2003 Annual Report
Dickinson Research Extension Center
1089 State Avenue
Dickinson, ND 58601
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Method of Determining Stocking Rate Based
on Monthly Standing Herbage Biomass Adjusted for the Effects of Drought Conditions
North Dakota State University
Dickinson Research Extension Center
Stocking rate is the number of animals (animal unit) for which a grassland unit (acre) can provide adequate forage for a specified length of time (month). Stocking rate depends on the amount of herbage biomass available to grazing animals, the time of year, the type of grazing system used, and the amount of forage consumed by livestock per month. Stocking rate is commonly presented as acres per animal unit month (AUM) or its reciprocal, AUM's per acre.
Forage dry matter intake of grazing animals is affected by the size of the cow. Large cows consume more forage than medium- and standard-sized cows. A more accurate estimate of daily or monthly forage demand of livestock on grazinglands can be determined with the metabolic weight of the animal than with its live weight. Metabolic weight is live weight to the 0.75 power. A 1000-pound cow with a calf is the standard, which is defined as 1.00 animal unit (AU) and has a daily dry matter allocation of 26 pounds of pasture forage. The metabolic weight of a 1200-pound cow with a calf is 1.147 animal unit equivalent (AUE), which has a daily dry matter allocation of 30 pounds of pasture forage. The metabolic weight of a 1400-pound cow with a calf is 1.287 animal unit equivalent (AUE), which has a daily dry matter allocation of 33 pounds of pasture forage. The amount of forage dry matter consumed in one month by one animal unit, a 1000-pound cow with a calf, is an animal unit month (AUM). The daily dry matter allocation for a cow with a calf on pasture is different from the daily dry matter requirement for just the cow during the same production periods. During the grazing season from May through November, the length of the average month is 30.5 days.
The mathematical process used to determine stocking rate from herbage biomass is presented in Table 1. The amount of herbage available during the grazing season is the average value of the mean monthly standing herbage biomass values for the grazing-season months. The mean monthly standing herbage biomass should be determined by clipping and weighing the dry herbage from each pasture and averaging the weights over several years. If these values are not available, the generalized values for western North Dakota native range (Table 2) can be substituted. The general monthly herbage values on the herbage weight curve in Table 2 are averages of herbage production on well-managed pastures during years with normal precipitation.
Each of the monthly herbage biomass values needs to be adjusted to reflect the differing effect of various grazing treatments on the quantity of herbage biomass produced. Time of year and intensity of defoliation vary with different grazing treatments; these variations affect plant biological mechanisms and result in the production of different amounts of herbage. The percentages of the potential amount of herbage biomass produced on various grazing treatments are summarized in Table 2. The monthly herbage biomass values for the months of the grazing season can be adjusted for differences in herbage biomass on specific grazing treatments by multiplying monthly herbage weight by the percentage of potential herbage produced on a particular grazing treatment.
Each of the monthly herbage biomass values also needs to be adjusted during the perennial plant recovery period to reflect the percentage of herbage weight reduction that resulted from the effects of drought conditions. The percent reduction that results from the effects of drought conditions can be determined by a comparison of normal-year herbage biomass and drought-year or recovery-year herbage biomass. If these data are not available, estimations based on several years of experience can be used. The estimate of percent herbage reduction needs to be converted to percent of herbage biomass produced. The percent of herbage produced is determined by subtracting the percent reduction in herbage from 100%. The monthly normal-year herbage biomass values adjusted for the effects of grazing system are multiplied by percent of herbage produced to adjust for the effects of drought conditions.
The average monthly herbage biomass is determined by adding the adjusted monthly herbage biomass for the months of the grazing season and dividing the sum by the number of grazing-season months.
Not all of the average herbage biomass for the planned months of the grazing season is consumable forage. Perennial plants must retain a portion of the leaf material to conduct photosynthesis and provide carbohydrates and other products necessary to sustain healthy and productive growth. The amount of leaf material the plant must retain ranges from 40% to 60%, depending on grassland type (40% on the shortgrass, 50% on the midgrass, and 60% on the tallgrass prairies). When specific values for percent proper utilization are not known for the grassland type, an average value of 50% may be used. This value implies that the plant retains half the herbage and half the herbage is available for utilization.
Not all standing herbage available for proper utilization is ingested by grazing animals. Grazing livestock consume only about 50% of herbage available for utilization. The remainder of the utilized herbage is broken from the plant, soiled by animal waste, consumed by insects and wildlife, and lost to other natural processes. Data to allow comparison of forage-harvest efficiency on different grazing systems are not available. However, the quantity of herbage ingested by livestock would be expected to increase with improvement in efficiency of harvest from some grazing systems.
The differences in daily dry matter intake for cows of different weights are used to adjust stocking rates for cow size. The standard 1000-lb cow requires 26 lbs of dry matter per day. The dry matter intake for lighter or heavier cows can be determined by multiplying 26 lbs by the animal unit equivalent (Table 3), which is based on the metabolic weight of the cow.
For determination of stocking rate, the available monthly forage for intake value adjusted for grazing system and drought conditions is divided by the amount of dry matter per cow per month--the daily amount of dry matter per cow adjusted for cow size multiplied by the number of days per average month (30.5 days). This stocking rate is presented as AUM's per acre. The reciprocal can be determined by dividing that number into 1 to give acres per AUM. This is the number of acres required to provide forage for one month for a cow with a calf.
The number of cows to turn onto the grazing system is determined by dividing the total number of acres in the pastures of a grazing system by the acres per AUM value and dividing this total number of AUM's by the number of months of the grazing season. The number of cows that can graze the pastures subtracted from the total number of cows in the herd will show the forage shortfall.
The stocking rate value determined by this mathematical process is based on the average monthly standing herbage biomass for the grazing-season months and has been adjusted for percentage utilization, percentage forage intake, grazing system, cow size, and the effects of drought conditions.
I am grateful to Amy M. Kraus for assistance in preparation of this manuscript. I am grateful to Sheri Schneider for assistance in production of this manuscript.
Table 1. Process to determine stocking rate from monthly herbage biomass adjusted for drought conditions
Adjustment for grazing treatment
Monthly herbage biomass value X % of potential herbage produced on appropriate grazing treatment
Adjustment for effects of drought conditions
Monthly herbage biomass value adjusted for grazing treatment X (100 % - % herbage reduction)
Average monthly herbage biomass
Sum of adjusted monthly herbage biomass for the months of the grazing season
÷ number of grazing season months
Adjustment for size of cow
26 lbs daily dry matter intake X Animal Unit Equivalent for appropriate cow size (from Table 3).
Average monthly herbage biomass adjusted for grazing treatment and for drought conditions
X % proper utilization (50%)
X % consumed (50%)
= lbs of available monthly forage
÷ dry matter per AUM (26 lbs X AUE X 30.5 days)
1.0 ÷ AUM/ac = ac/AUM
The number of cows that can graze the available forage
Total number of acres in grazing system
÷ number of grazing season months
Table 2. Generalized Standing Herbage Biomass for Well-Managed Native Rangeland during Normal Precipitation years
Table 3. Animal Unit Equivalent (AUE) based on metabolic weight (live animal weight 0.75 ).
Equivalent yx 0.75
(% of 1000 lbs)
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