SWARD HEIGHT AND YIELD RELATIONSHIPS IN GRAZED PASTURES
James R. Gerrish1
Abstract
Estimation of available pasture is a useful aid in making grazing management decisions.
Forage yield can be approximated from sward height with reasonable accuracy if precautions
are taken to define the sward being estimated. The yield:height relationship in pastures is
affected by stand density, species composition, and sward height. Grazing management
affects all of these parameters. Differences in predicted yield occur in continuously stocked
(CS) versus rotationally stocked (RS) pastures. At shorter heights, RS pastures tended to be
higher yielding at comparable heights while CS pastures tended to be higher yielding at taller
heights. Greater stand thinning occurred in CS pastures when grazed below 4 in. and more
dead material accumulated in CS pastures when allowed to grow above 8 in., compared to
RS pastures. Prediction of available forage from height can be reasonably accurate when
adjustment is made for species composition, stand density, and sward height.
Introduction: Estimation of available forage dry matter is an important tool for both livestock
managers and pasture researchers. The need for balancing forage supply with animal
requirements is crucial skill for successful graziers. 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 were analyzed to determine how much variance exists
in the sward height:forage yield relationship between two different grazing management
regimes.
Materials and Methods: Dry matter yield and mean sward height data were collected in a
grazing study conducted at the University of Missouri-Forage Systems Research Center.
Pastures were either continuously or rotationally stocked 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
(Festuca arundinacea Schreb.), Kentucky bluegrass (Poa pratensis L.), and orchardgrass
(Dactylis glomerata L.) were the primary grasses with red clover (Trifolium pratense L.),
white clover (T. Repens L.), and birdsfoot trefoil (Lotus corniculatus L.) being the dominant
legumes.
Continuously stocked pastures were sampled bi-weekly along four defined transect
lines with sample three sites per transect. Four paddocks out of the 12 paddocks in each
rotationally stocked pastures 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. Clipped samples were
oven dried (155°F) and total dry matter yield (DMY) was determined. 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 quadratic functions.
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 & 3). 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.
Linear prediction equations resulted in much greater variance between CS and RS
pasture yields than did the quadratic prediction equations. Whereas linear predicted yield at
12 in. for CS was 900 lb/acre higher than for RS, the difference in quadratic predicted yields
at the same height was less than 300 lb/acre. The standard error of estimate tended to be
lower for all quadratic equations compared to linear equations.
Measured GDMY/acre-inch was significantly higher for RS pastures at lower sward
heights (Figures 2 & 4). Stand density and species composition was determined using a
point-step method at three times during the grazing season. At high stocking rate treatments
for both CS and RS pastures, more bare ground hits were recorded later in the grazing
season. Percentage of bare ground was somewhat greater in CS pastures compared to RS
resulting in lower GDMY/acre-inch for CS pastures.
As stand density is a critical factor in the height:yield relationship, any use of height
as a predictor of yield should probably be tempered with an adjustment factor for stand
density. In a previous study, Gerrish (1982) 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 management education sessions (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.
Conclusions: The data reported in this paper were collected as basic data in a grazing
management study and so do not cover the full range of possibilities in factors affecting
height:yield relationships. The results do indicate that grazing management does in fact affect
height:yield relationships
Literature Cited:
Gerrish, J. R., 1982. A prediction model for pasture composition and yield. p. 102. Agronomy
Abstracts.
Table 1. Estimated dry matter yield per acre-inch for various forages at three stand density
levels.
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Stand Density
---------------------------------------
Forage 60 - 75% 75 - 90% > 90%
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------------ lb/acre-inch--------------
Tall Fescue + N 250 - 350 350 - 450 450 - 550
Tall Fescue + Legumes 200 - 300 300 - 400 400 - 500
Smooth Bromegrass + 150 - 250 250 - 350 350 - 450
Legumes
Orchardgrass + Legumes 100 - 200 200 - 300 300 - 400
Bluegrass + 150 - 250 300 - 400 500 - 600
White Clover
Mixed pasture 150 - 250 300 - 400 400 - 500
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Figure 1. Predicted dry matter yield for rotationally stocked pastures.
Figure 2. Predicted green forage dry matter yield per acre-inch for rotationally stocked pastures.
Figure 3. Predicted green forage dry matter yield for continuously stocked pastures.
Figure 4. Predicted green forage dry matter yield per acre-inch for continuously stocked pastures.
1 Research Assistant Professor, University of Missouri - Forage Systems Research
Center, 21262 Genoa Rd., Linneus MO 64653.
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