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SWARD HEIGHT
AND YIELD RELATIONSHIPS
IN GRAZED PASTURES

Jim Gerrish

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 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 & 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 relationships.

Table 1.  Estimated dry matter yield per acre-inch for various forages at
three stand density levels.
===========================================================================
                              Stand Density                  
---------------------------------------------------------------------------
     Forage                   60 - 75%       75 - 90%       > 90%   
---------------------------------------------------------------------------
                             ------------- lb/acre-inch-------------
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.

EXAMPLE:

    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 grazed.

    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" equivalents.


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