Semiarid Southern Great Plains Research Articles

Grazing summer cover crops mix impact on carbon[sbnd]nitrogen cycling, soil water, and wheat yields

Summer fallow practice is common in the US semiarid Southern Great Plains ecoregion. Its primary objective is to store water to enhance winter wheat production for grain and grazing. Prior research on replacing fallow with legume cover crops (CC) (green manure) reported a yield loss through soil moisture depletion for winter wheat. Recent research has shown the potential of grass CC to enhance soil physical properties compared to legume CC. Cost-benefit analyses of CC make it difficult to justify their adoption. Grazing, however, offers an opportunity to mitigate CC costs. The impact of a grazed CC mix on stored soil water, CN cycling, intercropping, and wheat yields was evaluated. Although CC used soil moisture early in the growing season and nitrate+nitrite-N, they recharged soil moisture by the critical wheat seeding and dormancy-breaking periods. In the summer of 2014, net mineralization in CC increased nitrate+nitrite-N by 10-fold compared to 2013 concentrations, resulting in no significant differences in yields in 2015. Nitrate+nitrite-N was reduced by 28% in 2015 compared to the preceding year due to CC growth and net nitrate+nitrite-N immobilization in residues, resulting in 25% less yield in CC treatments in 2016 compared to no-till (NT) fallow. Grazed and ungrazed CC enhanced organic N by up to four times that in NT fallow and sequestered particulate organic N and C by 7–17% and 6–10%, respectively, in micro-aggregates. Despite reducing biomass by 56%, grazing did not differ from most ungrazed measured soil parameters, offering the potential for mitigating CC production costs.

Excess energy and photosynthesis: responses to seasonal water limitations in co-occurring woody encroachers of the semi-arid Southern Great Plains

Woody plant areal encroachment is pervasive throughout the Southern Great Plains, USA. The ability of woody plants to dissipate excess solar radiation - dynamically over the day and sustained periods without recovery overnight -is key for maintaining photosynthetic performance during dry stretches, but our understanding of these processes remains incomplete. Photosynthetic performance and energy dissipation were assessed for co-occurring encroachers on the karst Edwards Plateau (<i>Juniperus ashei</i>, <i>Prosopis glandulosa</i>, and <i>Quercus fusiformis</i>) under seasonal changes in water status. Only <i>J. ashei </i>experienced mild photoinhibition from sustained energy dissipation overnight while experiencing the lowest photochemical yields, minimal photosynthetic rates, and the highest dynamic energy dissipation rates at midday during the dry period - indicating susceptibility to photosynthetic downregulation and increased dissipation under future drought regimes. Neither other encroacher experienced sustained energy dissipation in the dry period, though <i>P. glandulosa</i> did experience marked reductions in photosynthesis, photochemical yields, and increased regulatory dynamic energy dissipation.

Net greenhouse gas balance with cover crops in semi-arid irrigated cropping systems

Climate smart agriculture has been emphasized for mitigating anthropogenic greenhouse gas (GHG) emissions, yet the mitigation potential of individual management practices remain largely unexplored in semi-arid cropping systems. This study evaluated the effects of different winter cover crop mixtures on CO2 and N2O emissions, net GHG balance (GHGnet), greenhouse gas intensity (GHGI), yield-scaled GHG emissions, and soil properties in irrigated forage corn (Zea mays L.) and sorghum (Sorghum bicolor L. Moench) rotations. Four cover crop treatments: (1) grasses, brassicas, and legumes mixture (GBL), (2) grasses and brassicas mixture (GB), (3) grasses and legumes mixture (GL), and (4) a no-cover crop (NCC) control, each replicated four times under corn and sorghum phase of the rotations, were tested in the semi-arid Southern Great Plains of USA. Results showed 5–10 times higher soil respiration with cover crop mixtures than NCC during the cover crop phase and no difference during the cash crop phase. The average N2O-N emission in NCC was 44% lower than GL and 77% lower than GBL in corn and sorghum rotations. Cash crop yield was 13–30% greater in cover crop treatments than NCC, but treatment effects were not observed for GHGnet, yield-scaled emissions, and GHGI. Integrating cover crops could be a climate smart strategy for forage production in irrigated semi-arid agroecosystems.

LESSER PRAIRIE-CHICKEN (TYMPANUCHUS PALLIDICINCTUS) USE OF MAN-MADE WATER SOURCES

The lesser prairie-chicken (Tympanuchus pallidicinctus) occurs in the semiarid southern Great Plains, a region prone to periods of drought. Researchers generally believe that lesser prairie-chickens are able to satisfy their water requirements through preformed water and metabolic processes, but also know that they experience low survival and reproductive success during periods of drought. We used motion-sensing cameras to assess lesser prairie-chicken visits to man-made free water sources over a 48-month period from March 2009 to February 2013 in west Texas. Our objective was to examine temporal patterns of water use by lesser prairie-chickens, and to explore life history phenology and environmental conditions that may influence the species' use of free water. We documented 1,439 visits to water sources by lesser prairie-chickens. Their use of water sources was high during the winter months (December–February; 92 visits per 100 trap days) but the highest average visit rate to water sources occurred during the lekking-nesting life stage (March–May; 146 visits per 100 trap days). Water use was lower during the brood-rearing stage (June–August; 71 visits per 100 trap days) and lowest during the brood dispersal and independence stage (September–November; 19 visits per 100 trap days). Water use was strongly associated with dew point (P < 0.0001) and temperature (P = 0.0002) but was not associated with precipitation (P = 0.1037). These data indicate life-cycle stage (e.g., lekking-nesting) and reduced availability of preformed water may influence use of free water sources by lesser prairie-chickens. Current climate models predict the region of the study area will experience increases in temperature and decreases in frequency of precipitation. The combined effect of this would be reduced environmental moisture. If the prediction of increasing aridity in the region holds true, man-made water sources may become a tool for conservation of the species.

Winter canola yield and nitrogen use efficiency in a semiarid irrigated condition

AbstractWinter canola (Brassica napus L.) is an economically viable alternative crop for the semiarid southern Great Plains of the United States. Although land area under winter canola has increased in recent years, an independent guideline on fertility recommendation and their effects on crop growth and soil health is not available for the semiarid southern Great Plains region. This study aimed to evaluate winter canola growth, yield components, and nitrogen use efficiency (NUE) with different nitrogen application timings (NATs) in a semiarid irrigated condition. A 2‐yr study was designed with two canola varieties (Riley and 46W94) and four NATs. Nitrogen treatments were F100 (100% of nitrogen applied in fall), S100 (100% of nitrogen applied in spring), FS50 (50% of nitrogen applied in fall and 50% in spring), and FSB25 (25% of nitrogen applied in fall, 25% in spring, and 25% before flowering). The winter dry matter accumulation was greater with the F100 N application, whereas the forage quality was better with the FS50 treatment. The highest seed yield of 2,539 kg ha–1 was from S100, which was not significantly different from FS50 and FSB25. The N use efficiency was the highest under FSB25, which uses 25% less N, followed by S100, FS50, and F100 treatments. The FS50 with 50% of N application in fall and 50% in spring was the most efficient N timing treatment when winter biomass accumulation, seed yield, and seed oil and protein contents were considered.

Nitrous Oxide and Methane Emissions from Beef Cattle Feedyard Pens Following Large Rainfall Events

HighlightsNitrous oxide and methane emissions were measured from a commercial beef feedyard following large rainfall events.Nitrous oxide emissions dropped below detection levels for ten days following a 77 mm rainfall event.Daily N2O and CH4 emissions followed a diel pattern, peaking at manure temperatures of 36°C to 38°C.Results will be used to refine empirical models for predicting GHG emissions from open-lot feedyards.Abstract. More than six million beef cattle are fed annually in feedyards on the semiarid Southern Great Plains (SGP). Manure deposited on the open-lot pen surfaces contributes to greenhouse gas (GHG) emissions. Nitrous oxide (N2O) and methane (CH4) are GHGs linked to climate change, and both have global warming potentials greater than carbon dioxide (CO2). Two sampling campaigns were conducted in 2019 to quantify N2O and CH4 emissions from open-lot pen surfaces. The occurrence of large, unforecast rainfall events during both campaigns provided an opportunity to compare GHG emissions from the dry manure before rainfall and from the wetted pen surface for one to two weeks following precipitation. Temporal variability was quantified by continuous sampling using six to eight automated flux chambers, a multiplexer system, and real-time analyzers. Spatial variability was quantified using a recirculating portable chamber on a 5 × 8 grid. Nitrous oxide emissions dropped below detection levels for ten days after the precipitation event. Nitrous oxide emissions were related to nitrification or other aerobic processes. Methane emissions dropped below detection levels for five days after the precipitation event and then increased to pre-rainfall levels by day 8. When present, N2O and CH4 emissions followed a diel pattern, with the highest emissions occurring during the afternoon when manure pack temperatures at the 25 mm depth were 36°C to 38°C and ambient temperatures were 31°C to 32°C. Average CH4 emissions from the feedyard pen surface were 96-fold lower than estimated enteric CH4 emissions. The results of this field research will be used to refine empirical models for predicting annual N2O and CH4 emissions from open-lot beef cattle feedyards on the semiarid SGP. Keywords: Beef cattle, Flux chamber, Greenhouse gas, Manure, Nitrous oxide, Rainfall.

Efficacy of Cover Crops on Weed Suppression, Wheat Yield, and Water Conservation in Winter Wheat–Sorghum–Fallow

ABSTRACTReducing the fallow period by using cover crops in a winter wheat (Triticum aestivum L.)–sorghum [Sorghum bicolor (L.) Moench]–fallow system has the potential to suppress weeds and improve wheat yield, yet limited information is available from the semiarid southern Great Plains (SGP) of the United States. This study determines the effects of spring‐planted cover crops on weed suppression and winter wheat yields in semiarid environments. The study used a randomized complete block design with eight cover crop treatments and three replications. The cover crop treatments consisted of fallow (weeds controlled with herbicide), pea (Pisum sativum L.), oat (Avena sativa L.), canola (Brassica napus L.), pea + oat, pea + canola, pea + oat + canola, and six‐species mixture of pea + oat + canola + hairy vetch (Vicia villosa L.) + forage radish (Raphanus sativus L.) + barley (Hordeum vulgare L.). Both winter wheat and cover crops received supplemental irrigation. The cover crops showed potential to suppress weeds and maintain good ground cover during summer. Specifically, weed biomass in the oat and oat mixtures with legumes and brassicas were 73 to 85% less than in fallow during 2018. Cover crops showed little effect on wheat yield, yield components, and water use efficiency. Although long‐term studies of cover crops effects on multiple ecosystem services may help to select the most effective cover crop on wheat yield and water use efficiency in the SGP, this study revealed benefit of cover cropping through weed suppression.

Model consensus projections of US regional hydroclimates under greenhouse warming

We investigate ensemble-mean (‘consensus’) values of resolution-weighted CMIP5 multi-model simulations of 1976–2005 summer regional hydroclimates, and of their projected 2070–2099 changes under three progressively more severe representative concentration pathways greenhouse scenarios. Uncertainties in these consensus values are estimated from the cross-ensemble scatter. We analyze differences among 30 year present-day and future consensus summer hydroclimates that are averaged over three disparate regions of the United States: the semi-arid Southern Great Plains, the arid Southwest, and the humid Southeast. Our study considers the impact of several scenarios of greenhouse forcing on the regional averages of both single hydroclimatic variables and on ratios of variables which are indicative of continental drying, as well as the partitioning of surface moisture or available energy into their respective subcomponents. In all three study regions, there is a projected robust increase in surface temperature as the severity of the greenhouse scenario increases; but the regional-average hydroclimatic changes are comparatively uncertain, and often are not proportional to the change in surface warming. There is, however, a projected robust increase in continental drying that is manifested by several complementary measures, but that differs in magnitude by region. The prospect of future continental aridification should be viewed with some caution, however, since it may be a result of various shortcomings in current-generation climate models or in the specified greenhouse scenarios.

Grazing and Tillage Effects on Soil Properties, Rain Infiltration, and Sediment Transport during Fallow

Core Ideas Soil loss tended higher (&gt;40%) with grazing, but tillage increased soil loss significantly. Infiltration increased with tillage and did not vary with grazing during either fallow phase. Combined grazing and NT management decreased infiltration without tillage to disrupt compaction. Depletion of the High Plains Aquifer that supplies irrigation water for the semiarid Southern Great Plains may necessitate dryland production of crops like wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] using the wheat–sorghum–fallow (WSF) rotation. Reduced crop productivity without irrigation can be offset by intensifying the WSF rotation with cattle (Bos taurus) grazing. Biomass removal and soil trampling without tillage, however, may reduce rain infiltration and storage during fallow that increases with no‐tillage (NT) over stubble‐mulch (SM) tillage. Our objective was to quantify grazing and tillage effects on infiltration, sediment transport, and aggregate stability during fallow periods after sorghum and wheat. Using a split plot randomized complete block design, we compared wet aggregate size distribution of a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll; 0.0–0.05 m) and sediment yield in runoff of simulated rain during the fallow‐like idle period after wheat and the sorghum fallow. Sediment concentration and yield for both fallows were numerically larger (&gt;40%) with grazing. Sediment concentration from SM tillage increased significantly over NT for either fallow, but soil loss differed only for wheat fallow. Mean final infiltration rate (IRf) and amount (IA60) did not differ significantly with grazing during either fallow, but IA60 was usually 20% lower for grazed than for ungrazed paddocks. Because SM tillage after grazed wheat significantly (P &lt; 0.01) increased IRf and IA60 over NT, occasional SM tillage to disrupt compaction from trampling may increase water conservation for dryland cropping systems combining grazing with NT residue management.

Land–Atmosphere Coupling at the Southern Great Plains Atmospheric Radiation Measurement (ARM) Field Site and Its Role in Anomalous Afternoon Peak Precipitation

Abstract The multimodel Global Land–Atmosphere Coupling Experiment (GLACE) identified the semiarid Southern Great Plains (SGP) as a hotspot for land–atmosphere (LA) coupling and, consequently, land-derived temperature and precipitation predictability. The area including and surrounding the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) SGP Climate Research Facility has in particular been well studied in the context of LA coupling. Observation-based studies suggest a coupling signal that is much weaker than modeled, if not elusive. Using North American Regional Reanalysis and North American Land Data Assimilation System data, this study provides a 36-yr (1979–2014) climatology of coupling for ARM-SGP that 1) unifies prior interdisciplinary efforts and 2) isolates the origin of the (weak) coupling signal. Specifically, the climatology of a prominent convective triggering potential–low-level humidity index (CTP–HIlow) coupling classification is linked to corresponding synoptic–mesoscale weather and atmospheric moisture budget analyses. The CTP–HIlow classification defines a dry-advantage regime for which convective triggering is preferentially favored over drier-than-average soils as well as a wet-advantage regime for which convective triggering is preferentially favored over wetter-than-average soils. This study shows that wet-advantage days are a result of horizontal moisture flux convergence over the region, and conversely, dry-advantage days are a result of zonal and vertical moisture flux divergence. In this context, the role of the land is nominal relative to that of atmospheric forcing. Surface flux partitioning, however, can play an important role in modulating diurnal precipitation cycle phase and amplitude and it is shown that soil moisture and sensible heat flux are significantly correlated with both occurrence and intensity of afternoon peak precipitation.

Factors Influencing Premetamorphic Body Mass of Two Polymorphic Spadefoot Species in Cropland and Grassland Playas

Spadefoot Toads, Spea bombifrons and Spea multiplicata, are often the most abundant amphibian species in playa wetlands in the semi-arid Southern Great Plains. Tadpoles of these species are morphologically indistinguishable, but both can develop omnivore and carnivore morphotypes. We tested the influence of species and morphotype on body mass of premetamorphic Spea tadpoles and the influence of land use (cropland vs. native grassland) and desiccation stress (water depth and daily rate of water-depth loss) on body mass of S. bombifrons carnivores and omnivores and on body mass of S. multiplicata and S. bombifrons omnivores. Spea bombifrons carnivores were larger than omnivores only in relatively deep playas. A decrease in water depth resulted in less body mass for carnivores, but omnivore body mass did not change. Spea bombifrons omnivores had larger body mass than did S. multiplicata omnivores at low and average playa depth. Spea bombifrons omnivores did not alter body size, whereas S. multiplicata omnivores were smaller when depth decreased. Generally, in average and deep playas, tadpoles were larger in cropland than grassland playas, but in shallow waters, cropland and grassland tadpoles had similar, smaller body mass. Also, cropland tadpoles had reduced body size, whereas grassland tadpoles had similar body mass when depth decreased. Our study documents body size divergence within the same morph in the two sympatric Spea species and species divergence within the omnivore morph depending on desiccation stress. Future studies should investigate whether these species may be mediating competition through both morph development and body size.

Human impacts on terrestrial hydrology: climate change versus pumping and irrigation

Global climate change is altering terrestrial water and energy budgets, with subsequent impacts on surface and groundwater resources; recent studies have shown that local water management practices such as groundwater pumping and irrigation similarly alter terrestrial water and energy budgets over many agricultural regions, with potential feedbacks on weather and climate. Here we use a fully-integrated hydrologic model to directly compare effects of climate change and water management on terrestrial water and energy budgets of a representative agricultural watershed in the semi-arid Southern Great Plains, USA. At local scales, we find that the impacts of pumping and irrigation on latent heat flux, potential recharge and water table depth are similar in magnitude to the impacts of changing temperature and precipitation; however, the spatial distributions of climate and management impacts are substantially different. At the basin scale, the impacts on stream discharge and groundwater storage are remarkably similar. Notably, for the watershed and scenarios studied here, the changes in groundwater storage and stream discharge in response to a 2.5 °C temperature increase are nearly equivalent to those from groundwater-fed irrigation. Our results imply that many semi-arid basins worldwide that practice groundwater pumping and irrigation may already be experiencing similar impacts on surface water and groundwater resources to a warming climate. These results demonstrate that accurate assessment of climate change impacts and development of effective adaptation and mitigation strategies must account for local water management practices.

Residue and Long-Term Tillage and Crop Rotation Effects on Simulated Rain Infiltration and Sediment Transport

Increased precipitation storage as soil water is crucial to dryland production of wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] on the semiarid southern Great Plains. At the USDA-ARS Conservation and Production Research Laboratory at Bushland, TX, surface runoff from a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) is typically greater with no-till (NT) than stubble-mulch (SM) tillage under the 3-yr wheat–sorghum–fallow (WSF) dryland crop rotation. Our objective was to quantify the effects of NT or SM tillage with bare or retained residue on infiltration of simulated rain, sediment transport, and related aggregate stability for continuous wheat (CW) and WSF rotation plots established in 1983. Compared with bare soil, the retained wheat residue cover increased mean 60-min cumulative infiltration across all tillage and rotation combinations by >25 mm and decreased soil loss. Cumulative rain infiltration and total soil loss did not vary significantly (P < 0.05) with tillage, but the mean infiltration rate at 60 min was 15.0 mm h−1 less for NT than SM. The CW rotation typically increased infiltration regardless of residue cover or tillage compared with the WSF rotation, which we attributed to greater aggregate stability. The greater aggregate stability for CW may have decreased soil loss compared with WSF, which was greatest for bare SM tilled sites. We conclude that residue cover significantly increases rain infiltration over bare soil conditions independent of any tillage or rotation treatment effects. Nevertheless, increased rain infiltration due to residue cover was not sustained in the absence of soil disturbance for NT.

Long‐Term Effects of Profile‐Modifying Deep Plowing on Soil Properties and Crop Yield

Increasing precipitation storage as plant‐available soil water increases dryland crop yields on the semiarid southern Great Plains. The dominant soil in this region is a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll), which features dense subsoil layers that limit infiltration and root proliferation. Deep (∼0.7 m) plowing may eliminate these layers, but the duration of treatment efficacy must be sufficient to recoup costs. Our objective was to quantify the long‐term effects of deep plowing on soil bulk density (BD), penetration resistance (PR), ponded infiltration, and crop yield. In fall 1971, paired 24‐ by 460‐m level conservation bench terrace plots at the USDA‐ARS Conservation and Production Research Laboratory, Bushland, TX (35°11′ N, 102°5′ W) were moldboard plowed to 0.7 m or untreated and, subsequently, maintained with conventional sweep tillage. Grain sorghum [Sorghum bicolor (L.) Moench] was grown intermittently during the next 34 yr for 15 paired yield comparisons. Compared with the control, deep plowing significantly decreased BD and PR and increased water infiltration. Long‐term mean grain yield of 2.86 Mg ha−1 with deep plowing was greater (P &lt; 0.05) than the 2.61 Mg ha−1 yield in control plots. Approximately 89% of the 15‐yr 3.75 Mg ha−1 cumulative yield increase with deep plowing occurred during 4 yr with large early growing season rains. We attributed this yield increase with deep plowing to greater internal drainage that reduced crop injury during infrequent floods and not greater root growth through the dense subsoil. Thirty years after deep plowing, high water intake suggests that flow‐limiting subsoil layers had not yet redeveloped. The sustained deep‐plowing benefit extends the period to recoup the 1971 plowing costs of US$160 ha−1

Guar Tolerance to Postemergence Herbicides

Guar production in the United States is limited to a relatively small region in the semiarid southern Great Plains of Texas and Oklahoma. The lack of POST broadleaf herbicides is a potential limiting factor to increased production. A greenhouse study was initiated in 2001 at the Texas A&M Research Center near Vernon, TX to evaluate guar tolerance to 10 POST herbicides typically used in soybean or cotton. Guar seedlings were grown in pots, and herbicides with appropriate adjuvants were applied to 3-wk-old seedlings at the registered rate (1×) and twice (2×) the registered rate for soybean or cotton. The study was repeated twice, with six replications in each run. Twenty-eight d after treatment (DAT), visual injury and aboveground dry weight of viable biomass were recorded for each plant. Significant differences (P = 0.05) were noted among herbicides for visual injury and viable biomass. Little or no differences in visual injury and aboveground dry weight were observed between the control (no herbicide ap...

Erosion Potential of a Torrertic Paleustoll after Converting Conservation Reserve Program Grassland to Cropland

Extensive cropland areas were covered by the Conservation Reserve Program (CRP) in the semiarid southern Great Plains. Because soils were highly erodible, would erosion again become a problem when CRP land was converted to cropland? The erosion potential due to tillage methods used to convert CRP grassland to cropland was determined on Pullman clay loam (Torrertic Paleustoll). Tillage methods were no‐, sweep, disk, and moldboard + disk tillage with CRP grass retained or removed (mowing and baling), and grass burning followed by sweep or disk tillage. Wind erosion potential was based on percentage of &gt;0.84‐mm diam. and mean weight diameter (MWD) of dry aggregates at 2 to 3 yr after converting to cropland. Water erosion potential was based on MWD and percentage of &lt;0.25‐mm water‐stable aggregates, and water stability of 1‐ to 2‐mm aggregates at crop planting and harvest. Few differences due to tillage methods were significant. For dry aggregates, more than 60% were &gt;0.84‐mm diam. and MWD was &gt;10 mm with all tillage methods, indicating a low wind erosion potential. Wet aggregate stability and MWD values at some sampling times indicated water erosion could occur. Although erosion potential was low, continued use of residue‐incorporating tillage could lead to greater potentials. Because of initially low potentials, CRP land on Pullman and similar soils could be converted to cropland by any tillage method. Then, a conservation tillage system (e.g., no‐tillage) could be implemented before erosion by wind or water became a serious problem.

Effect of Tillage Timing on Herbicide Toxicity to Field Bindweed

When can tillage be resumed after herbicide treatment of field bindweed (Convolvulus arvensis L.) without reducing control ? This question was answered with experiments on a clay loam soil in the semi-arid southern Great Plains in mid-summers of 1979, 1985, and 1986 when field bindweed vigor was visually rated as good, fair, or poor, respectively. Plant vigor was controlled by availability of soil water. Highest control, about 95%, of field bindweed 9 mo after application of 2,4-D, glyphosate, and dicamba was aehieved when vigor was good. Control with picloram + 2,4-D was not affected by plant vigor. When field bindweed growth was rated as good, control with dicamba, glyphosate, and a mixture of 2,4-D and picloram was not reduced when sweep tillage was delayed for 1 day after treatment (DAT). To achieve maximum control with 2,4-D, tillage had to be delayed 7 DAT. When plant vigor was fair, control was not reduced with picloram + 2,4-D when tillage was delayed 2 DAT. Glyphosate and dicamba gave maximum control after 3 DAT delay, and 2,4-D required a 7 DAT delay for maximum effectiveness. With poor plant vigor, picloram + 2,4-D, a herbicide combination that persisted in the soil, gave best control when tillage was delayed 2 DAT. Dicamba, a less persistent herbicide, required a 7 DAT delay for maximum controL Control was not better than zero 9 mo after treatment with glyphosate or 2,4-D when plants had poor vigor.

Crop Rotation and Tillage Effects on Organic Carbon Sequestration in The Semiarid Southern Great Plains

Limited information is available regarding soil organic carbon (SOC) distribution and the total amounts that occur in dryland cropping situa­ tions in semiarid regions. We determined crop rotation, tillage, and fer­ tilizer effects on· SOC distribution and mass in the semiarid southern Great Plains. A cropping system study was conducted for 10-years at Bushland,TX, to compare no-till and stubblelTlUlch management on four dryland cropping systems: continuous wheat (CW) (Triticum aestivum L.); continuous grain sorghum (CS) (Sorghum bicolor [L.] Moench.); wheat/fal­ low/sorghum/fallow (WSF); and wheat/fallow (WF). Fertilizer (45 kg N ha- 1 ) was added at crop planting to main plots. Subplots within each tillage and cropping treatment combination received no fertilizer. Ten years after treatment initiation, soil cores were taken incrementally to a 65-cm depth and subdivided for bulk density and SOC determination. The no-till treatments resulted in significant differences in SOC distrib­ ution in the soil profile compared with stubblemulch tillage in all four crop rotations, although differences were largest in the continuous crop­ ping systems. Continuous wheat averaged 1.71% SOC in the surface 2 cm of soil compared with 1.02% SOC with stubblemulch tillage. Continuous sorghum averaged 1.54% SOC in the surface 2 crn of soil in no-till com­ pared with 0.97% SOC with stubblemulch tillage. Total SOC content in the surface 20 cm was increased 5.6 t C ha- 1 in the CW no-till treatment and 2.8 t C ha- 1 in the CS no-till treatment compared with the stub­ blemulch treatment. Differences were not significantly different between tillage treatments in the WF and WSF systems. No-till management with continuous crops sequestered carbon in comparison to stubblemulch management on the southern Great Plains. Fallow limits carbon accu­ mulation.

Ridge tillage for continuous grain sorghum production with limited irrigation

Improved soil and water conservation are basic goals for crop production systems in the semi-arid southern Great Plains (SGP) of the USA and similar regions, and managing crop residues on the soil surface plays a key role in attaining these goals. Residue production by dryland grain sorghum ( Sorghum bicolor (L.) Moench) is often too low in semi-arid regions to improve soil and water conservation, but much greater amounts are produced by irrigated sorghum. However, irrigation water may be limited, as in the SGP, and a limited-irrigation approach is sometimes used for sorghum, which responds well to timely irrigations. This study, conducted on Pullman clay loam (fine, mixed, thermic Torertic Paleustoll) from 1986 to 1992, determined effects of residue management treatments involving ridge tillage on continuous production of two grain sorghum hybrids with limited irrigation. Treatments were conventional (ConvT) tillage, which involved stalk shredding, disking, and rebuilding of the ridges soon after sorghun harvest, and two ridge tillage methods (residues shredded early (at harvest) — ConsTE; residues shredded late (at planting) — ConsTL). Soil water storage between crops was greatest with the ConsTL treatment and least with the ConvT treatment. Soil water use by sorghum was greater (66 mm) with ConsTL than with ConvT (51 mm). Mean initial plant populations and panicles harvested were greater with ConvT than with other treatments, but grain yields were less with ConvT (4.21 Mg ha −1) than with ConsTE (4.42 Mg ha −1) and ConsTL (4.52 Mg ha −1). Mean stover yields and seed weight (mg per seed) were not affected by tillage treatments. Although differences were relatively small, mean grain and stover yields were greater for the ‘Pioneer’ cultivar (4.53 and 5.68 Mg ha −1) than for the ‘Funk’ cultivar (4.24 and 5.25 Mg ha −1). Mean water use efficiency differences among treatments and between hybrids were small. Ridge tillage was better than conventional tillage for continuous grain sorghum production with limited irrigation in the semi-arid SGP. Ridge tillage that retained residues on the surfaces until planting increased soil water storage due to reduced evaporation and increased snow trapping, which resulted in the greater yields than where residues were incorporated with soil soon after crop harvest.

Tillage Effects on Dryland Wheat and Sorghum Production in the Southern Great Plains

AbstractWinter wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] are important, well‐adapted grain crops for the semiarid southern Great Plains. They often are grown in rotation on dryland to reduce the risk of failure associated with annual cropping and to overcome the low precipitation‐use efficiencies associated with crop‐fallow systems. This study on Pullman soil (Torrertic Paieustoli) determined the effects of 12 tillage systems ranging from no‐tillage (herbicides only) to sweep tillage (no herbicides) on water storage and use, crop growth, yields, and yield components for dryland wheat and sorghum grown in rotation. Tillage systems did not affect mean water storage during fallow nor mean water use by either crop. All yield, growth, and yield component factors differed among growing seasons. Wheat grain and straw yields were not affected by tillage. Mean wheat grain yield was 2.92 Mg ha−1. Mean sorghum grain yield was greatest (3.91 Mg ha−1) for the system of reduced tillage after sorghum and no‐tillage after wheat, and least (3.48 Mg ha−1) for the system of reduced tillage after each crop. Sorghum stover yields were not affected by tillage. Number of panicles harvested was greatest (233 000 ha−1) and least (212 000 ha−1) for the same treatments that resulted in the greatest and least grain yields. Sorghum seed weights and test weights were not affected by tillage. This study showed that a wide range of tillage systems is adaptable for a dryland wheat‐grain sorghum cropping system for the semiarid southern Great Plains.