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Drought and its relationship to dynamics of primary productivity and production of grazing animals

C. Wayne COOK * and Phillip L. SIMS **

(*) C. Wayne Cook: Head of Department of Range Science, Colorado State University, U.S.A.

(**) Phillip L. Sims: Research Professor, Department of Range Science, Colorado State University, U.S.A.


Introduction
Drought and vegetation changes
Drought and forage yield
Drought and livestock responses
Management flexibility to cope with drought
Bibliography


SUMMARY

This paper presents results of over 40 years of experience on rangelands in the central and western parts of the United States. The principal theme is the relationship of drought to vegetation production and, hence, the productivity of grazing animals. The management ideas and results presented are not to be considered as proposed solutions to the problems arising in Africa from the recent severe drought. On the other hand, the ideas presented are management schemes applied to U.S.A. rangeland to assure sustained productivity of rangelands and grazing animals through cyclic climatic conditions that include periodic droughts. Hopefully, African scientists and decision-makers can glean from these experiences useful approaches toward solutions of Africa's range management problems. The approaches. if they are to truly provide solutions, must be applied in the context of African traditions and socio-economic conditions and the national priorities of each country.

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 forage yield. Furthermore, large fluctuations in forage production from year to year are the norm across rangelands of the world. Therefore, perhaps the first consideration for managing rangelands is to develop the controls in the numbers and distribution of livestock. Secondly, the livestock industry must build flexibility into operations allowing for a decreasing of animal numbers when forage is limited and a building up of numbers when forage is plentiful, in order to survive the effects of a high variability in annual forage production.

Introduction

Effects of drought upon plant welfare vary from reductions in size and vigor to actual death of plants. Coupland (9) and Albertson et at. (2), presented excellent reviews of the effects of climate upon the forage yield and floristic composition of the Great Plains rangelands.

Low rainfall is usually the cause of drought, but high temperatures may also be involved. Drought as related to precipitation may be a result of several growing days without precipitation, low seasonal precipitation, or abnormally low annual precipitation for a particular year or even for a period of years.

The U.S. Weather Bureau defines drought as a period when rainfall is but 30 percent of average for 21 days or longer. Other such definitions state that drought occurs when annual precipitation is 75 percent of normal, or monthly precipitation is 60 percent of normal (27).

It is generally understood that drought conditions along with alternate periods of high precipitation appear rather regularly over time throughout the range areas of the world. In a period of 15 to 20 years, range areas would expect a series of high forage producing years and likewise a series of drought years with herbage production far below normal (22, 10).

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, precipitation is indeed limiting. Most dominant species on arid range lands have developed adaptations to cope with intermittent periods of deficient soil water.

Plants of arid rangelands of the world 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 for the remainder of the year; 2) drought evading, by remaining small or restricting growth when moisture is limiting; 3) drought enduring, plants which may grow very little or not at all for an entire year or more, yet remain alive to renew growth when rain arrives; and 4) drought resisting, by withstanding arid conditions by accumulating water in the plant as a stored reserve. There are many drought resisting plants in grasslands of the world that renew growth following dormancy even though there is no apparent soil water available (31).

Some of the xerophytic adaptations of plants are: a decrease in size of all cells, including guard cells; a thickened cell wall; a strongly developed palisade and mesophyll; an increase in number of stomata per unit area; and a rise in osmotic pressure. Some 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 said 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 water or atmospheric conditions which promote rapid water loss. Anatomical and physiological adaptations common to xerophytes have evolved under many different degrees and 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 range areas of the world, 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 so far failed to present methods of pre-dieting forage yield from existing or pre-existing soil and climatic factors. However, with the aid of high speed computers and expert analysis it should not be too long 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 water along with other parameters related to plant growth. This may involve form of precipitation, distribution over certain periods, soil water to varying depths at the beginning of the growing season, atmospheric temperatures, and evapo-transpiration-precipitation ratios.

Some scientists feel that it is better, for managing and using the forage resource, to predict droughts, 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 inestimable value for the range manager if he could forecast drought incidence and intensity at least a season or even a year in advance.

The solution to low herbage yield during drought periods may lie in man's ability to control weather through cloud seeding. Even though this does not appear promising at the moment, it may someday be developed to the degree that it will at least moderate unfavorable rainfall periods.

It should be pointed out that extreme drought conditions or droughts of long duration seldom cover more than a particular region of the western U.S.A. range area. Therefore, the entire livestock industry of the west never suffers a poor production year, and thus, 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 1800's (12). However, the effect of climate upon permanent change in vegetation composition, along with other related factors such as livestock grazing, fire and small herbivores, has been immensely confounded.

The frequency and duration of drought are both important in determining the severity of the effect of climate upon botanical composition. It is common knowledge that most plants that inhabit rangelands are subject to stresses of limited precipitation at some time during the annual growth cycle, whether for a period of a few weeks or a few months. This might be a normal climatic rhythm or it might 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 long periods of deficient soil water (14).

Studies in mixed prairie in northern central plains during the drought of the 1934-1937 period showed profound changes in height growth and species composition, but few dominant species completely died in most plant communities (32, 3). In some cases, however, a few species were lost in some plant communities. Hurtt (15), Lommasson (19), and Lang (18) found that even some deep-rooted woody plants died and were almost totally absent after the drought in 1939.

In a study by Albertson and Tomanek (1) that covered a period from 1932 to 1961 it was found that the drought of the 30's caused a loss of plant species in the short grass communities to the extent that it was dominated by only one species, 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 was much less than in the short grasses. This is to be expected, since the tall and mixed grasses had more favorable site conditions. However, on the extreme western portion of the mixed grasses between the short and mixed grasses, the tall grass species were completely eliminated from many valley bottoms as a result of drought and dust. In the mixed prairie of the Central Great Plains, the drought in which the precipitation was less than one-half of normal for a period of 6 years caused the vegetation even in protected areas to change markedly (30). Peak numbers of grasshoppers and rabbits added to this devastation.

During an extended drought in the late fifties and another during the early sixties in south Texas, severe damage to the vegetation occurred as a result of deficient moisture and grazing (7). Mortality of range plants was high as a result of subnormal precipitation during these two dry periods. The plant population was not destroyed, however, and recovery was satisfactory, provided ranges were protected from heavy grazing during favorable years.

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, that 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 (22, 13). These authors found that basal area of perennial grasses decreased 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.

On the Santa Rita Experimental Range in southern Arizona, where the U.S. Forest Service since 1903 has studied the semidesert grass-shrub range at about 2,900 feet elevation, with an average rainfall of about 10 inches annually, which varies rather markedly from year to year, about 80 percent of the herbage production is composed of annuals that vary from 250 lbs. or more per acre in wet years to virtually nothing in dry years. The perennial grasses, even though they produce only 20 percent of the grass herbage yield, are more stable and vary less with wet and dry years (22, 21).

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 (17).

In the annual-plant range types in California, Bently and Talbot (5) found that relative amounts of the different species not only varied according to below-average precipitation, but also because of the distribution of rainfall during a drought year. For this reason there were drought years that caused a reduction in the quantity of some annual grasses, but because of rainfall distribution an actual increase of other annual fortes or grasses was observed.

It is generally acknowledged that both soil types and past grazing use affect change in species composition, and this is emphasized during dry years.

In arid ranges where severe droughts occur intermittently there is damage to the habitat through severe wind erosion. 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 some is left pedestalled.

Since drought is characterized by a deficiency of soil water, factors such as mulch, which implement 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. Weaver (29) found that nearly all of the mulch cover disappeared in the prairie after seven 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 displays rather marked fluctuation in 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 varied widely about a mean of 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 was composed of annuals, which accounted for most of the variation. The average yield from perennial grasses was about 20 pounds per acre (21). In an earlier study on the same experimental area the yield of forage on these semidesert ranges varied from about 125 pounds of forage per acre during dry years to about 725 pounds during favorable years (26).

In a desert-grass shrub type in New Mexico, forage production during the best years was four to five times that in the poorest (28, 22).

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 (13).

Table 1 - Vegetation range type, state, and variation in annual precipitation and forage yield for various studies in the arid rangelands of the western United States

Vegetation type

State

Variation in annual precipitation

Ratio High/Low

Variation in forage yield

Ratio High/low

Duration of study

High

Low

High

Low

(inches)

(lbs./A)

Semidesert grass-shrub

Arizona

19.0

3.0 *

6.3

300

15

20.0

10 years

Semidesert grass-shrub

New Mexico

17.3

3.1

5.6

807

114

7.1

7 years

Salt-desert shrub

Utah

11.1

3.8

2.9

468

75

6.2

12 years

Palouse prairie

Oregon

16.6

6.1

2.7

930

280

3.3

10 years

Sagebrush grass

Idaho

15.9

7.9

2.0

1,065

489

2.2

20 years

Short grass prairie

Kansas

31.2

9.2

3.4

5,580

1,250

4.5

18 years

Mixed grass prairie

North Dakota

17.9

12.2

1.5

3,123

12.4

2.6

15 years

(*) Estimated from Weather Bureau data.

In a study of the salt-desert shrub ranges at the Desert Range Experiment Station in Central Utah, Hutchings and Stewart (17) found that herbage yield ranged from 75 pounds to 468 pounds per acre. The higher yielding years produced 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 about one-half of normal. Annual precipitation from October to October varied from 3.8 inches to 11.1 inches. These authors found a close relation 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 (24). 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 1,065 to 489 pounds of air-dry herbage per acre. There was a rather high correlation between herbage production and annual precipitation (6).

Seeded foothill range in the Intermountain area in central Utah showed that grass yield varied from about 400 pounds to about 850 pounds per acre over a 10-year period, as a result of climatic fluctuation (11).

Weaver and Albertson (30), in the Central Great Plains, discovered that where it normally required 12 acres for an animal unit, it required 30 to 50 acres following drought. In many areas there was virtually no grazing capacity. Densities in the short grass communities decreased from 89 percent to 22 percent from 1934-1939. This great drought of 1934 to 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 changes.

At the Central Plains Experimental Range in the northern shortgrass 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 1,027 pounds (4).

Sims (23), in a 12-year study from 1957-1968 in the mixed prairie rangelands of the sandhills in northeastern Colorado, found that annual precipitation varied from 9.9 inches to 19.4 inches, and forage production varied from 1,079 pounds to 1,573 pounds per acre. The forage yield on these sandhill ranges is more stable than the upland hardlands, and therefore the sandhill ranges are assessed at much higher values.

In a grazing study, from 1956 to 1971, at Mandon, North Dakota, in the northern Great Plains mixed-prairie type, the herbage production varied from 1,214 pounds to 3,123 pounds per acre; and annual precipitation for these years was 12.12 inches and 17.92 inches, respectively. As would be expected, the forage yield follows closely the precipitation, since most of the moisture is received from April through September, while plants are growing (20).

In west-central Kansas, on a clay upland range site dominated by native shortgrass species, Launchbaugh (1974) found in a recent study that covered 18 years (1956 to 1973) that forage production varied from 5,580 pounds per acre during wet years (31.12 inches of annual precipitation) to only 1,250 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 receipts 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, and in the northwest only about 15 percent are subnormal (25). Data in Table 2 present 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 that figure. This agrees with what can be expected in forage yield over long time periods. Both annual precipitation and forage yield may be expected to vary as much as 300 to 400 percent over a period of years.

Table 2 - Maximum and minimum annual precipitation recorded over a 35-year period in the range area. (Data from U.S. Weather Bureau, 1937.)

Station

Maximum precipitation (inches)

Minimum precipitation (inches)

Ratio of maximum/minimum

Abilene, Tex.

46.43

10.85

4.28

Baker, Ore.

14.18

6.39

2.22

Boise, Idaho

18.10

7.95

2.28

Cheyenne, Wyo.

22.68

10.85

2.09

Denver, Colo.

22.96

7.75

2.96

Dodge City, Bans.

54

10.12

3.22

Fresno, Calif.

16.47

0.90

18.30

Helena, Mont.

19.63

6.28

3.13

Lander, Wyo.

21.56

8.32

2.59

Modena, Utah

16.67

6.80

2.45

Phcenix, Ariz.

19.73

3.03

6.51

Rapid City, S.D.

27.14

7.51

3.61

Reno, Nev.

11.30

4.32

2.61

Salt Lake City, Utah

21.69

10.34

2.10

Santa Fe, N.M.

21.52

5.03

4.28

Spokane, Wash.

23.28

9.92

2.35

Valentine, Neb.

28.91

11.13

2.60

Williston, N.D.

20.00

6.13

3.26

Yuma, Ariz.

11.41

0.47

24.28

Drought and livestock responses

Studies on seeded foothill range in Utah showed that gains from all age classes of cattle from April 15 to July 1 followed closely the average forage production (11). Even on mountain ranges used for summer grazing, poor animal responses were encountered during drought years (Stoddart, 1944). Studies on arid ranges of the southwestern United States showed that during drought years the forage became dry, and poor animal responses were obtained as a result of nutritional deficiencies (22, 26).

Grazing studies in the Palouse prairie in Oregon and in the shortgrass plains of Colorado showed that steers gained 0.3 and 0.42 pounds per day during drought years, and 2.7 and 1.75 pounds per day during favorable years in each study, respectively (24, 4).

Steer gains on mixed prairie sandhill ranges of northeastern Colorado showed only a slight tendency to rary in daily gains with respect to dry and wet years. The poorest daily gains were made during the driest year (1.41 pounds per head), but the best gains (2.04 pounds per head) were made during a year that received about average precipitation (23).

In the shortgrass prairies of west-central Kansas, steer gains varied from 0.79 pounds per day to 1.79 pounds per day. There was, however, little relationship of animal response to forage yield as it was affected by wet and dry years. This could have been a result of adjusting grazing period to forage yield during each year of the study.

In the northern Great Plains mixed prairie at Mandon, North Dakota, yearling steer gains over a 16-year period followed rather closely the climatic conditions and the quantity of forage produced. This would be expected, since high rainfall years would tend to provide green growing vegetation 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 more than 2.0 pounds per day during the more favorable years (20).

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; but during normal forage production years, where a mixture of desert grasses and shrubs prevail, supplements were needed only during inclement weather (8).

Poor individual gains during drought years may perhaps be the result of closer grazing because of low herbage yield, and the effects of forage being dry and dormant and consequently lower in nutrients.

Management flexibility to cope with drought

Allowable use of plants during drought must be conservative, otherwise permanent damage will occur. Studies in Utah (8) and in New Mexico (22) 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 important for sustained yield of forage.

The need for flexibility in operations to cope with drought has been met in several ways, but none have been designed for the extreme fluctuations in forage yield. It has been suggested that cattle ranchers maintain an all-age operation. This means that steer calves and a plentiful supply of replacements would 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 it is not a substitute for conservative grazing and flexible management practices.

It has been suggested by many investigators (25, 22, 2]) that the breeding cow herd be maintained at about 70 percent of the carrying capacity, based on normal growth years, and the excess capacity during average or above-average years be used by younger animals that are marketable at almost any season.

The 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 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. Another alternative is to lease grazing land in distant areas where drought has not occurred.

In years past it was a practice of the Forest Service to stock the range so that overuse because of dry years would not occur more than one year out of four (25). At present it is the policy of the Government land management agencies to stock conservatively during even average years, so that vigor and reserves could be accumulated for heavier use during subnormal forage-producing years. In most cases the contract with the grazing permittee calls for some time adjustments as to 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.

Bibliography

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Figure 1 - Annual precipitation for 17 western states west of the 100th meridian.


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