Drought has been an irregular environmental factor on grasslands from their beginning, as has fire.
Periods with rainfall shortage are normal weather conditions for the Northern Plains. Long-term precipitation records (118 years) for western North Dakota show that a 6 month perennial plant growing season, mid April to mid October, rarely has adequate rainfall each month. Only 6% of the growing seasons occur without water deficiency, that’s only 3 years in a 50 year period.
The average 6 month perennial plant growing season has 2 months with water deficiency. This should be considered as the typical growing season for the Northern Plains. The frequency of water deficiency occurrence is not distributed evenly across the growing season months. Frequency of water deficiency has occurred 17% in April, 14% in May, 10% in June, 38% in July, 53% in August, 50% in September, and 47% in October.
Normal rainfall for an area is between 125% and 75% of the long-term mean (LTM), which is often determined for the growing season and for the year. In western North Dakota, normal rainfall occurs during 67% of the growing seasons, and 18% of the growing season have rainfall greater than 125% of LTM. Most of the growing seasons, 85%, receive rainfall at 75% or greater than the long-term mean.
Drought conditions occur during 15% of the growing seasons in western North Dakota and receive less than 75% of LTM, that’s about 2 drought seasons every 13 years. Most of the growing seasons with drought conditions are considered to be moderate, with precipitation at less than 75% but greater than 50% of LTM. Moderate drought conditions occur during 12% of the growing seasons, or 1 moderate drought every 8 years.
Severe droughts are usually wide spread and everyone remembers them. Severe droughts have occurred during 3% of the growing seasons and receive rainfall at less than 50% of LTM, or about 1 severe drought every 29 years.
Livestock producers usually endure forage deficiency problems even during moderate droughts. Low rainfall is the most obvious detrimental factor. However, the additional factors of higher soil evapotranspiration rates, deteriorating soil structure, decreasing soil organism activity, and reduction of ecosystem health status all negatively affect forage production. If the reduction in rainfall were the only factor causing decreased forage production, the percent reduction in forage below normal levels would be the same percentage as the reduction in precipitation.
The major cause for the quantity of reduction in forage production greater than the percent reduction in precipitation is the low quantity of available mineral nitrogen in the soil. Wight and Black were able to determine that the minimum quantity of 100 lbs of available mineral nitrogen per acre was required in order to sustain forage production at the biological potential level for rangelands in the Northern Plains.
Wight and Black found that the precipitation use efficiency mechanism (lbs of herbage produced per inch of precipitation received) within rangeland grasses improved when soil mineral nitrogen was available at quantities of 100 lbs/ac or greater. Inhibitory deficiencies of mineral nitrogen in rangelands that had less than 100 lbs of available mineral nitrogen caused the weight of herbage produced per inch of precipitation received to be reduced an average of 49.6% below the weight of herbage production per inch of precipitation on the rangeland ecosystems that had 100 lbs/ac or greater of mineral nitrogen.
The quantity of available mineral nitrogen in the soil also directly effects the rate of ecosystem recovery following a growing season with low rainfall. Traditionally managed rangelands generally have 50 to 60 lbs/ac of mineral nitrogen and require 1 full growing season with reduced stocking rates to recover from a moderate drought and require 2 growing seasons to recover from a severe drought.
Biologically effective management strategies that have 100 lbs/ac or greater of available mineral nitrogen require 1 growing season to recover from a severe drought but the ecosystem can recover without reducing stocking rate following a moderate drought.
Intact grassland soils (never plowed) are not deficient in nitrogen. Unbroken grassland soils contain between 3 to 8 tons, with most at 5 to 6 tons, of organic nitrogen per acre, which is not available for grass plant use. With this great quantity of organic nitrogen, intact grassland soils should never need the application of supplemental nitrogen. The organic nitrogen must be transformed into mineral nitrogen in order to be usable by grass plants. Soil microorganisms in the rhizosphere around perennial grass roots can transform organic nitrogen into mineral nitrogen. However, the rhizosphere microbes are achlorophyllous and cannot fix their own carbon energy.
Grass lead tillers fix surplus carbon energy during their vegetative growth stages between the 3.5 new leaf stage and the flower (anthesis) stage which occurs between 1 June and 15 July each year. This surplus carbon energy can be released from the grass tillers and exudated through the roots into the rhizosphere, to the soil microbes, if cattle grazing is specifically managed so the animals partially defoliate 25-33% of the aboveground leaf biomass on 60-80% of the lead tillers. This biologically effective grazing management is described in a refereed book chapter that can be downloaded for free @ https://hdl.handle.net/10365/28801.
Drought conditions are going to come again, livestock producers should make some preparations. Manage the grazinglands to transform 100 lbs/ac of mineral nitrogen or greater in the soil, and store about 12% of the hay forage as silage during each normal year in order to have adequate feed during the drought year to keep the good genetics in the cow herd intact and permit the grassland ecosystem to recover during the following growing season.
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