Inadequate amounts and distribution of rainfall are most often the major limiting factors to crop production in the southern Great Plains. This is especially true for summer crops, whether grown in mono‐ or double‐cropping situations. This study was conducted at the Oklahoma Vegetable Research Station, Bixby, OK from 1976 to 1987 on a Wynona silt loam soil (Cumulic Haplaquolls) with 0 to 1% slope. The objective was to determine the effects of long‐term double‐cropping on the potential for sustaining grain yields of wheat [Triticum aestivum (L.) em. Thell], soybean [Glycine max (L.) Merr.], and grain sorghum (Sorghum bicolor L. Moench) produced on the same land under rainfed conditions. Over a 12‐yr period monocropped wheat averaged 3050 compared with 2510 and 2450 kg ha−1 when double‐cropped with soybean and grain sorghum, respectively. Conventionally tilled monocropped soybean and grain sorghum and no‐till double‐cropping of both soybean and grain sorghum after wheat produced grain 11 out of 12 yr. Monocropped soybean averaged 2470 compared with 1930 kg ha−1 for no‐till double‐cropped soybean. Monocropped grain sorghum averaged 5130 compared with 4200 kg ha−1 for double‐cropped grain sorghum. During the years of near 30‐yr average rainfall amounts and distribution (5 out of 11), yields of double‐cropped soybean and grain sorghum were competitive with those of monocropped soybean and grain sorghum. These results indicate that yields of double‐cropped wheat, soybean, and grain sorghum can be sustained over long periods of time. In eastern Oklahoma double‐cropping on a deep medium textured soil produced more total grain that resulted in more efficient use of climatic, land, labor, and equipment resources when compared with monocropping.

In wheat [Triticum aestivum (L.) Thell] producing regions with adequate frost‐free days to permit double‐cropping, water is often the most limiting factor in producing the second crop. To more fully utilize climatic resources and land after the harvest of wheat grain, a field study was conducted on a Wynona silty clay loam (Cumulic Haplaquolls) to analyze the effects of tillage and row spacings on soil water content and yields of monocropped (MC) and double‐cropped (DC) wheat and soybeans [Glycine max (L.) Merr.]. Double‐cropped wheat (DCW) yielded 2,210 compared to 2,530 kg/ha for monocropped wheat (MCW). Conventional tillage double cropped soybeans (CT‐DCS) in 50‐cm rows yielded 1,610 compared to 1,460 kg/ha in 75‐cm rows. No‐tillage double‐cropped soybeans (NTDCS) in 50‐cm rows yielded 1,730 compared to 1,550 kg/ ha in 75‐cm rows. Conventional tillage monocropped soybeans (CT‐MCS) in 50‐cm rows yielded 2,010 compared to 1,880 kg/ha in 75‐cm rows. Water content in 120 cm of soil profile was not significantly affected during the soybean growing period by tillage and row spacings; however, lack of moisture in the soil‐seed environment resulted in poorer stands and lower yields for CT‐MCS in 1976. Water storage efficiency under wheat from the fully ripe stage of growth to late September averaged 28% for MCW. More grain production and more effective use of annual precipitation and soil stored water were achieved when wheat and soybeans were DC.

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