In search of sustainable agricultural systems for the Llano Estacado of the U.S. Southern High Plains

Abstract

Crop production on the Llano Estacado of the Texas High Plains has used precipitation and supplemental irrigation with water pumped from the Ogallala aquifer at rates that have far exceeded recharge for many years. Over 20% of the U.S. cotton (Gossypium hirsutum L.) crop is produced currently in this once vast grassland. Most of this cotton is produced in monoculture systems that are economically risky and contribute to wind-induced erosion and depletion of ground water resources. Although large numbers of cattle are found in this region, little integration of livestock and crop production exists. Integrated crop–livestock systems could improve nutrient cycling, reduce soil erosion, improve water management, interrupt pest cycles, and spread economic risk through diversification. Two whole-farm scale systems compared (1) a cotton monoculture typical of the region; and (2) an alternative integrated system that included cotton, forage, and Angus-cross stocker beef steers (initial body weight 249 kg). Steers grazed the perennial warm-season grass ‘WW-B. Dahl’ old world bluestem [Bothriochloa bladhii (Retz) S.T. Blake] in sequence with rye (Secale cereale L.) and wheat (Triticum aestivum L.) from January to mid-July when they were sent to the feedyard for finishing. Grass seed were harvested from bluestem in October. Cotton in the alternative system was grown in a two-paddock rotation with the wheat and rye. Cotton was harvested from both systems in October. At the end of 5 years, the alternative system reduced needs for supplemental irrigation by 23% and for nitrogen fertilizer by 40% compared with the conventional cotton monoculture. Fewer chemical inputs including pesticides were required by the alternative system. Soil with perennial grass pasture was lower in predicted soil erosion and was higher in soil organic carbon, aggregate stability, and microbial biomass than soil where continuous cotton was grown. Profitability was greater for the alternative system until cotton lint yields reached about 1500 kg ha⁻¹ for the continuous cotton system. Differences between the systems became larger as depth to ground water increased. Systems that are less dependent on supplemental irrigation and less consumptive of non-renewable resources and energy-dependent chemical inputs appear possible, but further improvements are required to ensure sustainability of agricultural systems for the future in the Texas High Plains.

Introduction

The Llano Estacado of the Southern High Plains region of the U.S. is both a very ancient and a very young region; old in terms of evolution of its geology and ecology and young in terms of conversion of this ecosystem by European settlement to a drastically altered, crop dominated ecosystem. This region is also a story about water. Overlying the southern end of the Ogallala Aquifer, water for irrigation has enabled this area to become one of the largest regions of intensive agricultural production on earth. Today, that water is disappearing at an alarming rate. The Ogallala Aquifer is the most intensively used aquifer in the U.S. providing 30% of the total withdrawals from all aquifers for irrigation (Maupin and Barber, 2005). Without question, the future for this region will not be a continuation of past practices. Water in sufficient quantities will not be available to support the irrigation practices and cropping systems that came to characterize this region during the last century. If the economy and productivity of this region are to be sustained into the future, new practices and agricultural systems that are less consumptive of natural resources are essential. In 1997, a multidisciplinary team of individuals representing university scientists, practitioners, industries, government agencies, and local businesses designed research to compare an integrated crop/livestock system with a monoculture cotton (Gossypium hirsutum L.) production system. Currently, between 20 and 25% of the cotton produced in the U.S. is grown in this region, primarily in monoculture systems. Thus, the objective was to test whether the integrated system provided more sustainable production in terms of profitability, irrigation water use, other energy-dependent inputs, and impact on natural resources than the cotton monoculture. This research is ongoing and some results have been published (Allen et al., 2005, Acosta-Martinez et al., 2004, Collins, 2003). These results are reviewed and presented in this paper along with preliminary results from later years.

To understand the challenges and design solutions for this region, it is necessary to appreciate both its history and its physiography. The Great Plains of North America comprises the vast interior region to the east of the Rocky Mountain chain and extends from Texas into Canada. The Llano Estacado or ‘Staked Plains’ of Texas is geographically part of the Great Plains and is located at the southern end primarily in Texas. It is best described as a nearly level, treeless, semiarid region (Webb, 1931). The relative flatness of the land is one of its most dramatic features. Slightly tilted, elevations range from 1372 m in the west to 823 m in the southeast but the vast expanse of the region makes this change imperceptible (Urban, 1992). The name is suggested to derive from the possibility that early Spanish explorers were so disoriented by the featureless landscape that they drove stakes into the ground to mark their route (Brooks et al., 2000).

The High Plains are a remnant of a vast alluvial plain formed by sediments deposited by ancient rivers that flowed eastward from the then-forming Rocky Mountains (Weeks and Sun, 1986). The source of these sediments was debris for even as the mountains were being thrust upwards the forces of erosion were beginning to tear them down. These sediments were deposited in a smooth coalescing alluvial-fan of vast extent (Johnson, 1931). The uplift of the Rocky Mountains occurred around 65 million years ago. Around 10 million years ago, during the Quaternary or late Tertiary age, the water-bearing sands of the Ogallala Aquifer were deposited across the region. This is the major water-bearing unit of the Texas High Plains (Weeks and Gutentag, 1984). The formation is found at varying depths at or near the land surface throughout most of the region with at least 80% of the water found within 120 m of the surface (Urban, 1992). The quality of this water is suited for most purposes and its presence has shaped the use and economy of the region during the past century.

Climatically, the region is semiarid, having a dry steppe climate with mild winters. Annual precipitation over the area ranges from about 355 mm in the west to about 559 mm at the eastern edge. Precipitation occurs often as thunderstorms that occasionally produce tornadoes, high winds, and hail. Intensity of rainfall events leads to runoff and localized flooding. About two-thirds of the annual rainfall occurs just before or during the growing season (Fig. 1). May and September generally receive peak rainfall amounts with very dry conditions prevailing during winter from October through April. The region has one of the highest percentages of sunny days in the continental U.S. but killing frosts can occur in spring following periods of mild temperatures that, along with wind and hail, can stress both plants and livestock. The nearly constant wind that sweeps across the plains has desiccating effects on young crops and contributes to wind erosion of soils. March to May is the windiest period generally and coincides with the planting time for crops that predominate in the region.

Soils of the Llano Estacado are mostly sandy loams, clay loams, with some areas of clayey loams of uniform texture (Livingston, 1952). Created from the Quaternary Aeolian sands and loess sheet of the Blackwater Draw Formation (Lee et al., 1994), generally they are fine-textured and very susceptible to wind and water erosion. At varying depths throughout the region a thick layer of caliche, a clayey-limey stratum, occurs that limits root penetration (Brooks et al., 2000).

Vegetation on the Llano Estacado evolved as a grassland variously characterized as mixed-prairie, short-grass prairie, and in some locations as tall-grass prairie (Gould, 1975). Eventually, the major species that evolved included buffalograss (Buchloe dactyloides) along with blue grama (Bouteloua gracilis) and sideoats grama (Bouteloua curtipendula). Low total seasonal precipitation and/or uneven distribution, and frequent fires contributed to the development of this grassland system. Although virtually treeless, sand sagebrush (Artemisia filifolia) and mesquite (Prosopis glandulosa) are common invaders, especially in the southern areas. These unique grasslands supported a wide diversity of animals. Although most of the large Pleistocene mammals became extinct between 11,000 and 10,000 years ago, in more modern times, the Llano Estacado was home to bison (Bison bison), pronghorn (Antilocapra americana), wolves (Canis lupus), coyotes (Canis latrans), and with Spanish exploration during the 1600s, the horse (Equus caballus) was reintroduced into its ancestral home.

Human occupation of this region is known to date back over 11,000 years (Brooks et al., 2000). Paleolithic hunters inhabited these plains to the eastern slopes of the Rockies. These ‘Big Game Hunters’ were followed by woodland and river-valley cultures that subsisted on small game and wild plants. Around 1500 years ago, evidence of a more agrarian society exists where the people grew corn (Zea mays), beans (Phaseolus spp.), squash (Cucurbita spp), and tobacco (Nicotiana tabacum) along the river bottoms although hunting continued to be important. In more recent time, the Llano Estacado became home to Apaches, Comanches, and the Kiowas. Apaches probably arrived in this region about 1500 (Hyde, 1959). The fierce Comanche dominated the Southern High Plains by the middle of the 18th century (Noyes, 1993).

With the arrival of Spanish explorers in the 16th century, the fate of the region was irrevocably altered. These early explorers brought with them horses and guns and the course of history began to change. The introduction of the horse greatly improved the mobility of the native peoples and the Comanche particularly became the great horsemen of the plains (Noyes, 1993). Alternating among trade, cooperation, conquest, and division, the forces of European settlement were gradually imposed into this region. Although under treaty with the U.S. Government, hunters entered the region and began massive slaughter of the vast herds of buffalo to supply demands of Eastern markets for buffalo hides. In 1874, the same year that barbed wire was patented, Indian tribes gathered in one last but unsuccessful effort to drive the encroaching buffalo hunters from their homelands (Izzard, 1993). By the 1880s the Native Americans effectively had been moved to reservations to the north. Cattle had replaced the buffalo as the predominant grazing animal and the region became dominated by large cattle ranches.

Until the late 1880s, would-be settlers shunned the Texas High Plains primarily because of lack of reliable and adequate water. By the late 1800s people entering the region brought with them the technology to hand-dig wells and pump the water from the aquifer that by then was known to exist just a few feet below the surface of the land. Windmills provided the means to pump water in sufficient quantities for livestock and for growing of food and also supplied water to replenish the steam locomotives that had begun to cross the region. By the 1930s and 1940s, with rural electrification as well as gasoline engines, irrigated agriculture expanded in the region. The water was thought to be an inexhaustible supply recharged with snow-melt from the Rocky Mountains. By the 1970s, 69% of the total irrigated crop land in Texas was located in the High Plains (Urban, 1992). The Texas High Plains also had become home to a vast cattle feeding industry that today accounts for about 25% of all the cattle on feed in the U.S.

Studies conducted during the 1970s began to reveal the decline in water in the aquifer and concerns mounted over the future use of this resource. Improved irrigation technologies developed during the last part of the 20th century greatly reduced the waste of water pumped for irrigation but more wells continued to be built adding to the total withdrawal of water. By the close of this century, a realization was inescapable that water was becoming a major concern to the sustainability of the systems that had been implemented in this region over the past 100 years. Today, over 95% of the water extracted from the aquifer is used for irrigated agriculture. Withdrawal rates documented by the High Plains Underground Water District in the 15 counties surrounding Lubbock, Texas during the period of 1993–2003 showed a drop of 399 mm per year in the aquifer (High Plains Underground Water Conservation District No. 1, 2003). Recharge to the aquifer is minimal, however, with most estimates suggesting less than 25 mm/year (Brooks et al., 2000, Gutentag et al., 1984) to a maximum estimate of 50 mm/year in some areas (High Plains Underground Water District, personal communication). Less than 1% of the precipitation percolates through the root zone effectively making this a non-rechargeable water source (Brooks et al., 2000).

Thus, today, the Llano Estacado reflects the results of millions of years of geomorphologic processes, at least 12,000 years of occupation by Native Americans, but only a century or so of the impact of modern civilization. Over millions of years, plants and animals evolved in this region to form a stable, interdependent, and resilient ecosystem (Brooks et al., 2000). Native Americans followed nomadic and locally agrarian lifestyles over thousands of years of occupation. During the last 100–125 years, settlement largely by Europeans has resulted in disturbance of nearly every hectare of this once vast grassland. Water, once considered to be an inexhaustible resource, is now known to be declining at a rate that has already left many wells dry and crop production increasingly vulnerable.

Today, over 5 million ha of the High Plains have been developed for cultivation, with nearly half of the area being irrigated with water from the Ogallala aquifer. About 20–25% of the total U.S. cotton crop is produced in this region with over 1.5 million ha planted annually (USDA, 2003, TASS, 1999). Over the past 20 years, a cotton monoculture has developed due to low grain prices, increased cost of irrigation, the U.S. government farm programs of 1985 and 1990, and historically a lack of insect pests. Recent changes in agricultural policy raise questions about the continued profitability and financial viability of current cropping practices. Cotton has been the main row crop with dependable economic profit. However, the cotton monoculture also has increased producers’ cash-flow problems and their vulnerability to crop failure due to lack of diversification of agricultural enterprises. Traditional cotton production systems allow nearly 50% of the total water applied to be wasted to runoff and evaporation, reduce natural soil tilth and fertility, and increase susceptibility to wind-induced soil erosion. Cover crops are grown primarily in the Southern High Plains in irrigated cotton systems where the soil has a high sand content, and blowing sand is a serious constraint to early cotton establishment. The cost in water of cover crops can be high, and they have no recognized economic benefit other than to reduce wind damage to the subsequent cotton. In areas where blowing sand is less of a problem or under dryland cotton production, cover crops are rarely grown. The potential for grazing cover crops by livestock is virtually unknown.

Texas leads all other states in the U.S. in numbers of beef cattle with the High Plains region including 8 of the 10 leading counties (USDA, 2003, TASS, 1999). The Texas High Plains also has the highest concentration of cattle in feedlots in the U.S. (TASS, 1999). About 5.5 million head of stocker cattle are shipped into the High Plains each autumn, primarily to graze wheat (Triticum aestivum L.) before entering feedlots. With grazing centered largely around wheat, marketing strategies for cattle are severely limited. Changes in farm programs now allow for use of other small grains and provide opportunities to extend grazing seasons.

Integrating crop and livestock production can improve total productivity and nutrient management while reducing needs for pesticides (Allen et al., 1994, Allen et al., 1997, Fontenot et al., 1997). Crop rotation and use of cover crops have long been known to reduce erosion and improve productivity. The importance of crops, forages, and livestock to the Texas High Plains highlights the need to develop systems that enhance profitability, improve conservation of soil and water resources, and expand marketing opportunities for a more sustainable agricultural system. Due to its impact on natural resources, the cotton monoculture currently in place is not a sustainable system. Although the native grassland/grazing system was sustainable for millions of years, the level of production is inadequate to generate either sufficient income or animal products to make this a viable option. As grazing systems are intensified there is a parallel increase in management, water use, fertilizer, pesticides, and other inputs. The need is to find viable systems that can be integrated for crop, forage, and livestock production such that their complimentary benefits would allow for a more sustainable use of water and soil while maintaining an appropriate level of agricultural production.

Research that began in 1997 is being conducted to compare and evaluate productivity, profitability, and impact on natural resources of (1) a cotton monoculture system managed by current, recommended technology and (2) an integrated cotton–forage/livestock system. Both systems used a sub-surface drip irrigation system to maximize efficiency of irrigation water use. This paper presents a review of this ongoing research.