Effect of the inclusion of legumes within perennial grasses on water footprint, economic value, and methane emission
Radicke, Kathryn Lindsay
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The Ogallala Aquifer is depleting faster than its recharge rate due to intense irrigation practices in the semi-arid Southern High Plains region. Some producers with reduced well output are opting to convert their cultivated cropland to pasture. The warm season grass ‘WW- B.Dahl’ old world bluestem [Bothriochla bladii (Retz) S.T. Blake] (Dahl) supports good stocker growth at 0.8 to 1.0 kg average daily gain. The inclusion of leguminous forage can improve the forage nutritive value even under limited irrigation or dryland conditions and reduce water footprint while enhancing animal performance. In addition, climate change is predicted to aggravate stressors on agricultural production globally, including in the semi-arid U.S. Southern Plains. In addition to water shortages from the Ogallala Aquifer, rises in temperatures will force agriculturalists to convert from irrigating crops to dryland cropping or grassland systems feeding cattle. Agriculture contributes about 9% of total U.S. greenhouse gas emissions, with enteric methane from fermentation in livestock making up about 55% of the 9% as CO2 equivalents. Methane is a byproduct of ruminal fermentation in cattle and associates a loss of energy to the atmosphere rather than muscle or milk production. Higher quality forages have been shown to decrease methane production within the rumen. Legumes are lower in total fiber (neutral detergent fiber, NDF), digest quickly, and potentially decrease the amount of methane emitted from animals in the diet compared with grass-only diets. Not only does reduced water footprint and enteric methane emissions increase sustainability of agriculture in the Texas High Plains, soil is essential in ecosystem functions, particularly, soil-water dynamics, nutrient cycling, temperature regulation, and gas exchange. Effective management is necessary in improving soil health and can provide a buffer to agricultural systems facing global climate change while continuing to regulate air, nutrient, and water cycling. Where there is high soil disturbance from trampling, improving soil health by using adequate grazing deferment periods is important to sustain optimum soil function. A field study in the Ogallala Aquifer region of the Texas High Plains (New Deal, TX) was conducted to measure water footprint, enteric methane emissions, soil compaction from grazing, and economic value of stocker steers consuming forages with and without legumes through two summer grazing trials. Six tester steers grazed 6 treatment pastures (3 blocks of 2 treatments) throughout the summer to collect animal performance data for the water footprint study and the economic analysis. Methane emissions were then collected continuously for five 24 hour periods from 18 random steers (3 per pasture) using the SF6 technique. Data indicate that the inclusion of legumes increased animal liveweight gain in 2018 and 2019, reduced water footprint in 2019, and showed no statistical difference in methane emissions per head among treatments, however, in 2019 a 10 g steer-1 day-1 decrease was observed. Management of these systems determines its ability to perform functions and processes effectively and sustainably, especially in the Southern High Plains (SHP) where producers are faced with threats to these ecosystems such as land degradation and resource scarcity, because they rely heavily on irrigation, tillage, and chemical inputs to be productive. Grazing stockers with legumes can reduce overall water use, potentially decrease impact on environment, and increase profitability with respect to individual liveweight gain compared to grass only scenarios.