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Winter wheat (Triticum aestivum L.) is the most common dryland crop grown in the central Great Plains. Producers in this region include fallow in the rotation to minimize yield variability due to erratic precipitation. However, fallow degrades soil quality by increasing erosion potential and loss of organic matter. Fortunately, minimum‐till production systems and residue management improve water use efficiency by plants, thus producers can crop more frequently. We evaluated eight rotations comprised of various sequences of winter wheat (W), corn (Zea mays L.) (C), proso millet (Panicum miliaceum L.) (M), sunflower (Hettanthus annum L.) (S), and fallow (F) in comparison to W‐F at Akron CO. Our goal was to identify rotations that can replace W‐F to minimize the frequency of fallow. The soil was a Weld silt loam (Aridic Paleustoll). Continuously cropping with W‐C‐M and W‐M almost doubled total grain yield compared with the conventional system of W‐ F. Other rotations such as W‐C‐F, W‐C‐S‐F, and W‐C‐M‐F yielded >60% more on an annualized basis than W‐F. Winter wheat yield increased with longer time intervals between wheat crops. Sunflower yielded the most when grown only once every 4 yr; more frequent cropping favored diseases. Sunflower reduced yield of the following crop, especially during dry years. Yield variability was highest with corn and sunflower, whereas proso millet showed the least variability. Producers can manage yield variability by diversifying crops in the rotation, as annualized yield variability of W‐M and W‐C‐M was similar to W‐F. With residue maintenance and minimum tillage, producers can crop more frequently, thus increasing land productivity while minimizing the frequency of fallow in this semiarid region. Research Question Since the 1930s, winter wheat‐fallow has been the prevalent crop rotation for the semiarid Central Great Plains. Because available water is usually the most limiting resource, producers rely on fallow to minimize the impact of erratic precipitation on grain production. However, fallow degrades soil quality by increasing erosion and loss of organic matter. Development of minimum‐till production systems has altered the water relations in our agroecosystems. Minimizing tillage leaves more crop residue on the soil surface, subsequently increasing precipitation storage and water use efficiency of crops. Thus, with minimum‐till systems, more intensive cropping is possible in the central Great Plains. This study evaluated cropping systems composed of various sequences of winter wheat (W), corn (G), proso millet (M), sunflower (S), and fallow (F), including continuous cropping. Our goal was to identify rotations that may be successful alternatives to W‐F. Literature Summary With reduced‐till systems, several crops have been successful in a wheat‐summer crop‐fallow rotation in this region, including proso millet, corn, and grain sorghum. In addition, longer rotations with three crops in 4 yr, such as W‐C‐M‐ F, are also successful and have increased land productivity by 70%. Another p...

wheat yields were reduced by 79 kg ha Ϫ1 for every centimeter that soil water at wheat planting was reduced by Many dryland producers in the central Great Plains of the USA sunflower (Helianthus annuus L.) ahead of wheat in express concern regarding the effect that elimination of fallow has on soil water content at winter wheat (Triticum aestivum L.) planting rotation. In southwestern Kansas, Norwood (2000) simiand subsequent yields. Our objectives were to quantify cropping sys-larly showed lower winter wheat yields when the previtem effects (fallow weed control method and crop sequence), including ous crop was sunflower or soybean compared with corn corn (Zea mays L.) (C) and proso millet (Panicum miliacium L.) (M), or grain sorghum [Sorghum bicolor (L.) Moench]. These on soil water at winter wheat planting and subsequent grain yield, and reductions in wheat yield were related to lower soil to determine the frequency of environmental conditions which would water at planting. Lyon et al. (1995) showed that soil cause wheat yield to drop below 2500 kg ha Ϫ1 for various cropping water at planting was strongly correlated with yield of systems. Crop rotations evaluated from 1993 through 2001 at Akron, short season summer crops [pinto bean (Phaseolus vul-CO, were W-F, W-C-F, W-M-F, and W-C-M (all no-till), and W-F garis L.), proso millet] but only weakly related to yield (conventional till). Yields were correlated with soil water at planting: of long season summer crops (sunflower, grain sorghum, kg ha Ϫ1 ϭ 373.3 ϩ 141.2 ϫ cm (average and wet years); kg ha Ϫ1 ϭ 897.9 ϩ 39.7 ϫ cm (dry years). Increasing cropping intensity to two corn). They attributed this result in part to shorter seacrops in 3 yr had little effect on water content at wheat planting and son crops having more soil water available at the critical subsequent grain yield, while continuous cropping and elimination of reproductive growth stage than longer season crops, fallow reduced soil water at planting by 11.8 cm and yields by 450which used much of the initial soil water for stover to 1650 kg ha Ϫ1 , depending on growing season precipitation. No-till production and did not have it available for grain develsystems, which included a 12-to 15-mo fallow period before wheat opment.planting nearly always produced at least 2500 kg ha Ϫ1 of yield underIn addition to differences in previous crop water use, normal to wet conditions, but no cropping system produced 2500 kg soil water content at wheat planting can also be affected ha Ϫ1 under extremely dry conditions. by differences in tillage and crop residue effects on precipitation storage efficiency. Precipitation storage efficiency increases as tillage is reduced during the sum-ARS, Central Great Plains Res. Stn., 40335 County Road GG, Akron, quent effects on grain yield, and (ii) determine fre-CO 80720; R.L. Anderson, USDA-ARS, Northern Grain Insects Res. quency of environmental conditions that cause wheat Lab.,

Changes in soil water retention of the surface soil brought about by tillage can significantly alter the amount of rain water that infiltrates into the root zone and is available for plant growth. Soil tillage generally increases porosity and changes the pore‐size distribution, leading to changes in the soil water retention curve and hydraulic conductivities. The objective of this study was to investigate some simple ways of estimating the soil water retention curve of a tilled soil from that of an untilled soil, knowing the change in soil porosity or bulk density due to tillage. The study of literature and empirical analysis of the available data indicated: (i) under field conditions the tillage did not significantly change the air‐entry value of the soil; (ii) tillage increased the absolute value of the slope of the log‐log relationship below the air‐entry value; and (iii) the changes due to tillage in the retention curve occurred only in the larger pore‐size range, approximately between the air‐entry pressure head value and 10 times the air‐entry value. Assuming these observations hold in general, two simple methods of estimating the water retention curve of a tilled soil from that of its untilled condition are proposed. The first method is a simple imposition of the Brooks and Corey function between the air‐entry value and 10 times this value. The second method assumes that the change in soil water content at a given pressure head in the above range of pressure heads was inversely proportional to the value of the pressure head. The tests on four pairs of measured water retention curves on three different soils showed that these methods provided good approximations.