Agroecology of three integrated crop-livestock systems in the Texas High Plains

Technological advances have enabled agriculture to feed and clothe a growing global population with great success. It is now imperative that we remain productive while halting the natural resource degradation that often accompanies high productivity. In the semi-arid Texas High Plains, sustainability of current agricultural practices is threatened by depletion of the Ogallala aquifer and soil erosion. Perennial grasses, the region’s historic vegetation, can build soil rather than lose it and require little or no supplemental irrigation. We designed and tested three agroecosystems that integrated crop and livestock production using a base of perennial forages. All systems were designed to decrease water withdrawals from the Ogallala aquifer relative to conventional irrigated agriculture. The experiment covered a total of 42 ha in a randomized block design with three blocks. The non-irrigated system, DRY (evaluated from 2004 to 2008), included a paddock of native perennial grasses and a rotation of cotton (Cynodon dactylon [L.] Pers.) and foxtail millet (Setaria italica [L.] P. Beauv.). The buffer-irrigated system, LOW (evaluated from 2009 to 2011), was the same as DRY except that LOW added an irrigated paddock of ‘WW-B. Dahl’ old world bluestem (Bothriochloa bladhii [Retz] S.T. Blake; hereafter bluestem), which was harvested for grass seed as well as grazed. The limit-irrigated system, MED (evaluated from 2007 to 2011), included two irrigated paddocks of bermudagrass that were grazed and harvested for hay, and one paddock of bluestem that was grazed and harvested for grass seed. Angus beef stocker steers (Bos taurus L.; initial BW: 245 kg) sequence-grazed each system. We evaluated these systems with a variety of criteria, including forage and animal production, water use, crop yields and quality, forage quality, soil C fractions, soil penetration resistance, forage species stability, economic performance, fossil fuel energy use, and C emissions associated with fossil fuel use. We found that each system had strengths and weaknesses. Annual irrigation water use by each system was: DRY, 0 mm; LOW, 44 mm; and MED, 229 mm. The MED system was the most productive, yielding 446 kg steer live weight gain ha-1, compared with 107 for LOW and 57 for DRY. Bermudagrass yielded the most animal unit grazing days ha-1 (644; bluestem: 246; native grasses: 90; annuals: 41) of any forage and was the highest quality forage with the exception of the annual, foxtail millet. Bermudagrass also had the highest soil concentration of particulate organic matter C (3.1 g kg-1 soil in top 5 cm; native grass: 2.5; bluestem: 2.1), an indicator of higher potential nutrient mineralization and soil C accumulation. The annual fields had the lowest levels (1.3 g kg-1 soil in top 5 cm); however, the MED system was the worst economic performer and emitted the most C (774 kg C ha-1; LOW: 226; DRY: 205) as a consequence of fossil fuel use. In contrast, DRY produced the least amount of grazing (72 animal unit grazing days ha-1; LOW: 76; MED: 434), but was the most profitable and used no irrigation water. Although productivity of DRY and LOW were less than MED, LOW and MED beef carcasses were of equivalent quality (68% USDA Choice; DRY was not evaluated) and cotton lint from DRY and LOW received price premiums in all years (mean $0.024 and 0.106 kg-1, respectively).
The lessons learned from this experiment should be used to design future agroecosystems that conserve soil and water while producing agricultural goods. Relative to annual and perennial non-irrigated paddocks, applying irrigation and N to introduced forages increased productivity and accelerated soil C sequestration, but the additional inputs were not justified economically. It may be more profitable to integrate, at the farm or landscape scale, large areas of non-irrigated native grasses in combination with smaller areas of intensively farmed, high-value crops that receive irrigation and fertilizer. Ley farming, which includes long rotations of perennial grass with annual crops, is one option that may hold promise for the future.