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The soybean cyst nematode (SCN), Heterodera glycines, is a major soybean yield-limiting factor, and the use of resistant cultivars is one of the most effective means to manage the nematode. During the past decade, a number of resistant cultivars in maturity groups I and II have been developed and made available to growers. A total of 47 resistant cultivars and nine susceptible cultivars were evaluated at 15 SCN-infested field sites and two noninfested sites during 1996 to 1998 in Minnesota. As expected, more nematodes developed on susceptible cultivars than on resistant cultivars. Egg density on susceptible cultivars increased by 1.9- to 10.6-fold during the growing season at 12 sites and did not change at the other three sites. Average egg density decreased over time for resistant cultivars at all sites, except where the initial egg density was low (≤455 eggs per 100 cm3 soil). Nematode reproduction factors (Rf = egg density at harvest/egg density at planting) for individual resistant and susceptible cultivars were highly consistent across the eight sites where initial SCN density was more than 1,000 eggs per 100 cm3 soil. Resistance, however, varied among the cultivars, with the average Rf of individual resistant cultivars across the sites ranging from 0.3 to 1.7. Resistant cultivars produced an average yield of 3,082 kg/ha compared with 2,497 kg/ha by susceptible cultivars at eight of 10 sites where egg density at planting was greater than 700 eggs per 100 cm3 soil. In contrast, no difference in yield was observed between resistant and susceptible cultivars at sites where egg density at planting was lower than 500 eggs per 100 cm3 soil. Yield differences between resistant and susceptible cultivars increased with increasing initial SCN egg density. In six fields infested with initial densities of more than 5,000 eggs per 100 cm3 soil, resistant cultivars produced 28.4% (676 kg/ha) more yield on average than susceptible cultivars. Soybean yield increased when cultivars with increasing resistance to the SCN (lower Rf or females formed on roots) were grown in fields infested with SCN. Average relative yield (yield of a cultivar/average yield of all resistant cultivars at a site) of individual resistant cultivars across all SCN-infested sites ranged from 0.76 to 1.10. Yield consistency of soybean cultivars was low among the different sites, indicating that many other factors affected yield. Our results suggest growing resistant cultivars is an effective method to manage SCN in Minnesota while minimizing yield loss due to SCN.

Nematicides have been used to control different nematodes including SCN (Schmitt et al., 1983;Noel, 1987; The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, Sasser and Uzzell, 1991; Smith et al., 1991). Nematicides, is a destructive pest of soybean, Glycine max (L.) Merr. Field research was conducted at Waseca and Lamberton, MN, from 1996 through however, are not acceptable for control of SCN because 1999 to evaluate the effect of crop sequence on SCN population they are not cost effective and potentially have negative density and on crop yields. Cropping sequence treatments were (i) environmental impacts. monocultures of corn (Zea mays L.), SCN-resistant soybean, and Crop rotation is an effective strategy for SCN man-SCN-susceptible soybean; (ii) susceptible soybean rotated with 1, 2, agement, especially in the southern USA. A number of or 3 yr of corn; (iii) resistant soybean rotated annually with corn; and studies have demonstrated that growing nonhost crops (iv) rotation of corn-resistant soybean-corn-susceptible soybean. Egg reduced SCN population densities and increased soydensity was determined at sowing and harvest, and crop yields were bean yield (Young and Hartwig, 1992; Koenning et al., determined each year. In general, yields of resistant soybean were 1993;Trevathan and Robbins, 1995;Weaver et al., 1995; higher than susceptible soybean. Resistant soybean in annual rotation Young, 1998;Howard et al., 1998). Two or more years with corn produced the highest yield, and susceptible soybean in monoculture produced the lowest yield among all treatments. A longer of a nonhost crop resulted in low or undetectable SCN period of corn in rotation resulted in higher yield of subsequent densities and acceptable soybean yields (Francl and susceptible soybean in most instances. Yields of corn following corn Dropkin, 1986;. Koenning et al. (1993) were lower than following soybean. Egg density at the start of the study demonstrated that 1 yr of a nonhost crop was sufficient was 6994 and 14 000 eggs 100 cm Ϫ3 soil at Waseca and Lamberton, to control SCN with no additional benefit beyond 2 yr respectively. Average Pf/Pi (egg density at harvest/egg density at of growing a nonhost crop. Soybean cyst nematode sursowing) was 0.59 (0.23-0.86) for corn, 0.49 (0.21-0.73) for resistant vival rate is higher in northern region than in southern soybean, and 3.3 (0.74-9.91) for susceptible soybean for all treatments regions of the USA (Riggs et al., 2001). Consequently, at the two sites across the 4-yr period. After 3 yr of corn, egg density The experiments were initiated at two field sites located in southwest (Lamberton) and south central (Waseca) Minne-Senyu Chen, Univ. of Minnesota Southern Research and Outreach sota in 1996. The Waseca site had been in an annual corn-Center, 35838 120th Street, Waseca, MN 59093; Paul M. Porter, Dep. soybean rotation prior to 1996 and with susceptible soybean in 1995. At Lamberton site, susceptible soybean had been Univ., Brookings, SD 57007; and Ward C. Stienstra, Dep. of Plan...

Winter cover crops have produced mixed impacts on crop yields in frigid semiarid environments. Our objective was to determine the influence of winter cover crops and landscape positions on corn (Zea mays L.) yield losses due to water and N stress, soil biology, and gene expression. Following wheat (Triticum aestivum L.), cover crop mixtures were drill seeded into three no-till fields. In the following spring, four N fertilizer rates were applied and corn was no-till seeded. The cover crop had fundamentally different impacts on corn yields at Trail City and Andover in 2011. Both sites were water stressed and responded to N fertilizer; however, at Andover 2011 the cover crop reduced corn yields, while at Trail City 2011 the cover crop did not impact yields. The yield reduction at Andover 2011 was attributed to high cover crop production ( >2000 kg ha -1 ), which reduced soil water 15% in the 30-to 60-cm soil depth in November 2010, and in the following growing season increased corn yield loss due to water stress from 1610 to 2950 kg grain ha -1 . Associated with the yield reduction was down expression of two mineral nutrient genes (NCBI, AF325723.1 and APO05869.3) and one gene associated with energy processing (AF325723.1). The cover crops reduced the relative amount of soil NO 3 at four of the six landscape positions and increased the relative bacteria/fungi ratio in 2011. These findings indicate that cover crops after winter wheat can be an important mechanism for scavenging N in semiarid frigid environments; however, adopting techniques that generate high cover crop yields can adversely influence the following cash crop. Abbreviations: NCBI, National Center for Biotechnology Information; SOC, soil organic carbon; YLNS, yield loss due to nitrogen stress; YLWS, yield loss due to water stress.Winter cover crops can increase yields, improve soil health, reduce nutrient losses, improve nutrient recycling, and reduce the risk of plant diseases (Tisdall and Oades, 1982;Angus et al., 1994;Bending and Lincoln, 1999;Mazzola, 2004;Smeltekop et al., 2002;Ruffo et al., 2004;Snapp et al., 2005; Bollero, 2005, 2006;Clark et al., 2007;Hoffbeck et al., 2008;Monteserrat et al., 2010;Green-Tracewicz et al., 2011;Blanco-Canqui et al., 2012;Bich, 2013). However, capturing these benefits in frigid, semiarid environments is complicated by a short growing season and the cover crop utilizing water that otherwise would be available to the crop. In semiarid environments, there are conditions where cover crops are beneficial and others where they are not. For example, water stress in cash crops was increased by replacing fallow with a green manure in Montana's arid and semiarid regions (Army and Hide, 1959), while nutrient losses were reduced by cover crops in irrigated systems (Snapp et al., 2005). To improve cover crop recommendations in frigid, semiarid regions, a better understanding of how interactions among available soil water, available soil nutrients, and cover crop production impacts the cash crop's yield is needed. Our ...

Techniques for measuring soil organic C (SOC) turnover in production fields are needed. The objectives of this study were to propose and test nonisotopic and 13 C stable isotopic techniques for assessing SOC turnover. Based on SOC equilibrium and mass balance relationships, an equation was derived: NHC/SOC initial 5 [1/(SOC 3 k NHC )] (dSOC/dt) 1 k SOC /k NHC , where dSOC/dt is the annual change in SOC, NHC is nonharvested C returned to soil, k SOC is the annual mineralization rate of SOC, and k NHC is the annual mineralization rate of NHC. This equation was used to calculate maintenance rates. An isotopic approach based on simultaneously solving the equations was developed to determine C budgets:and (iv) SOC final 5 SOC retained 1 PCR incorp , where e is the Rayleigh fractionation constant, PCR incorp is the amount of NHC incorporated into SOC final with D PCR being the associated 13 C discrimination (D) value, and SOC retained is the amount of SOC initial retained in the soil after mineralization with D SOC retained being the associated D value.Isotopic and nonisotopic approaches were tested on a production field where aboveground corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields were measured with a yield monitor and soil samples collected from a slightly offset grid (30 m) in 1995 and 2003 were analyzed for organic C and D. The nonisotopic approach showed that maintenance rates increased with SOC and that an accurate measure of NHC was required to calculate maintenance requirements. Sensitivity analysis of the isotopic approach showed that calculated budgets were sensitive to 13 C discrimination during SOC mineralization. If 13 C discrimination during SOC and NHC mineralization did not occur (e 5 0), then 14.9 and 7.6% of the SOC measured in 1995 (SOC initial ) was mineralized, and 7420 and 2780 kg C ha 21 of NHC were incorporated into SOC final in the 523.4-to 527.3-and 527.3-to 529.2-m elevation zones, respectively. If 13 C discrimination occurred (e 5 22.52%) during SOC mineralization, then the calculated amount of SOC mineralized and the amount of new C incorporated into SOC were reduced.