To improve understanding of relationships between crop diversity, weed management practices, and weed population dynamics, we used data from a field experiment and matrix models to examine how contrasting crop rotations affect velvetleaf. We compared a 2-yr rotation system (corn–soybean) managed with conventional rates of herbicides with a 4-yr rotation (corn–soybean–triticale + alfalfa–alfalfa) that received 82% less herbicide. In November 2002, a pulse of velvetleaf seeds (500 seeds m−2) was added to 7- by 7-m areas within replicate plots of each crop phase–rotation system combination. Velvetleaf seed, seedling, and reproductive adult population densities, seed production, and seed losses to predators were measured during the next year. Velvetleaf seed production was greater in the 4-yr rotation than in the 2-yr rotation (460 vs. 16 seeds m−2). Averaged over 12 sampling periods from late May to mid-November 2003, loss of velvetleaf seeds to predators also was greater in the 4-yr rotation than in the 2-yr rotation (32 vs. 17% per 2 d). Modeling analyses indicated that velvetleaf density in the 4-yr rotation should decline if cumulative losses of seeds produced in the soybean phase exceeded 40%. Achieving such a level of predation appears possible, given the observed rates of velvetleaf seed predation. In addition, no tillage occurs in the 4-yr rotation for 26 mo after soybean harvest, thus favoring seed exposure on the soil surface to predators. Models that included estimates of seed predation indicated that to prevent increases in velvetleaf density, weed control efficacy in soybean must be ≥ 93% in the 2-yr rotation, but could drop to 86% in the 4-yr rotation. These results support the hypothesis that diverse rotations that exploit multiple stress and mortality factors, including weed seed predation, can contribute to effective weed suppression with less reliance on herbicides.

Recent attention has focused on the use of perennial, warm-season grasses as renewable energy crops. Th e objective of this study was to assess the eff ects of N fertilization on partitioning of biomass and nutrients between above-and belowground plant components by four warm-season grass species in Iowa. In 2006-2007, established stands of big bluestem (Andropogon geradii Vitman), switchgrass (Panicum virgatum L.), indiangrass [Sorghastrum nutans (L.) Nash], and eastern gamagrass [Tripsacum dactyloides (L.) L.] were fertilized with 0, 65, 140, or 220 kg N ha −1 in the spring and harvested following frost in the fall. Dependent on grass species and year, yield response to N was linear or quadratic. Optimum yield aft er 2 yr was 13.5 Mg ha −1 at 140 kg N ha −1 for all grasses except eastern gamagrass, which demonstrated lower yield and a consistent linear N response. Nitrogen inputs had pronounced but grass-specifi c eff ects on root biomass and nutrient partitioning. For big bluestem and switchgrass, 140 kg N ha −1 maximized root biomass and favored allocation of nutrients to roots over shoots. In contrast, for indiangrass and eastern gamagrass, root biomass and root nutrient allocation were adversely aff ected by N inputs. For all grasses, 220 kg N ha −1 shift ed allocation of nutrients to shoots over roots. Selection of crops and management practices that optimize yield, and maintain a high level of resource partitioning to roots at low to intermediate N input rates will promote the development of productive and effi cient bioenergy systems.

We conducted a 9-ha field experiment near Boone, IA, to test the hypothesis that yield, weed suppression, and profit characteristics of low-external-input (LEI) cropping systems can match or exceed those of conventional systems. Over a 4-yr period, we compared a conventionally managed 2-yr rotation system {corn (Zea mays L.)/soybean [Glycine max (L.) Merr.]} with two LEI systems: a 3-yr corn/soybean/small grain + red clover (Trifolium pratense L.) rotation, and a 4-yr corn/soybean/small grain + alfalfa (Medicago sativa L.)/alfalfa rotation. Synthetic N fertilizer use was 59 and 74% lower in the 3-and 4-yr systems, respectively, than in the 2-yr system; similarly, herbicide use was reduced 76 and 82% in the 3-and 4-yr systems. Corn and soybean yields were as high or higher in the LEI systems as in the conventional system, and weed biomass in corn and soybean was low (≤4.2 g m −2 ) in all systems. Experimentally supplemented giant foxtail (Setaria faberi Herrm.) seed densities in the surface 20 cm of soil declined in all systems; supplemented velvetleaf (Abutilon theophrasti Medik.) seed densities declined in the 2-and 4-yr systems and remained unchanged in the 3-yr system. Without subsidy payments, net returns were highest for the 4-yr system ($540 ha −1 yr −1 ), lowest for the 3-yr system ($475 ha −1 yr −1 ), and intermediate for the 2-yr system ($504 ha −1 yr −1 ). With subsidies, differences among systems in net returns were smaller, as subsidies favored the 2-yr system, but rank order of the systems was maintained. Disciplines Agronomy and Crop Sciences | Statistics and Probability CommentsThis is an article from Agronomy Journal 100 (2008): 600, doi:10.2134/agronj2007.0222. Posted with permission. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. O ne of the key questions facing agriculturalists in the 21st century is how to produce adequate amounts of food, feed, and farm income while protecting and improving environmental quality (Robertson and Swinton, 2005). Th e need to answer this question is particularly acute in the midwestern United States, one of the largest regions of intensive, rain-fed agriculture in the world. Crop production in this region currently relies heavily on synthetic N fertilizer and herbicides to manage soil fertility and weeds (National Agricultural Statistics Service, 2003, 2007a. Concomitantly, N and herbicides emitted from midwestern cropland are detected regularly in ground and surface waters, and are viewed by many analysts as important environmental contaminants that require improved management approaches (Goolsby et al., 1999;Dinnes et al., 2002;Gilliom et al., 2006). Th e midwestern United States has also been a major recipient of agricultural subsidy payments from the federal government (Environmental Working Group, 2007), and there are persistent questions concerning farm economic viability if these subsidies were removed due to global tra...

. Total dry matter production by triticale/corn and triticale/sorghumsudangrass was 25% greater than sole-crop corn, which in turn produced 21% more dry matter than triticale/sunn hemp. Potential ethanol yield was greatest for triticale/corn, which was estimated to have the capacity to produce 1080 L ha -1 more ethanol than sole-crop corn. Crop N uptake was greater in double-crop systems during April-June, greater in the sole-crop corn system during July-August, and greater again in double-crop systems during September-October. Relative to sole-crop corn, potentially leachable soil N was reduced in double-crop systems by 34 and 25%, respectively, in the spring (mid-April) and fall (late October). High nutrient density of biomass coupled with high productivity for triticale/corn and triticale/sorghum systems also resulted in the removal of 83, 41, and 177% more N, P, and K, respectively, compared with sole-crop corn. Sustained removal of large quantities of nutrient-dense biomass from double-cropping systems would necessitate increased fertilization or integration with nutrient recycling mechanisms.

Summary1. Post-dispersal seed predation is an important source of mortality for arable weed populations that can potentially contribute to ecologically based management strategies. The extent to which cropping practices influence rates of seed predation is not well established. 2. Removal of Abutilon theophrasti and Setaria faberi seeds by predators was measured in 2-year (maize/soyabean), 3-year (maize/soyabean/triticale + red clover) and 4-year (maize/soyabean/triticale + lucerne/lucerne) crop rotations in Iowa, USA, during 27 48-h sampling periods conducted during the 2003 and 2004 cropping seasons. 3. Predation of S. faberi seeds was equal (eight sampling periods) or greater (19 sampling periods) than predation of A. theophrasti seeds, but the influence of crops on seasonal predation patterns was generally similar for the two weeds. 4. Seasonal patterns in seed predation were crop-specific and complementary. In maize and soyabean, seed predation was low in spring, high in summer and low in autumn. In triticale-legume intercrops, seed predation was high in spring, low in summer and moderate in autumn. In lucerne, seed predation fluctuated from high to low, matching the periodic harvest and regrowth cycle of the crop. 5. Measurements of crop canopy light interception taken in 2004 were positively correlated with rates of seed removal for both A. theophrasti ( r = 0·54, P < 0·001) and S. faberi ( r = 0·25, P < 0·001), suggesting that vegetative cover promotes weed seed predation. 6. Synthesis and applications. The results indicate that crop vegetation can be managed to promote weed seed predation. Diversified farming systems that include a range of phenologically dissimilar crop species are likely to provide the greatest opportunities for weed seed destruction by predators.