce reproductive performance. This is a greater problem when immature
alfalfa is the only forage fed; it is easier to formulate diets with
immature alfalfa when a high NDF, low protein forage is included. A goal
of 40% NDF is often chosen for alfalfa because it is felt that this
optimizes profitability from a combination of factors influencing alfalfa
yield, supplement cost, and milk production. Some circumstances might
justify harvesting alfalfa earlier such as when the immature alfalfa will
be blended in a diet with a higher NDF, low protein forage. However,
harvesting immature alfalfa risks health of the plants and therefore the
productive life span of the alfalfa stand, particularly for spring harvest.
How do I attain my goal? There are several methods that can be used for
timing first alfalfa harvest to come closer to your quality goal than when
harvesting by calendar date. Cumulative temperature during growth may vary
greatly from year to year, resulting in large differences in quality on the
same date. In an experiment conducted in 1990 and 1991, we found that the
NDF content of spring harvest alfalfa varied up to 10 percentage units when
harvested on the same date from one year to the next. However, when the
difference in cumulative temperature between the years was accounted for
using growing degree-days (GDD), little difference in NDF content of
alfalfa was observed. Harvesting alfalfa by GDD is one of the methods that
can be used to get closer to your goal. Other methods include the scissors
cut, harvesting by maturity, or by a combination of maturity and height
using the PEAQ (Predictive Equations for Alfalfa Quality) method (see
below).
What are GDD and how are they calculated? Growing degree-days are a
temperature-derived index representative of the amount of heat that the plants
are exposed to, which in turn is directly related to the growth rate of growth
and development of the plant. GDD are calculated several ways, so it is
important to make sure GDD are calculated using the same formula that was used
to establish the relationship you will use to harvest alfalfa. For instance,
different base temperatures are used for different crops, and corn uses a
different method of calculation entirely. Alfalfa GDD used for recommendations
in this article are calculated by averaging the maximum and minimum temperature
for a given day (24-h period) then subtracting the base temperature of 41F to
get the number of GDDs for that day. Daily GDD for days with an average
temperature less than 41°F are set equal to 0. Finally, a seasonal total is
then obtained by summing the daily growing degrees from March 1st through the
current day. Example: if the maximum and minimum temperatures for one day
are 75F and 39F, respectively, the average is 57 and the growing degrees for
this day are 16 (57 - 41). Agricultural Meteorology Staff at MSU generate a
table of GDD using this calculation for alfalfa on Mondays and Thursdays
through mid-June of the growing season using data from weather stations
throughout Michigan. The alfalfa GDD results are broadcast on DTN and the MSU
Ag. Weather Web Site (at www.agweather.geo.msu.edu) as well as emailed to all
county extension offices in Michigan twice weekly. Note that because they
are calculated with a different method, the GDD published in the weekly MSU
Extension Crop Advisory Team Alert cannot be used for alfalfa harvest at
this time.
How close to can I get to my goal? It is important to realize that no
method will predict the NDF content of the alfalfa harvested perfectly.
There are many factors that affect the accuracy of prediction including the
amount of grass and(or) weeds in the field, proximity to the weather
station used, proximity to a large body of water, difference in elevation
from the weather station, adequacy of soil moisture, length of time
wilting, exposure to rain between cutting and chopping or baling, and
quality of fermentation for silage. Using GDD (base 41F), we can predict
NDF content within ± 3 percentage units 68% of the time and within 6
percentage units 90% of the time. Thus, if you are using GDD (base 41F)
to begin cutting alfalfa at 40% NDF, in 7 out of 10 years the alfalfa will
be between 37 and 43% NDF and in 9 out of 10 years it will be between 34
and 46% NDF. Although GDD cannot be expected to predict alfalfa NDF
perfectly, it is certainly better than harvesting by calendar date and it
seems to work as well or better than other methods in Michigan.
When should I begin harvesting? According to data collected in the upper
Midwest over several years, alfalfa averages 40% NDF at 750 GDD (base 41F).
It takes about 220 additional GDD to reach 45% NDF at GDD (base 41F).
Beginning harvest at 40% NDF will give about a 7 day window to complete
harvesting before the alfalfa reaches 45% NDF. If you are storing the
alfalfa in a horizontal silo or are feeding it in a diet with a high NDF,
low protein forage, start cutting at 680 GDD (base 41° F), which
corresponds to 38% NDF. Filling horizontal silos with layers of increasing
maturity will allow harvest to begin a little earlier because the layers of
alfalfa are blended as the alfalfa is removed from the silo. If a higher
fiber, lower protein forage is included in the diet it will provide the
necessary fiber and dilute the high protein content of the less mature
alfalfa.
What about alfalfa stands containing grass? Grasses have higher NDF
content than alfalfa when harvested at the same age and therefore older
alfalfa stands containing grass should be harvested first. Begin
harvesting the fields with the most grass first so that purer alfalfa
stands can be harvested at the appropriate NDF content. If wet weather
delays harvest and the fields with grass can be segregated for storage,
consider harvesting the purest stands of alfalfa first and harvest the
stands containing grass for the far-off dry cows and(or) heifers.
What about harvesting second and later cuttings of alfalfa? Predicting
alfalfa NDF using GDD cannot be done when there is inadequate soil moisture
during the growing season because GDD accumulate with little or no response
in plant growth. Because of this, GDD is highly related to quality only
for spring harvest alfalfa with adequate rainfall and not for subsequent
cuttings which nearly always have periods of inadequate soil moisture
during growth.
How much does it cost me to delay harvest? A lot! It costs as much as
$0.10 per cow per day for each unit of NDF increase past 40% NDF for
increased energy and protein supplement and for lost production from the
effect of lower NDF digestibility on dry matter intake. This amount is
even higher as production per cow increases because rumen fill becomes a
greater constraint on feed intake. Although there will be some increase in
yield by delaying first harvest, the cost of lower milk production and
supplemental energy and protein easily justify harvesting on time.
Should I continue planting corn or harvest alfalfa? When wet weather
results in delayed planting of corn it will usually pay to stop corn
planting to harvest alfalfa, particularly for high producing herds. Corn
yield is reduced approximately 1 bushel / acre per day when planting is
delayed in late May and alfalfa NDF increases about 0.6 units per day. For
a 100-cow dairy farm feeding alfalfa as the only forage, with 130 acres of
corn grain and corn valued at $2.50 per bushel, delaying planting 1 day
will cost $325. per year. However, assuming first cutting alfalfa is 40%
of the total annual yield, the cost of delaying alfalfa harvest for
supplemental feed and decreased milk production is over twice this amount.
What about measuring GDD on my farm? Although we can use data from nearest
weather stations for determining GDD, for increased accuracy it is best to
measure the minimum and maximum temperatures as close to the alfalfa field
as possible. You can use an inexpensive minimum / maximum thermometer and
read it every day from March 1 on, or you can purchase an electronic data
logger that records temperature automatically. Whichever you use it is
very important to place the temperature sensor correctly. The database that
was used to develop the NDF prediction equations from GDD used temperature
data from weather stations in which the temperature sensors are located in a
ventilated enclosure 5' above the ground. If an enclosure is not provided
with your sensor, a good, cheap alternative for a field enclosure is a small
wooden box (interior-grade plywood works well) mounted on a fence post, open
on one side and on the bottom, painted white with the open side facing north
and the open bottom towards the ground. Try to mount the thermometer off of
the surface of the enclosure (at least an inch) to facilitate free air flow
around the sensor. Also, never allow a thermometer to be exposed to direct
sunlight. Temperatures and resulting GDD sums taken from instruments that
are biased because of siting or exposure problems may be erroneous or
misleading.
What about other methods? Several alternative methods are sometimes used
to predict alfalfa quality: scissors cut programs, maturity, and the PEAQ
method. Scissors cut programs involve taking samples of alfalfa directly
from the field and sending the samples to a feed testing laboratory for
analysis. This method is thought by some to be the "gold standard" to
which all other methods should be compared because quality is actually
measured and not predicted from a formula. However, this method is subject
to other errors and might be no more accurate than the predictive methods.
The most significant potential error is that the NDF of the sample begins
to increase once the plant is clipped because sugars are converted to
carbon dioxide and water through respiration. Fresh alfalfa is very
unstable and there is no way to prevent respiration. Although cooling can
slow respiration down, the sample should not be frozen, as this will also
increase the NDF content. In addition, wet chemistry should be used
because NIRS (near infrared reflectance spectroscopy) equations for fresh
alfalfa are not generally available and NIRS predictions might be
inaccurate. This method is time consuming and costly, takes longer to get
results, and is not necessarily more accurate than using GDD.
Harvesting alfalfa by maturity is often used but it is difficult to
determine the average maturity of a field of alfalfa, particularly if there
is insect damage. This method takes some time and is no more accurate than
using GDD.
The PEAQ method requires sampling alfalfa stems from several locations in
an alfalfa field and determining length of the longest stem and maturity of
the maturest stem. These values are then inserted into a formula to
predict alfalfa NDF. Although this is not a difficult method, it takes
time to perform, is potentially subject to measurement errors, and has not
been shown to be better at predicting spring harvest alfalfa NDF than GDD
in two years of on-farm comparisons in several locations in Michigan.
Is this really the final word? Although we currently recommend using GDD
over other methods for spring harvest alfalfa, definitive studies comparing
the methods still need to be conducted over several years evaluating the
effect of sample preparation (hay vs. silage), analysis method (wet
chemistry vs. NIRS, and measurement of GDD on site. Because of this we
might change our recommendations, as more information becomes available.
For the time being, remember these points about using GDD to harvest
alfalfa:
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