those methods as they have been used from the Canadian Prairie Provinces to the southern Great Plains of Successful dryland crop production in the semiarid Great Plains the United States and the resultant effects on system of North America must make efficient use of precipitation that is often limited and erratic in spatial and temporal distribution. The purpose WUE. Additionally, differences in precipitation use effiof this paper is to review research on water use efficiency and precipita-ciency (PUE) between cropping systems across the Great tion use efficiency (PUE) as affected by cropping system and manage-Plains region are identified. ment in the Great Plains. Water use efficiency and PUE increase with residue management practices that increase precipitation storage METHODS FOR INCREASING PSE, efficiency, soil surface alterations that reduce runoff, cropping se-WUE, AND PUE quences that minimize fallow periods, and use of appropriate management practices for the selected crop. Precipitation use efficiency on Tillage Effects on PSE a mass-produced basis is highest for systems producing forage (14.5 kg ha Ϫ1 mm Ϫ1 ) and lowest for rotations with a high frequency of oilseed Precipitation storage efficiency increases as tillage incrops (4.2 kg ha Ϫ1 mm Ϫ1 ) or continuous small-grain production in the tensity is reduced during the summer fallow period. The southern plains (2.8 kg ha Ϫ1 mm Ϫ1 ). Precipitation use efficiency when increased soil water storage is a result of both maintaincalculated on a price-received basis ranges from $1.20 ha Ϫ1 mm Ϫ1 (for ing crop residues on the soil surface and reducing the an opportunity-cropped system with 4 of 5 yr in forage production number of times that moist soil is brought to the surface in the southern plains) to $0.30 ha Ϫ1 mm Ϫ1 {for a wheat (Triticum as tillage intensity is reduced. Data from winter wheataestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench]-fallow fallow systems at North Platte, NE (Smika and Wicks, system in the southern plains}. Throughout the Great Plains region, 1968), and Sidney, MT (Tanaka and Aase, 1987), show PUE decreases with more southern latitudes for rotations of similar fallow PSE increasing from under 25% to around 40% makeup of cereals, pulses, oilseeds, and forages. Forage systems inas tillage intensity decreased from moldboard plow to the southern Great Plains appear to be highly efficient when PUE is computed on a price-received basis. In general across the Great no-till (Fig. 1, top). Data collected at Bushland, TX, fol-Plains, increasing intensity of cropping increases PUE on both a mass-lowed a similar trend with PSE increasing from 15% with produced basis and on a price-received basis.

No abstract

The water‐limited environment of the semiarid Central Great Plains may not produce enough cover crop biomass to generate benefits associated with cover crop use in more humid regions. There have been reports that cover crops grown in mixtures produce more biomass with greater water use efficiency than single‐species plantings. This study was conducted to determine differences in cover crop biomass production, water use efficiency, and residue cover between a mixture and single‐species plantings. The study was conducted at Akron, CO, and Sidney, NE, during the 2012 and 2013 growing seasons under both rainfed and irrigated conditions. Water use, biomass, and residue cover were measured and water use efficiency was calculated for four single‐species cover crops (flax [Linum usitatissimum L.], oat [Avena sativa L.], pea [Pisum sativum ssp. arvense L. Poir], rapeseed [Brassica napus L.]) and a 10‐species mixture. The mixture did not produce greater biomass nor exhibit greater water use efficiency than the single‐species plantings. The slope of the water‐limited yield relationship was not significantly greater for the mixture than for single‐species plantings. Water‐limited yield relationship slopes were in the order of rapeseed < flax < pea < mixture < oat, which was the expected order based on previously published biomass productivity values generated from values of glucose conversion into carbohydrates, protein, or lipids. Residue cover was not generally greater from the mixture than from single‐species plantings. The greater expense associated with a mixture is not justified unless a certain cover crop forage quality is required for grazing or haying.

oilseed crop produced in the USA, canola is the dominant oil crop in Canada. The cool climatic conditions Oilseed crops are grown throughout the semiarid region of the characteristic of the Canadian prairies provide an ideal northern Great Plains of North America for use as vegetable and industrial oils, spices, and birdfeed. In a region dominated by winter environment for Brassica spp. oilseeds and flax (Table and spring wheat (Triticum aestivum L. emend. Thell.), the accep-2) while the climate found in the USA is better suited tance and production of another crop requires that it both has an to the warm season crops like soybean and sunflower. agronomic benefit to the cropping system and improve the farmers' In the northern Great Plains, soybean is a relatively economic position. In this review, we compare the adaptation and new crop finding a place in semiarid cropping systems rotational effects of oilseed crops in the northern Great Plains. Canola with the development of early maturing, low heat-unit (Brassica sp.), mustard (B. juncea and Sinapis alba L.), and flax cultivars (Miller et al., 2002). As a result, the vast major-(Linum usitatissimum L.) are well adapted to cool, short-season conity of soybean production in both the USA and Canada ditions found on the Canadian prairies and northern Great Plains occurs in wetter regions east of the Great Plains. Howborder states of the USA. Sunflower (Helianthus annuus L.) and safflower (Carthamus tinctorius L.) are better adapted to the longer ever, for the other oilseed crops listed in Table 1, the growing season and warmer temperatures found in the northern and majority of production occurs within the northern Great central Great Plains states. Examples are presented of how agronomic Plains. practices have been used to manipulate a crop's fit into a local environ-Diversification within cereal-based cropping systems ment, as demonstrated with the early spring and dormant seeding can be critical to breaking pest infestations that are management of canola, and of the role of no-till seeding systems in common with monoculture (Bailey et al., 1992, 2000; allowing the establishment of small-seeded oilseed crops in semiarid Elliot and Lynch, 1995; Holtzer et al., 1996; Krupinsky regions. Continued evaluation of oilseed crops in rotation with cereals et al., 2002). Results of crop rotation studies in the Great will further expand our understanding of how they can be used to Plains revealed that where oilseeds are adapted, their strengthen the biological, economic, and environmental role of the region's cropping systems. Specific research needs for each oilseed

Growing a legume cover crop in place of fallow in a winter wheat (Triticum aestivum L.)–fallow system can provide protection against erosion while adding N to the soil. However, water use by legumes may reduce subsequent wheat yield. This study was conducted to quantify the effect of varying legume termination dates on available soil water content at wheat planting and subsequent wheat yield in the central Great Plains. Four legumes [Austrian winter pea, Pisum sativum L. subsp. sativum var. arvense (L.) Poir.; spring field pea, P. sativum L.; black lentil, Lens culinaris Medikus; hairy vetch, Vicia villosa Roth.) were grown at Akron, CO, as spring crops from 1994 to 1999. Legumes were planted in early April and terminated at 2‐wk intervals (four termination dates), generally starting in early June. Wheat was planted in September in the terminated legume plots, and yields were compared with wheat yields from conventional till wheat–fallow. Generally there were no significant differences in available soil water at wheat planting due to legume type. Soil water at wheat planting was reduced by 55 mm when legumes were terminated early and by 104 mm when legumes were terminated late, compared with soil water in fallowed plots that were conventionally tilled. Average wheat yield was linearly correlated with average available soil water at wheat planting, with the relationship varying from year to year depending on evaporative demand and precipitation in April, May, and June. The cost in water use by legumes and subsequent decrease in wheat yield may be too great to justify use of legumes as fallow cover crops in wheat–fallow systems in semiarid environments.