Linneus, Linn County
* September 25, 2013
* October 1-3, 2013
21262 Genoa Road
Linneus, MO 64653
Phone: 660 895-5121
FAX: 660 895=5122
AND YIELD RELATIONSHIPS
IN GRAZED PASTURES
Introduction: Estimation of available forage dry matter is an important tool
for livestock managers in making decisions about pasture allocation. A
simple, inexpensive method for estimating available forage is to measure
height and use an estimate of forage yield per acre-inch as a multiplier to
calculate standing available forage. However, forage yield per acre-inch is
not a constant value but depends upon sward density, species composition, and
height. Grazing management can affect all three sward parameters, so there
is question whether the same yield:height relationships hold true for
continuously and rotationally stocked pastures. Data collected from one
grazing season at FSRC were analyzed to determine how much variance exists in
the sward height:forage yield relationship between continuously and
rotationally grazed pastures. We have collected yield:height measurements
for a number of years and have tried to develop useful tools to help graziers
make pasture allocation decisions.
Materials and Methods: Dry matter yield and mean sward height data were
collected in a grazing study conducted at FSRC during the 1996 grazing
season. Pastures were either continuously (CS) or rotationally (RS) grazed
at four different stocking rates from April 25 through August 31, 1996.
Swards consisted of mixed cool-season grasses and legumes with some presence
of both annual and perennial warm-season species. Tall fescue, Kentucky
bluegrass, and orchardgrass were the primary grasses with red clover, white
clover, and birdsfoot trefoil being the dominant legumes.
Continuously grazed pastures were sampled bi-weekly along four defined
transect lines with three sample sites per transect. Four paddocks out of the
12 paddocks in each rotationally grazed pasture were sampled during each
grazing cycle before and after each grazing period. Each paddock had three
sample sites. Within each sample site in both CS and RS pastures, three
quadrats (3.25 ft2) were clipped at 1/2-in.height. A correction factor for
dead material was subtracted from the total yield and dry matter yield of
green forage (GDMY) was calculated.
At each quadrat site, mean sward height was determined using a simple
yardstick. Green forage DMY for each quadrat was divided by the measured
height at that site and GDMY/inch was calculated. Yield X height
relationships were determined through linear regression using both linear and
Results and Discussion: For the type of pasture used in this study,
height:yield relationships were significantly different for CS and RS
pastures (Figures 1 & 2). At sward heights less than 4 in., CS pastures
typically were lower yielding than RS pastures at comparable heights. Above
4-in. height, CS pastures were predicted to have higher yields than RS
pastures. This difference is likely due to reduced stand density occurring
in CS pastures when grazed below 4 in.. If continuous stocking is light
enough to allow growth to be significantly greater than 6 to 8 in., dead
material rapidly accumulates in the sward resulting in higher total DMY but
reduced GDMY. The correction factor used in this study for dead tissue was
a fixed quantity and was not adjusted for the greater dead material
percentage at higher sward yields.
Total DMY/acre-inch was significantly higher for RS pastures at lower
sward heights, but GDMY/acre-inch at sward heights less than 4 inches was
actually higher for CS pastures (Figure 2). Percentage of bare ground was
somewhat greater in CS pastures compared to RS resulting in lower DMY/acre-
inch for CS pastures. Also, CS pastures had very little dead residue below
4 inches while RS pastures had significantly more dead residue below 2 to 3
inches because RS pastures were rarely grazed below that level. As stand
density is a critical factor in the height:yield relationship, any use of
height as a predictor of yield should be tempered with an adjustment factor
for stand density. In a previous study, we reported the successful use of
multiple regression prediction equations which used both visual estimates of
species composition and ground cover coupled with sward height to predict
both GDMY and component yields.
In a practical application, producers are not likely to use such complex
techniques to estimate forage yield. Another approach to addressing the
stand density and species composition factors is to develop a table of forage
yield per acre-inch for different pasture types at different stand densities.
We have successfully used such a table in field exercises with producers in
grazing schools at FSRC (Table 1). Each pasture type X stand density cell
shows a range in expected GDMY/acre-inch. The lower yield can be used for
taller swards while the higher yield can be used for shorter swards. As an
aid to producer use, NRCS in Missouri has produced sward sticks with this
table printed on the sides of a square yard stick. The yard stick can be
used for determining stand density with the point-step method, measure sward
height, and serve as a reference table for yield information. These results
indicate that grazing management does in fact affect height:yield
Table 1. Estimated dry matter yield per acre-inch for various forages at
three stand density levels.
Forage 60 - 75% 75 - 90% > 90%
Tall Fescue + N 250 - 350 350 - 450 450 - 550
Tall Fescue + Legumes 200 - 300 300 - 400 400 - 500
Smooth Bromegrass + Legumes 150 - 250 250 - 350 350 - 450
Orchardgrass + Legumes 100 - 200 200 - 300 300 - 400
Bluegrass + White Clover 150 - 250 300 - 400 500 - 600
Mixed pasture 150 - 250 300 - 400 400 - 500
Height:yield relationships can be taken a step farther as a simple
method for allocating pasture without worrying about estimating actual yield
or intake requirements. If we compute the average for each stand density, we
come up with 215, 333, and 450 lb/acre-inch for thin, average, and thick
pasture, respectively. If we assume that a 1000 lb lactating cow will
consume around 3% of her bodyweight, we can figure a "cow-day" to be
approximately equal to 30 lb of forage consumed. If we divide 30 lb of
forage/cow day into the lb of forage /acre inch, we find that the "cow-day"
yield of thin, average, and thick pasture to be about 7, 10, and 15
cow-days/inch of pasture consumed.
This becomes a simple method of allocating pasture: 1) Look at the
pasture density and determine it to be thin, average, or thick; 2)measure or
estimate the height of the pasture to be allocated, 3)subtract from the
total height the height of stubble you want the animals to leave, 4)multiply
the difference between starting height and ending height by the cow-days/inch
to figure available cow-days/acre, and 5) divide the number of cows in the
herd by cow days/acre to figure how much area should be allocated.
Step 1. We look at the grass and say, "This is average grass" which
gives us a cow-day/inch factor of 10.
Step 2. We measure the height to be 8".
Step 3. We would like to leave a 3" residual, so 8" - 3" = 5" to be
Step 4. 5 inch grazed X 10 cow-days/acre-inch = 50 cow-days/acre.
Step 5. If we have 100 cows, we should allocate 2 acres/day.
If you are going to use a flexible paddock system where temporary fences
are strung between two permanent subdivision fences, it is very desirable to
set the main fences at a spacing that allows for simple calculation of acre
increments. It is very handy to have the lineposts in the permanent fences
set at known intervals that mark fractional acre increments. For example, if
the permanent strips are 300 ft wide, then lineposts at 50 ft intervals mark
about 1/3 acre strips. Forward planning at fence building time can make
future operation of the system much simpler.
This example works fine for 1000 lb cows, but what about other classes
of livestock. As grazing management is an imprecise science due to ever
changing conditions, we are only looking for an approximation, not
perfection. If cows are anywhere in the 900 to 1200 lb range, the simple
approach used above will probably be adequate. If you have livestock of
different weights, you can add up the total estimated weight of all the
livestock in the herd, divide by 1000 and be fairly close to "cow-day"
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