Stored soil water and growing season precipitation generally support early season growth of grain sorghum (Sorghum bicolor L. Moench) in dryland areas but are insufficient to prevent water stress during critical latter growth stages. The objective of this study was to determine if growing plants in clumps affected early season growth and subsequent grain yield compared to uniformly spaced plants. We hypothesized that growing grain sorghum plants in clumps would result in fewer tillers and less vegetative growth so that more soil water would be available during the grain-filling period. Results from 3 yr at Bushland, TX, and 1 yr at Tribune, KS, showed that planting grain sorghum in clumps of three to six plants reduced tiller formation to about one per plant compared to about three for uniformly spaced plants. Grain yields were increased by clump planting by as much as 100% when yields were in the 1000 kg ha 21 range and 25 to 50% in the 2000 to 3000 kg ha 21 range, but there was no increase or even a small decrease at yields above 5000 kg ha 21 . Our results suggest that planting grain sorghum in clumps rather than spaced uniformly conserves soil water use until later in the season and may enhance grain yield in semiarid dryland environments.Abbreviations: LAI, leaf area index; PET, potential evapotranspiration.

Time, fertilizer, tillage, and cropping systems may alter soil organic carbon (SOC) levels. Our objective was to determine the effect of long-term cropping systems and fertility treatments on SOC. Five rotations and two N fertility levels at three Iowa sites (Kanawha, Nashua, and Sutherland) maintained for 12 to 36 yr were evaluated. A 75-yr continuous corn (Zea mays L.) site (Ames) with a 40-yr N-P-K rate study also was evaluated. Soils were Typic and Aquic Hapludolls and Typic Haplaquolls. Four-year rotations consisting of corn, oat (Avena saliva L.), and meadow (alfalfa [Medicago sativa L.], or alfalfa and red clover [Trifolium pratense L.]) had the highest SOC (Kanawha, 32.1 g/kg; Nashua, 21.9 g/kg; Sutherland, 27.9 g/kg). Corn silage treatments (Nashua, < 18.9 g/kg; Sutherland, <23.2 g/kg) and no-fertilizer treatments (Kanawha, 25.3 g/kg; Nashua, <20.9 g/kg; Sutherland, <23.5 g/kg) had the lowest SOC. A corn-oat-meadowmeadow rotation maintained initial SOC (27.9 g/kg) after 34 yr at Sutherland. Continuous corn resulted in loss of 30% of SOC during 35 yr of manure and lime treatments. SOC increased 22% when N-P-K treatments were imposed. Fertilizer N, initial SOC levels, and previous management affected current SOC levels. Residue additions were linearly related to SOC (Ames, r 2 = 0.40; Nashua, r 2 = 0.82; Sutherland, r 2 = 0.89). All systems had 22 to 49% less SOC than adjacent fence rows. Changing cropping systems to those that conserve SOC could sequester as much as 30% of C released since cropping began, thereby increasing SOC. T HE ATMOSPHERIC CC>2 CONCENTRATION has gained much attention for its potential contribution to global warming. Agriculture affects atmospheric COz concentrations through consumption of fossil fuels, clearing of forested lands for food production (U.S. Congress, 1991; Wallace et al., 1990), and alteration of SOC levels by agricultural management practices. Agricultural fuel consumption and N fertilizer production release 35.4 Tg C yr~' into the atmosphere. Based on current production practices, the Council for Agricultural Science and Technology (1992) estimates another 2.7 Tg C yr~' are released into the atmosphere from cultivated soils in the USA alone. Changes in SOC can be attributed to crop species grown, cropping systems (including rotations), residue management practices, fertilizer applications, tillage practices, and other management factors (Havlin et al., 1990; Unger, 1968). Anderson et al. (1990), Bauer and Black (1981), and Havlin et al. (1990) independently showed that SOC losses were directly related to tillage intensity. Manure applications modified the tillage-SOC relation, increasing SOC even with high-intensity conventional tillage (Anderson et al., 1990). Crop rotations may retard SOC losses relative to those observed in

The 30-s Wingate anaerobic test (30-WAT) is the most widely accepted protocol for measuring anaerobic response, despite documented physical side effects. Abbreviation of the 30-WAT without loss of data could enhance subject compliance while maintaining test applicability. The intent of this study was to quantify the validity of the 20-s Wingate anaerobic test (20-WAT) versus the traditional 30-WAT. Fifty males (mean +/- SEM; age = 20.5 +/- 0.3 years; Ht = 1.6 +/- 0.01 m; Wt = 75.5 +/- 2.6 kg) were randomly selected to either a validation (N = 35) or cross-validation group (N = 15) and completed a 20-WAT and 30-WAT in double blind, random order on separate days to determine peak power (PP; W kg(-1)), mean power (MP; W kg(-1)), and fatigue index (FI; %). Utilizing power outputs (relative to body mass) recorded during each second of both protocols, a non-linear regression equation (Y (20WAT+10 )= 31.4697 e(-0.5)[ln(X (second)/1174.3961)/2.6369(2)]; r (2) = 0.97; SEE = 0.56 W kg(-1)) successfully predicted (error approximately 10%) the final 10 s of power outputs in the cross-validation population. There were no significant differences between MP and FI between the 20-WAT that included the predicted 10 s of power outputs (20-WAT+10) and the 30-WAT. When derived data were subjected to Bland-Altman analyses, the majority of plots (93%) fell within the limits of agreement (+/-2SD). Therefore, when compared to the 30-WAT, the 20-WAT may be considered a valid alternative when used with the predictive non-linear regression equation to derive the final power output values.

Cover crop use is being widely promoted throughout the entire United States because of the potential benefits related to protecting and improving the soil. However, in semiarid environments such as the western and central Great Plains (where water is the single most limiting factor to crop production), cover crop water use may result in significant yield loss in following crops such as winter wheat. This article explores why many of the benefits associated with cover crop use may not be seen in this water‐limited environment. Earn 1.5 CEUs in Crop Management by reading this article and completing the quiz at http://www.certifiedcropadviser.org/certifications/self-study/699.

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