Dickinson Research Extension Center
1089 State Avenue
Dickinson, ND 58601
Plants Have Smaller Forage Reductions
during Precipitation Shortages
L. Manske PhD, Range Scientist
Amy M. Kraus, Composition Assistant
Thomas C. Jirik, Agriculture Communication Editor
Dakota State University
Dickinson Research Extension Center
Effective grazing management can help minimize herbage reductions during periods of below- normal precipitation, says a North Dakota State University range scientist.
"Most of western North Dakota is experiencing below-average herbage production on perennial grass spring pastures, native rangeland pastures, and grass and alfalfa haylands. Estimates of 25 to 60 percent reduction below normal herbage biomass are common," notes Lee Manske, range scientist at NDSU's Dickinson Research Extension Center. "Climatic conditions have been receiving most of the blame for these reductions, although herbage production is affected by management practices as well as by the level of precipitation in relation to normal amounts."
The majority of the state's western region was dry during late summer and fall
of 2001 and cool during the spring of 2002. The dry fall resulted in reduced
fall tiller growth, with an average of fewer than 1.5 leaves per tiller. Tillers
of cool-season grasses require about 2.5 fall leaves for normal herbage production
the following year. Cold nights and cool days during the spring resulted in
a moderate reduction in the daily grass growth rate. The combination of a dry
fall and cool spring is responsible for a portion of the reduction in herbage
biomass in the region, Manske says.
In addition, parts of western North Dakota received less-than-normal precipitation
this spring. Large areas south of Interstate 94 have various levels of water
stress and several areas in south central North Dakota are experiencing drought
or near-drought conditions. The below-normal precipitation further restricted
herbage production on pastures and haylands in these areas and created additional
"The amount of herbage biomass reduction resulting from just the combination
of restricted growth of last season's fall tillers and this spring's slightly
reduced rate of grass growth has not been great enough to change grazing dates
or stocking rates on pastures that have been properly managed and have healthy
plants," Manske says. "The annual development of healthy grass plants occurred
within the normal range of dates during the spring of 2002, with a delay of
only 3 to 7 days from the average date. However, in some areas large reductions
in herbage occurred because the dry fall and cool spring magnified existing
problems caused by detrimental management practices."
The quantity of herbage biomass produced is related to plant size and plant
density. These two characteristics are directly affected by the level of plant
health, which is determined by the biological effectiveness of the grazing management
practices used, Manske notes. Consequently, management practices that do not
meet the biological requirements of the plants retard plant processes. The resulting
deterioration in the level of plant health is manifested as decreased plant
density and diminished plant size that lead to reduced herbage production.
Herbage reduction percentages caused by detrimental grazing management practices
such as grazing before the third-leaf stage, grazing throughout the entire season,
or grazing during the fall usually vary between 40 and 60 percent below the
potential herbage biomass. The greatest reductions of herbage production observed
in western North Dakota have occurred on domesticated grass spring pastures
that were hayed during the summer and/or grazed during the fall, on native rangeland
summer pastures that were grazed during the fall, and on domesticated grass
and alfalfa haylands that were hayed late and/or grazed during the fall.
"The long-term solution to management-caused herbage reduction problems is to
implement beneficial management strategies that meet the biological requirements
of the plants," Manske says. He recommends three management practices that improve
Herbage weight of perennial plants increases from early season through May,
June, and July until peak herbage biomass, which occurs during the last couple
weeks of July. Herbage weight then decreases as plants age and dry, Manske explains.
The amount of herbage biomass produced by healthy plants is related to precipitation
levels during January through July, which affect plant size and plant density.
Herbage reduction caused by low precipitation is usually proportional to the
levels of precipitation below the normal range. An estimate of the amount of
herbage reduction low precipitation causes in healthy plants can be determined
by a comparison between the local long-term mean precipitation received during
January through July and the current year's precipitation for that period. The
range of normal precipitation is plus or minus 25 percent of the long-term mean.
The procedure to estimate percent reduction in peak herbage biomass caused by
below-normal precipitation requires just three simple calculations: first, the
monthly precipitation for January through July is totaled to give the current
seasonal precipitation; then, this precipitation amount is divided by the local
long-term January through July precipitation amount to determine the current
seasonal precipitation as a percentage of the long-term mean precipitation;
next, that percentage is subtracted from 75 percent, which is the low-normal
long-term precipitation value.
"The resulting estimated percentage of reduction in biomass that below-normal
precipitation has caused in peak herbage biomass provides a guideline for the
percent reduction in stocking rate needed for the remainder of the grazing season--until
mid October--on pastures that have been properly managed and have healthy plants,"
For example, if the January through July seasonal precipitation amount is 65
percent of the long-term mean, the estimated 10 percent reduction from normal
herbage biomass would suggest a 10 percent reduction in stocking rate--assuming
the proper stocking rate was being used. This method does not determine the
amount for stocking rate adjustments required on pastures managed by practices
that diminish the health status of plants below potential levels.
The long-term mean monthly precipitation amounts for numerous locations are
available on the National Weather Service (NOAA) web site for North Dakota (www5.ncdc.noaa.gov/climatenormals/clim81/NDnorm.pdf).
For the current season's precipitation, amounts collected at individual ranches
and marked on the calendar can be used if a complete January through July data
set is available.
Another source for the current season's precipitation amounts for many locations is the NDAWN web site (http://ndawn.ndsu.nodak.edu). These data start in April because NDAWN does not collect data for precipitation that occurs as snow. The precipitation amounts for January through March and the amount of precipitation that falls as snow during other periods must be obtained from other sources. Current season's precipitation data that include snow moisture are available on the National Weather Service site (www.crh.noaa.gov/bis/OtherHydro.htm Click on "text" for the desired month's precipitation data).
The following list of locations includes the long-term January through July precipitation, the 2002 January through July precipitation as a percent of the long-term seasonal mean, and the estimated percent reduction in stocking rate needed on healthy pastures in that area: Williston (9.8 inches, 101 percent, 0 percent), Watford City (9.7 inches, 116 percent, 0 percent), Manning (10.3 inches, 139 percent, 0 percent), Dickinson (11 inches, 114 percent, 0 percent), Beulah-Hazen (11 inches, 86 percent, 0 percent), Bismarck (10.7 inches, 67 percent, 8 percent), Bowman (10.7 inches, 73 percent, 2 percent), Hettinger (10.5 inches, 54 percent, 21 percent), and Shields (11.5 inches, 49 percent, 26 percent).
If the percentages of reduction in herbage biomass produced on domesticated grass spring pastures, native rangeland pastures, or grass and alfalfa haylands are greater than the estimated percentage of herbage reduction reached by the comparison between the long-term and current seasonal precipitation amounts, the health of the plants is below the potential level because the management practices have not met the plant biological requirements. When management practices meet the biological requirements of the plants and the level of plant health is high, the percentages in herbage biomass reduction that occur during periods of below-normal precipitation are smaller and less problematic than reduction percentages on areas with diminished plant health.
"Dry falls and cool springs, water stress during growing season months, and summer drought are not abnormal climatic conditions in western North Dakota," observes Manske. "Plant health status, which is affected by management practices, can magnify or diminish the negative effects these reoccurring environmental conditions have on herbage production. Management strategies that sustain high levels of plant health help to ensure that the problems accompanying below- normal precipitation are minor incidents rather than catastrophes."
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