DROUGHT AND ITS RELATIONSHIP TO DYNAMICS OF
PRIMARY PRODUCTIVITY AND PRODUCTION
OF GRAZING ANIMALS

-by C. Wayne Cook

Summary

Drought not only causes a loss in abundance of plants, a reduction in ground cover, and a decrease in vigor of plants, but also results in a decrease in overall forage yield. Therefore, the livestock industry must build flexibility into operations in order to survive the effects of high variability in forage production from year to year.

The effects of intermittent drought upon vegetation are further complicated with past grazing use by livestock, insect, and rodent populations and, in some cases, dust from adjacent cultivated farmlands and deteriorated ranges. It is common knowledge that most plants that inhabit rangelands are subject to stresses of limited moisture sometime during almost every annual growth cycle. This may be a period of a few weeks or a few months. This might be a normal climatic rhythm, or it may be somewhat subnormal.

The species composition and dynamics of the primary producers of the rangeland ecosystems are determined largely by the ability of plant species to survive intermittent periods of deficient soil moisture. It has been found in many studies in the western portion of the United States that severe droughts markedly affect the density, vigor, and relative abundance of the native rangeland species. Many ecologists contend that even though most of the plants of the dominant species may die, drought rarely kills all of the plants of any one species in a plant community. A series of dry years can materially reduce the accumulation of mulch on the soil surface and, thus, detract from the amount of water entering the soil and the amount retained in the soil for future plant use. During drought years, nearly all of the cover may disappear and, in effect, create a xeric site that otherwise would be mesic in character with adequate mulch and normal precipitation.

Precipitation over the western range area of the United States shows that monthly and yearly moisture receipt vary widely from their means, and extended periods below normal are common. In the southwest part of the United States, 40 percent of the years are below normal; in the northwest portion, only about 15 percent are subnormal. In the desert areas of the West, forage yield varies from the best to the poorest years by as much as 10 times, and in the less arid foothill areas and on the plains by as much as 4 times. In general, both annual precipitation and forage yield may be expected to vary as much as 300 to 400 percent over a period of 10 years in the western range area.

Livestock responses from wet and dry years do not correlate in some cases with projected expectations; but in general, drought years produce decreased individual livestock gains and, of course, decreased livestock production per acre of land utilized. Individual animal response to a drought year can be a result of closer grazing and greater quantities of dormant forage in the diet.

Drought is by far the most serious physical hazard to the livestock industry of the world. During drought years, the herbage may be so deteriorated that it furnishes little more than a sustenance ration for the small herbivores that normally inhabit the area. Therefore, the livestock must be removed or die of starvation. Management systems that provide for flexibility in stocking rates by years, and by seasons within years, are a very vital part of successful ranching in the western United States.

The true test of any grazing system is its ability to sustain optimum vigor of plants, even during periods of below average precipitation. Observations regarding the vigor of grazed and ungrazed plants after extended drought periods show that the degree of loss of vigor and number of dead plants was substantially greater on continually grazed areas.

Modern management decisions with respect to maintaining a balance between primary production and the consumer population must recognize the fluctuation in herbage production resulting from varying climatic conditions. This recognition must consider the optimum vigor of forage plants and the health and welfare of the herbivore population being managed.

It has been suggested that cattle ranchers maintain an all-age operation. This means that steer calves and a plentiful supply of replacements will be held over during better than normal forage producing years; but during drought years, the entire calf crop would be sold as weaners. During extremely low forage producing years, no replacements would be held over, and the entire breeding herd would be discriminately culled. Feeding supplements during drought may be needed to salvage the basic breeding herd.

Sheep ranchers in the United States cannot hold over wether lambs during favorable years as the cattleman does with his steer calves. Therefore, the sheep rancher must cull his ewe herd heavily and keep no replacements during drought years. During favorable growing years, he would rebuild his breeding herd.

In all cases, conservative grazing that will allow for maintenance of high vigor among plants is good insurance against drought. The larger livestock operators in the western range areas of the United States generally possess ranches in more than one state so that breeding herds can be shipped from one ranch to another when localized drought seriously reduces the forage yield.

Introduction

Effects of drought upon plant welfare vary from reduction in size and vigor to actual death of the plants. Low rainfall is usually the cause of drought, but high temperatures may also be involved. Drought, as related to periods of low precipitation, may be a result of several growing days or even several growing weeks. Low seasonal precipitation and abnormally low annual precipitation for a particular year, or, even in some cases, for a period of years, may be the cause of destructive drought situations.

Drought has been defined as a period when rainfall is only about 30 percent of average for 21 days or longer. Other such definitions state that drought occurs when annual precipitation is only 75 percent of normal. It is generally understood that drought conditions, along with alternate periods of high precipitation, appear rather regularly over time throughout the range of grazing areas of the world. In a period of 15 to 20 years, range lands in arid areas should expect a series of higher than average forage producing years and, likewise, a series of drought years with herbage production far below normal.

It is acknowledged that many expressions of ecological communities are the result of plant tolerance to environmental extremes such as temperatures, wind, and soil moisture. In most range ecosystems of the world, moisture is intermittently limiting. Most dominant species on arid rangelands have developed adaptations to cope with intermittent periods of deficient soil moisture.

Plants of arid rangelands have been classified as: 1) drought escaping by completing their life cycle in a very short time when growing conditions are favorable, after which they become dormant the remainder of the year; 2) drought evading by remaining small, or restricting growth, when moisture is limiting; 3) drought enduring plants may grow very little or not at all for an entire year, yet remain alive to renew growth when rain arrives the following year; and; 4) drought resisting by withstanding arid conditions by accumulating water in the plant as a stored reserve.

Some of the xerophytic adaptations of plants are: 1) a decrease in size of all cells including guard cells; 2) a thickened cell wall; 3) a strongly developed palisade and mesophyll; 4) an increase in the number of stomata per unit area; and 5) a rise in osmotic pressure. Some drought-tolerant plants are able to control rate of transpiration by control of stomatal aperture and by means of a covering of resins or pubescence.

It is sometimes viewed that xerophytes are plants that are found only on desert areas; however, current thinking is that xerophytes occur on all arid rangelands. At least it can be said that most plants growing on arid ranges are xerophytic in their tolerance to deficient soil moisture or atmospheric conditions, which promote rapid water loss. Anatomical and physiological adaptations common to xerophytes have evolved under many different kinds of xeric environments. On saline desert sites, the plant growth is further impeded because of the difficulty of absorbing moisture against the osmotic pressure of high salt content in the soil.

Since extremes in climatic conditions are to be encountered throughout the arid range areas of the western United States, the ability of range ecosystem managers to cope with fluctuating climatic and herbage conditions is, indeed, difficult because such alternating cycles are not precisely predictable for management purposes. Research has been challenged to present methods of predicting forage yield from existing or preexisting soil and climatic factors. However, it may not be too long in the future before herbage yield in many areas can be determined with a high degree of accuracy by means of past weather features, climatic patterns, and soil moisture, along with other parameters related to plant growth. Variables will include the form of precipitation, distribution over certain periods, soil moisture to varying depths at the beginning of the growing season, atmospheric temperatures, and evapo-transpiration-precipitation ratios.

Some scientists feel that it is better to predict droughts for managing and using the forage resource rather than simply recognizing average herbage dynamics from season to season as a result of current soil and weather conditions. It would, of course, be of great value for the range manager if he could forecast drought incidence and intensity at least a season in advance.

It should be pointed out that extreme drought conditions or droughts of long duration seldom cover more that a particular region of the western range area. Therefore, the entire livestock industry of the West would never suffer a poor production year, and relief can be received by an interchange of grazing agreements among grazing areas (Figure 1).

Drought and Vegetation Changes

The debatable issue that climate alone causes permanent change in the range ecosystem has not been conclusively settled, even though weather records have been available since the early 1800s. However, the effect of climate on permanent change in vegetation composition, along with other related factors (i.e., livestock grazing, fire, small herbivores), has been immensely confounded.

The frequency and duration of drought are both important in determining the severity of effects of climate upon botanical composition. It is common knowledge that most plants that inhabit rangeland ecosystems are subject to stresses of limited moisture sometime during the annual growth cycle. This may be a period of a few weeks or a few months. This might be a normal climatic rhythm, or it may be somewhat subnormal. In addition to droughty periods that commonly appear sometime during the annual life cycle of a plant, there are drought spells that last several years. Thus, intensity and duration of drought may be identified either within months of the year or among years over time infinitum.

The species composition and dynamics of the primary producers of the range ecosystem are determined largely by the ability of plant species to survive rather long periods of deficient soil moisture along with over use of the herbage. Studies in mixed prairie types in the northern Central Plains during the severe drought of 1934-1937 showed profound changes in height growth and species composition. However, few dominant species completely died in most plant communities.

In a study by Albertson and Tomanek (1965) that covered a period from 1932 to 1961, it was found that the drought of the 1930s caused a loss of plant species in the shortgrass communities to the extent that it was dominated by only one species, buffalo grass, and even this species was reduced in quantity by less than a third of normal. These same authors found that the tall grasses and mixed grasses in the Central Plains states showed marked changes in percent species composition, but the extent of change and reduction in herbage cover were much less than in the short grasses. This is to be expected, because the tall and mixed grasses had more favorable site conditions.

In the semidesert grass-shrub ranges of New Mexico, it was found that basal area and species composition changed appreciably during periods of above- and below-average precipitation. It was also found that some species were less sensitive to changes in basal area during drought years than others, and, furthermore, various plant species reacted differently to dry and wet years on different sites. It was found that plant cover was closely related to weather cycles, while herbage yields were more related to growing conditions within years (Paulsen and Ares 1962, Herbel et al. 1972). These authors found that basal area of perennial grasses decreased by as much as 65 percent during dry years. Many individual plants died during drought, but recovery during favorable years came from regeneration of remaining crown tissue and new seedling establishment.

Individual species of the salt-desert shrub in Utah and Nevada respond differently to drought. Only a sparse amount of annuals make up the botanical composition. Generally, the dominants that are decreased most rapidly because of drought, likewise, respond most rapidly to recovery during favorable years. During even a two-year drought period, as much as 30 percent of plants of some dominant species died (Hutchings and Steward 1953).

It is generally acknowledged that both soil types and past grazing use affect change in species composition, and this is greatly emphasized during dry years. In arid rangeland ecosystems where severe droughts occur intermittently, there is severe damage to the habitat through wind erosion during drought periods. Large areas are sometimes left bare, and the drifting sands and dust accumulate in obstacles or on the leeward side of depressions. Thus, some plant life is actually smothered, and others are left pedestalled.

Since drought is characterized by a deficiency of soil moisture, factors such as mulch, which enhance the process of absorption, can ameliorate the severity of subnormal precipitation. A series of dry years can materially reduce the accumulation of mulch on the soil and, thus, detract from the amount of water entering the soil and, furthermore, allow greater evaporation from the soil surface. In many cases, nearly all of the mulch cover disappeared in the prairie after a few years of low precipitation.

Drought and Forage Yield

It is acknowledged that most dominant plant species on arid rangelands of the world have developed adaptations to cope with intermittent periods of deficient soil moisture, but all plant life under these conditions display rather marked fluctuation in their herbage yield from year to year and from season to season (Table 1).

On the semidesert grass-shrub range in southern Arizona, where annual rainfall varies widely by about 10 inches, the forage production varied from as much as 300 pounds per acre during good years to as low as 10 to 15 pounds during bad years. About 80 percent of this herbage yield is composed of annuals, which accounts for most of the variation. The average yield from perennial grasses is about 20 pounds per acre.

In a semidesert grass-shrub type in New Mexico on the Jornada Experimental Range during a study period from 1941 to 1957, it was found that annual precipitation ranged from 17.3 inches to 3.1 inches, and the three dominant perennial grass species yielded from 807 pounds to 114 pounds per acre (Herbel et al. 1972).

In a study of the salt-desert shrub ranges at the Desert Range Experiment Station in central Utah, Hutchings and Stewart (1953) found that herbage yield ranged from 75 pounds to as much as 468 pounds per acre. The higher yielding years were more than six times the minimum yield. The period of study was from 1935 to 1947. The lowest yield of 75 pounds per acre was the result of two consecutive years with precipitation less than one-half of the normal. Annual precipitation from October to October varied from 3.8 inches to 11.1 inches. These authors found a close relationship between precipitation and yield of desert herbage. This provided a basis for estimating the amount of forage available in the fall from the previous 12 months' precipitation.

In the Palouse prairie of Oregon, over a 10-year period, the annual precipitation varied from 6.1 to 16.6 inches. During this same period, herbage production varied from 280 pounds per acre to 930 pounds (Sneva and Hyder 1962). Annual growth of herbage was dependent upon the precipitation from September to June.

In the Snake River plains in central Idaho, where vegetation is predominantly a sagebrush-grass type, the annual precipitation varied from 15.9 to 7.9 inches over a 20 year period from 1934 to 1954, and forage yield varied from 1065 to 489 pounds of air dry herbage per acre. There was a rather high correlation between herbage production and annual precipitation (Blaisdell 1958).

In the Central Great Plains, Weaver and Albertson (1956) discovered that where it normally required 12 acres for an animal unit, it required 30 to 50 acres following a drought. In many areas, there was virtually no grazing capacity. Densities in the shortgrass communities decreased from 89 percent to 22 percent from 1934 to 1939. The great drought of 1934-1939 caused such drastic decreases in forage that thousands of animals died of starvation, and many ranchers went bankrupt because of their inability to adjust to these drastic changes.

At the Central Plains Experimental Range in the northern shortgrass range type in northern Colorado, the annual precipitation varied from 4.3 to 22.9 inches from 1932 to 1967. The herbage yield over this period varied from 145 pounds per acre to 1027 pounds (Bement 1968).

In west-central Kansas on a clay upland range site that was dominated by native shortgrass species, Launchbaugh (1974) found in a study that covered 18 years (1956-1973) that forage production varied from 5580 pounds per acre during wet years (31.21 inches of annual precipitation) to only 1250 pounds during dry years (9.2 inches of annual precipitation).

Precipitation collected by the U.S. Weather Bureau over the western range shows that monthly and yearly moisture received varies widely from the mean, and extended periods below normal were common. In the Southwest, 40 percent of the years are below normal, and in the Northwest, only about 15 percent are subnormal (Stoddart and Smith 1943). Data in Table 2 presents the low and high precipitation years for various parts of the arid range area of the western United States. It can be seen that precipitation from the best to the poorest years ranges from about 2.25 inches annually to as much as 10 times as much. This agrees with about what can be expected in forage yield over long periods of time. Both annual precipitation and forage yield may be expected to vary by as much as 300 to 400 percent over a period of years.

Drought and Livestock Responses

Studies on a seeded foothill range in Utah showed that gains from all age classes of cattle from April 15 to July 1 followed closely with the average forage production (Frischknecht and Harris 1968). Even on mountain ranges used for summer grazing, poor animal responses were encountered during drought years (Stoddart 1944). Studies on arid ranges of the Southwest showed that during drought years, the forage became dry, and poor animal responses were obtained as a result of nutritional deficiencies (Paulsen and Ares1962, USDA Forest Service1952).

Grazing studies in the Palouse Prairie in Oregon, and in the shortgrass plains, in Colorado showed that steers gained 0.3 and –0.42 pounds per day during drought years and 2.7 to 1.75 pounds per day during favorable years in each study respectively (Sneva and Hyder 1962, Bement 1968).

In the Northern Great Plains mixed prairie at Mandan, North Dakota, yearling steer gains over a 16-year period followed rather closely with the climatic conditions and the quantity of forage produced. This would be expected, as high rainfall years would tend to provide green growing forage throughout the spring and summer grazing season; whereas, dry years would tend to cause the forage, at least in the summer, to become dormant and less nutritious. Individual yearling steers during this 16-year study gained about 1.7 pounds per day during the dry years and 2.1 pounds per day during the more favorable years (Lorenz 1974).

On salt-desert shrub ranges in Utah, Idaho, and Nevada, it was found that sheep and cattle responded better during favorable forage production years. During drought years, livestock had to be supplemented heavily to prevent serious weight losses and correct nutritional deficiencies. During normal forage production years, where a mixture of desert grasses and shrubs prevail, supplements were needed only during inclement weather periods (Cook and Harris 1968).

Reasons for poor individual gains during drought years is potentially a result of closer grazing because of low herbage yield and the effects of the forage being dry and dormant and consequently lower in nutrients.

Management Flexibility to Cope with Drought

Grazing of plants during drought must be conservative; otherwise, severe damage will occur. Studies in Utah (Cook 1967) and in New Mexico (Paulsen and Ares 1962) showed that high death losses of forage plants occurred during dry years when associated with heavy utilization. Therefore, varying the stocking rate from season to season and from year to year to prevent excessive grazing during dry years is very important for sustained yield of forage over time.

Flexibility in operations to cope with drought has been met in several ways, but none can completely address the extreme fluctuations in forage yield that often occur. It has been suggested that cattle ranchers maintain an all-age operation. This means that steer calves and a plentiful supply of replacements will be held over during better than normal forage producing years; but during drought years, the entire calf crop would be sold as weaners. During extremely low forage producing years, no replacements would be held over, and the entire breeding herd would be discriminately culled. Feeding supplements during drought is a common practice, but is not a substitute for conservative grazing and flexible management practices.

It is suggested that the breeding cow herd be maintained at about 70 to 75 percent of the carrying capacity, based on normal growth years, and that the excess capacity during average or above-average years be used by younger animals that are marketable during most any season.

Sheep ranchers in the United States cannot hold over wether lambs during favorable years as the cattleman does with his steer calves. Therefore, he must cull his ewe herd heavily and keep no replacements during drought years.

In all cases, conservative grazing that will allow for maintenance of moderately high vigor among plants is good insurance against drought. The larger livestock operators in the range areas of the United States generally possess ranches in more than one state so that breeding herds can be shipped from one to another when localized drought seriously reduces the forage yield.

In years past, it was a practice of the Forest Service to stock the ranges so that over use, because of dry years, would not occur more than one year out of four. At present, it is the policy of the government land management agencies to stock conservatively during even average years so that vigor and reserves can be accumulated for heavier use during subnormal forage producing years. In most cases, the contract with the grazing permittee calls for some time adjustments when grazing will be allowed at the beginning of the grazing season, and when grazing will be terminated or lengthened at the end of the grazing season. This agreement allows for droughty years that normally would reduce plant vigor and forage yield to produce near normal forage growth without the usual vagaries that are associated with the intermittent dry years that appear frequently in the rangeland states.