Cover crops (CC) can contribute to production in pastures, but the diversity of CC mixtures and defoliation frequency (DF) may alter productivity. A 2-yr experiment conducted at Raymond, MS, quantified CC × DF effects on forage mass (FM) and nutritive value of winter CC and subsequent summer hay production. Treatments were factorial combinations of 10 CC (using several species of grasses, legumes, and brassicas) and three DF (harvested every 4 or 8-wk or cut and left as mulch) in a split-plot arrangement of a randomized complete block design experiment with three replications. Generally, mixtures with legumes had greater FM and better nutritive value. Summer hay production did not respond to difference in CC composition, however, harvesting of CC reduced summer hay but increased year-long FM. These results suggest that CC when harvested can contribute to forage production with improved nutritive value and can increase year-long FM, but summer hay production can benefit when the CC is left as mulch.

Herbicide resistance, regulations on pesticide use, and cost of pesticides are all challenges for managing weeds in production agriculture. The use of cover crops (CC) has emerged as a promising integrated weed management tool to aid in weed suppression. There are many questions concerning the best management practices to reap the most benefits from CC. Research was conducted to determine if the application of a pre-plant herbicide as well as the type of CC planted would increase CC biomass and subsequent winter weed suppression. Early planting and selecting a cereal rye or a cereal rye-containing mixture are the most important factors to obtain the greatest CC biomass production. Additionally, the combination of a CC and a pre-plant herbicide increased weed suppression compared to a no CC (winter fallow) treatment or CC without a pre-plant herbicide. The difference in Palmer amaranth emergence between a rolled cereal rye CC or one that is left standing was also examined along with termination timing to achieve different CC biomass levels. Overall, greater CC biomass suppressed more Palmer amaranth, but treatments of rolled or standing or termination timing did not affect weed suppression consistently. Light penetration data also showed that greater CC biomass led to a decrease in light penetration through the CC canopy, which could be a factor in reducing Palmer amaranth emergence particularly at the greater CC biomass accumulation levels. Additionally, studies were conducted to investigate the effect of cereal rye CC termination timing (i.e., "planting green" being CC terminated at the time of soybean planting or "planting brown" being CC terminated 2 weeks prior to planting) on Palmer amaranth suppression, as well as to determine how termination timing influences herbicide program optimization. A delay in emergence and growth rate of Palmer amaranth was documented in the CC containing plots when compared to the no CC plots, but no differences were observed between the termination timings. Additionally, significantly lower Palmer amaranth densities were observed under CC containing plots when compared to the no CC treatments. Within CC treatment options, the most economical option was planting green with a single postemergence herbicide application, but overall, no CC treatments were more economical programs. Finally, research was conducted to understand weed and corn competition for nitrogen when hairy vetch + cereal rye CC was present. A range of side dress nitrogen fertilizer rates, weedy versus weed free herbicide programs, and CC versus no-CC treatments were compared. Overall, yield did not differ among treatments. Ear leaf and grain nitrogen was generally greater under weed free, CC, and when fertilized at or above yield goals respective of location. Despite these findings, early season weed control in corn is still necessary to achieve maximum potential yield. These studies indicate that CC biomass is consistently the most important factor for achieving weed suppression, and that CC results can vary in response to environmental and management effects. More research is therefore necessary to evaluate the effects of CC over greater periods of time. / Doctor of Philosophy / Herbicide resistant weeds are a major challenge for farmers across the globe. With the increased number of weed species resistant to multiple herbicides and the restrictions on pesticide use, farmers need more tools to control weeds. The use of cover crops (CC) to suppress weeds can be a viable integrated weed management tool for farmers. Although there are multiple benefits associated with CCs, there are also many drawbacks. CCs are an additional input cost for farmers, and require a greater level of management when compared to conventional systems. There are also many questions concerning best management practices in order to reap the benefits of CCs. Previous research indicates CC biomass is the most important factor in achieving weed suppression. Research trials were conducted at Virginia Tech to determine whether CC species as well as the application of preplant herbicide at the time of CC planting would affect CC biomass production as well as weed suppression. Cereal rye-containing plots produced more biomass overall compared to hairy vetch alone or crimson clover and earlier plantings accumulated greater CC biomass. Additionally, planting earlier was significant for CC biomass accumulation while applying a pre-plant herbicide was not. Weed suppression varied by species but was more successful when both a CC and pre-plant herbicide were applied as compared to no CC and no pre-plant herbicide application. Termination timing and mechanism of CC termination are both known to impact potential CC benefits. Many farmers roll their CC at termination, but it is unclear whether rolling is necessary for weed suppression benefits and soybean yield. Palmer amaranth counts were collected at four and six weeks after planting, as well as yield data across a range of cereal rye CC biomass levels, rolled or left standing at termination, and planted green (CC terminated at the time of planting) versus brown (CC terminated 2 weeks prior to planting). Light penetration measurements were also collected to observe changes in light availability through a CC canopy. Overall, CC treatments reduced Palmer amaranth emergence when compared to no CC, and suppression increased with greater CC biomass. Yield did not differ among treatments, therefore CC management can be tailored to weed suppression efforts. There were clear patterns of light penetration reduction as CC biomass increased. This reduction in light penetration could be part of the reason for the decreased Palmer amaranth emergence with greater CC biomass. Overall, the most important factor affecting weed suppression is accumulation of greater CC biomass. Research was conducted to determine differences in Palmer amaranth suppression due to CC termination timing, as growers have experimented with planting green while traditionally, planting brown or no CC at all was more common. Palmer amaranth groups were created and followed throughout a 10-week period to examine density and growth rates, which allowed various herbicide programs to be simulated and compared. A delay in emergence and growth rate of Palmer amaranth was documented in the CC containing plots when compared to the no CC plots, but no differences were observed between the termination timings. Additionally, significantly lower Palmer amaranth densities were observed under CC containing plots when compared to the no CC treatments. The most effective programs varied by year, but amongst CC options, planting green with a single POST herbicide was optimal, but overall, no CC treatments were more economical due to the expenses associated with CC seed and planting. If a grower is planning on using a CC, the recommendation is therefore to plant green, as lower costs were associated with this practice. In addition to weed suppression benefits, the potential for leguminous CC such as hairy vetch to provide nitrogen for the cash crop is another reason that farmers may plant CCs. However, the effect of a CC on weed competition for nitrogen is still unclear, as the nitrogen released by the CC can also stimulate weed emergence while the CC itself is physically suppressing weeds through its biomass. A study was established to evaluate the effects of CC or no CC, weedy or weed free herbicide programs, and six nitrogen side dress fertilizer rates on yield. Overall, greater concentrations of nitrogen in the ear leaf and in grain were found under CC, weed free, and when fertilized-to-yield goal for the respective locations. Yield was not responsive to treatment effects, and agronomic optimum analysis indicated that a side dress nitrogen to yield goal is still the best option even under CC to achieve optimal yields. Use of CCs is another tool for farmers to implement in their weed control programs. Proper management of CCs can result in increased weed suppression and provide other benefits not examined in this document. However, varying results by location calls for further research to explore the intricacies of CC management and its effect not only on other weed species, but also on other major cash crops.

Doctor of Philosophy / Department of Agronomy / P. V. Vara Prasad / Leguminous cover crops systems have been envisaged as a critical component of sustainable agriculture due to their potential to increase soil productivity through cycling of carbon (C) and nitrogen (N) in agricultural systems. The objectives of this study were to evaluate the performance of leguminous summer cover crops; cowpea [Vigna unguiculata (L.) Walp.], pigeon pea [Cajanus cajan (L.) Millsp], sunn hemp (Crotalaria juncea L.) and double-cropped grain crops; grain sorghum [Sorghum bicolor (L.) Moench] and soybean [Glycine max (L.) Merr.] after winter wheat (Triticum aestivum L.) and to determine the effects of these crops and varying N rates in the cropping system on nitrous oxide (N[subscript]2O) emissions, growth and yield of succeeding grain sorghum and maize (Zea mays L.) crop, soil aggregation, aggregate-associated C, and N. Field and laboratory studies were conducted for two years. The cover crops and double-cropped grain crops were planted immediately after winter wheat harvest. The cover crops were terminated at the beginning of flowering. Nitrogen fertilizer (urea 46% N) rates of 0, 45, 90, 135, and 180 kg N ha[superscript]-1 were applied to grain sorghum or maize in fallow plots. Pigeon pea and grain sorghum had more C accumulation than cowpea, sunn hemp and double-cropped soybean. Pigeon pea and cowpea had more N uptake than sunn hemp and the double-cropped grain crops. Fallow with N fertilizer application produced significantly greater N[subscript]2O emissions than all the cover crops systems. Nitrous oxide emissions were relatively similar in the various cover crop systems and fallow with 0 kg N ha[superscript]-1. Grain yield of sorghum and maize in all the cover crop and double cropped soybean systems was similar to that in the fallow with 45 kg N ha[superscript]-1. Both grain sorghum and maize in the double-cropped soybean system and fallow with 90 kg N ha[superscript]-1 or 135 kg N ha[superscript]-1 gave profitable economic net returns over the years. The double-cropped grain sorghum system increased aggregate-associated C and whole soil total C, and all the cover crop and the double-cropped soybean systems increased aggregate-associated N and soil N pools. Inclusion of leguminous cover crops without N fertilizer application reduced N[subscript]2O emissions and provided additional C accumulation and N uptake, contributing to increased grain yield of the following cereal grain crop.

Master of Science / Department of Agronomy / J. Anita Dille / Kraig Roozeboom / Increasing numbers of herbicide-resistant weed species require alternative methods of weed suppression to be examined. This study quantified the interaction between various cover crop or herbicide systems and horseweed [Conyza canadensis (L.)] growth. Fall cover crops of winter wheat [Triticum aestivum (L.)], winter rye [Secale cereal (L.)], barley [Hordeum vulgare (L.)] and annual ryegrass [Lolium multiflorum (L.)] were seeded in November 2012 and 2013. Spring cover crop of oat [Avena sativa (L.)] was seeded in April 2013 or rye was seeded in March 2014. All cover crops were no-till seeded into grain sorghum stubble [Sorghum bicolor (L.) Moench]. Four herbicide treatments were fall or spring applied, with and without residual. The spring non-residual treatment was also applied to plots of winter rye. Cover crop plots were split and terminated with a roller crimper or glyphosate application prior to soybean [Glycine max (L.) Merr.] planting to determine the effect of termination method on treatment performance. Soybean was planted in June 2013 and May 2014 and mechanically harvested in October of both years. Horseweed density, biomass accumulation, and soybean yield data were quantified. Horseweed height, whole plant seed production, and seed subsamples were recorded in the untreated fallow control, winter wheat, and winter rye plots in 2014. Horseweed suppression by winter rye approached 90%, levels similar to suppression by herbicide systems. In both years, herbicide plots had less than half the horseweed biomass than any of the cover crop systems. In 2013, soybean yields in herbicide plots were at least 1,500 kg ha[superscript]-1, nearly more than double yields in cover crop plots. Soybean yields in 2014 were more consistent across treatments; barley and spring rye plots achieved yields equal to or greater than 2,000 kg ha[superscript]-1. Winter rye and winter wheat reduced horseweed seed production by 60% compared to the untreated fallow control, with no effect on individual seed weight. Seed production varied across plants, with the untreated control producing the greatest number of seeds. Cover crops were successful at reducing horseweed biomass, suppressing horseweed pressure, preserving soybean biomass, and protecting soybean yields when compared to a fallow untreated control.

Master of Science / Department of Agronomy / Peter J. Tomlinson / Growing cover crops (CCs) in rotation with cash crops has become popular in recent years for their many agroecosystem benefits, such as influencing nutrient cycling and reducing nutrient losses. This study aimed to (i) determine the long-term effects of no-till with CCs and varying nitrogen (N) rates on subsequent sorghum [Sorghum bicolor (L.) Moench] yield and yield components, (ii) assess how CCs affect the N dynamic in the soil-crop relationship during the growing season and N use efficiency (NUE) of sorghum, and (iii) define and evaluate important periods of nitrous oxide (N₂O) losses throughout the cropping system. Field experiments were conducted during the 2014-15 and 2015-16 growing season in a three-year no-till winter wheat (Triticum aestivum L.) – sorghum – soybean [Glycine max (L.) Merr] rotation. Fallow management consisted of a chemical fallow (CF) control plus four CCs and a double-crop soybean (DSB) grown after wheat harvest. Nitrogen fertilizer was subsurface banded at five rates (0, 45, 90, 135, and 180 kg ha⁻¹) after sorghum planting. On average, DSB and late-maturing soybean (LMS) provided one-third and one-half of the N required for optimum economic grain yield (90 kg N ha⁻¹), respectively; resulting in increased grain yield when compared to the other CCs and CF with 0-N application. Crimson clover (Trifolium incarnatum L.) and daikon radish (Raphanus sativus L.) had no or negative effects on sorghum yield and N uptake relative to CF across all N rates. Sorghum-sudangrass (SS) (Sorghum bicolor var. sudanese) significantly reduced N uptake and grain yield, even at higher N rates. Sorghum following CF had the lowest NUE at optimum grain yield when compared to all CC treatments, suggesting that CCs have a tendency to improve NUE. Cover crops reduced N₂O emissions by 65% during the fallow period when compared to CF; however, DSB and SS increased emissions when N was applied during the sorghum phase, indicating that N fertilization might be the overriding factor. Moreover, about 50% of the total N₂O emissions occurred within 3 weeks after N application, regardless of the cover crop treatment, indicating the importance of implementing N management strategies to reduce N₂O emissions early in the growing season. Overall, these results show that CC selection and N fertilizer management can have significant impacts on sorghum productivity and N₂O emissions in no-till cropping systems.

Increases in agricultural intensification over the past century have resulted in significant alterations to the rural landscape across the Midwest. Pollinators are essential to sustain natural and managed ecosystems. They are vital for food production and their declines have been linked, in part, to a rise in intensive agricultural practices. There is a recognized need among numerous stakeholders to build sustainability into the management of agroecosystems to protect both the biotic and abiotic resources of these systems. The use of cover crops is gaining interest among agricultural producers for benefits such as improving water quality and soil health. Cover cropping systems have the potential to provide floral resources to pollinators and suppress problematic driver weeds. The overall objective of this study was to quantify the effects of cover crops on plant and pollinator biodiversity within agricultural systems. This study aimed to characterize the pollinator diversity indicative of the patchwork mosaic forest-agroecosystem of Crab Orchard National Wildlife Refuge; evaluate the roles cover crop treatments play in supporting pollinator diversity and weed suppression benefits in a conventionally managed system; and provide the basis of recommendations for sustainable weed suppression tactics and for enhancing the quality of pollinator habitat within agricultural systems.

Orientador: José Eduardo Corá / Banca: Rogério Peres Soratto / Banca: Luís Reynaldo Ferraciú Alleoni / Banca: José Carlos Barbosa / Banca: Carlos Eduardo Angeli Furlani / Resumo: Em sistema de semeadura direta, os resíduos das culturas de entressafra são utilizados para protegerem a superfície do solo dos agentes erosivos e promoverem a ciclagem de nutrientes. O objetivo desse trabalho foi avaliar o efeito de sequências de culturas na quantidade, qualidade, decomposição e liberação de nutrientes dos resíduos vegetais de culturas de entressafra, bem como a cobertura do solo proporcionada pelos resíduos, na fertilidade do solo, nas frações particulada e associada aos minerais de C, nas substâncias húmicas do solo e na produtividade de milho e soja cultivados no verão, em região de clima tropical. O experimento foi conduzido em Jaboticabal, SP (48°15'22'' W e 21°18'58'' S), em um Latossolo Vermelho eutrófico. O delineamento experimental foi em faixas, com três repetições. Os tratamentos foram constituídos pela combinação de três sequências de culturas de verão (rotação soja-milho, monocultura de milho e monocultura de soja) com sete culturas de entressafra (milho, sorgo, girassol, crotalária, guandu, nabo forrageiro e milheto). O experimento foi iniciado em 2002 e o presente estudo se refere aos anos agrícolas 2008/2009 e 2009/2010. Avaliaram-se as quantidades de matéria seca e acúmulo de N, P, K, Ca, Mg e S pelas culturas de entressafra, bem como a dinâmica de decomposição e liberação dos nutrientes, por meio da utilização de sacolas de decomposição, com os seguintes períodos de avaliação: 15, 30, 60, 120 e 180 dias. A cobertura do solo foi avaliada após o manejo e ao final de cada ano agrícola. Amostras de solo foram coletadas nas camadas 0-10, 10-20 e 20-30 cm de profundidade em duas ocasiões, antes da semeadura das culturas de entressafra, em março, e de verão, em outubro, nos anos de 2008 e 2009. As amostras de solo foram submetidas às análises químicas para determinação dos teores... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In no-tillage system, the offseason crops residues are expected to maintain the soil covered in order to control soil erosion and to promote the nutrients cycling. The objective of this study was to evaluate the offseason crops dry matter quality and production, soil cover, residues decomposition and nutrient release and their effects on soil fertility, C content of humic substances, particulate and mineral associated organic matter fractions and summer maize and soybean yield. A field experiment has been carried out on a Rhodic Eutrudox at Jaboticabal, SP, Brazil (48°15'22'' W and 21°18'58'' S). A randomized split-block design with three replications was used. The treatments were the combination of three summer crops sequences (soybean-corn rotation, monoculture of corn and monoculture of soybean) with seven offseason crops (maize, grain sorghum, sunflower, sunn hemp, pigeon pea, oilseed radish and pearl millet). The cultivations started in 2002 and this study is related to 2008/2009 and 2009/2010 growing seasons. The offseason crops dry matter production and N, P, K, Ca, Mg and S accumulations were evaluated at management moment. The period of time to measure the residue decomposition and the nutrient release was 15, 30, 60, 120 and 180 days, using litter bags. The soil cover was evaluated after the offseason crops management and at the end of each growing season. The soil was sampled at 0-10, 10-20 and 20-30 cm soil depths in two occasions, before the offseason crops sowing, at March, and before the summer crops sowing, at October, in 2008 and 2009. Total organic C, pH, P, K, Ca, Mg and H+Al were determined in each soil samples. The soil samples obtained on October at 0-10 cm were submitted for physical and chemical organic matter fractionation, for particulate, mineral-associated, humic acid, fulvic acid and humin C content determination. The summer monoculture... (Complete abstract click electronic access below) / Doutor

Nitrogen availability in agricultural soils from fertilizer, plant residue inputs, and soil organic matter has important implications beyond crop yield. Legume winter cover crops and one fourth the recommended N rate on sweet corn resulted in yields equivalent to those at the recommended rate in the Willamette Valley of western Oregon. Cereal rye winter crops absorbed an average of 40 kg N/ha that otherwise would have been leached, but did not effectively replace fertilizer N. Cereal rye as a cover crop therefore shows an ability to immobilize N from fertilizer. This was further confirmed in an experiment with "N labeled urea where results showed that N derived from fertilizer in sweet corn or cereal rye plant residue was less available for crop uptake and loss from the system than inorganic N or N directly immobilized from fertilizer. Losses of N from fertilizer ranged from 40 to 73% of that which was in the soil over winter. Mineralization of organic matter N is an important process in N availability, especially when cover crops are used to replace fertilizer. Finding a general indicator or predictor of N mineralization in soils would help in reducing fertilizer N costs and leaching of inorganic N that is applied in excess of crop needs. In a screening of 17 biological and chemical properties of 19 differently managed soils from around the state of Oregon, a model using total soil N and ��-glucosidase activity provided the best model of mineralized N uptake by ryegrass. Biological activity is primarily responsible for the transformations that result in N availability in soils. Management of soils directly impacts soil biology, and results from multivariate analyses of biological and chemical parameters in differently managed soils showed that disturbance creates an overriding common biochemical state in soils. Beyond disturbance, vegetation and the nature of organic inputs also impart recognizable multivariate patterns in soils managed differently. These results suggest that indicators independent of soil type may be used to discern effects of management on agricultural soils. / Graduation date: 1999

Graduation date: 1999

Although legumes have been widely studied for their nitrogen-fixing ability, it is uncertain to what extent legume cover crops achieve their nitrogen-fixing potential under the climatic conditions encountered in western Oregon. Furthermore, it is unknown what factors control the proportions of legume cover crop N that are either sequestered into soil organic matter, or that contribute to the N requirements of the following summer crop. Soil was sampled in mid-September 1997, after harvest of a summer broccoli crop, from plots located at the North Willamette Research and Extension Center, Aurora, Oregon. Soil was sampled from main plots that had been either winter cover cropped with red clover (LN��� and LN���) or fallowed during the winter period (FN��� and FN���), and specifically from sub-plots in which the following summer crop had received either zero (N���) or an intermediate (N���) rate of N fertilizer as urea. Levels of total organic carbon (TOC), total Kjeldahl nitrogen (TKN), and readily mineralizable C and N were measured in both whole soil samples and in different aggregate-size classes (<0.25, 0.25-0.5 0.5-1.0, 1.0-2.0, and 2-5mm) prepared by dry sieving the soil. Aggregate size-class distribution was not affected by the cover crop treatment. Although there was no significant effect of cover crop treatment on either TKN or TOC levels in whole soil samples, TOC levels were consistently higher in the small aggregate size-classes <1 mm of the fallow than the legume treatment. There was a significantly higher level of mineralizable C in the <0.25 mm size class of the legume than the fallow treatment. There was a trend for the level of mineralizable N to be greater in soil from the legume than the fallow treatment. However, N fertilizer had a significant positive effect on the level of readily mineralizable N in both fallow and legume cover-cropped treatments, it had a negative effect on TKN levels among all aggregate-size classes. There were differences in the levels of mineralizable N measured among the aggregate-size classes, and immobilization of N between 20 and 40 days of incubation also differed among the aggregate-size classes. / Graduation date: 1999