Browsing by Author "Chris Reberg-Horton, Committee Member"

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    Allelopathy in Rye (Secale cereale)
    (2008-12-05) Brooks, Ashley Meredith; Jeff Thompson, Committee Member; David A. Danehower, Committee Chair; Chris Reberg-Horton, Committee Member; J. Paul Murphy, Committee Member
    Allelopathy is an ecological phenomenon in which chemicals produced by and released from a plant affect the germination or growth of another plant. A possible exploitation of allelopathy is the use of allelopathic cover crops for weed management. Organic farming systems can utilize allelopathy as an alternative to synthetic herbicides and conventional farming can reduce reliance upon pre-emergence herbicides. Rye (Secale cereale) is a cover crop species known to be allelopathic to many weeds. In addition to allelopathic activity, rye is a successful cover crop because of prolific biomass, high germinability and winter hardiness. The objective of this research was to investigate the potential to develop a rye cultivar with increased allelopathy through a conventional breeding approach. A population of 150 half-sib families of rye was grown in two North Carolina locations. Above ground tissue was utilized to assess rye allelopathic activity. To assess allelopathy in the population, we aimed to develop a greenhouse bioassay which utilized a rye incorporated soil media and redroot pigweed (Amaranthus retroflexus) as the indicator species. It is necessary to identify a screening protocol to quantify variation in allelopathic activity and to identify high performing lines. The greenhouse bioassay was fast, inexpensive and able to screen the large number of genotypes in the rye population. Results of redroot pigweed fresh weight biomass were reproducible and were utilized to estimate genetic parameters for allelopathy in the rye population. Estimates of genetic variation, genotype x environment interaction and narrow sense heritability help plant breeders develop an appropriate breeding program for the trait of interest. The estimates also give an idea of the rapidity at which progress can be made with selection. Genetic variation for rye allelopathy was not significant across locations but was significant within each location. Redroot pigweed fresh weight biomass was normally distributed indicating that allelopathy in rye is a quantitative trait. Heritability estimates were low on a per-plot basis and moderately low on an entry mean basis. A petri dish bioassay was also utilized to estimate genetic parameters for allelopathy in rye. Redroot pigweed germination and root length measures were utilized to quantify allelopathic activity. Genetic variation was not significant across locations for germination or root length. Analysis of variance within each location detected variation among the genotypes grown at the Kinston location but not at the Clayton location. Measures of redroot pigweed germination and root length were normally distributed. Heritability estimates were low on a per-plot basis and on an entry mean basis. This study demonstrates that allelopathy in rye is under genetic control and that it is a quantitative trait. Results suggest that a conventional breeding approach may be used for the development of a highly allelopathic rye cultivar.
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    Cover Crop Mulches for No-till Organically Managed Onion Production
    (2008-12-04) Vollmer, Emily R.; Nancy Creamer, Committee Chair; Chris Reberg-Horton, Committee Member; Greg Hoyt, Committee Member
    Combining the environmentally sound practices of reduced tillage and cover crop use with organic vegetable production systems requires management choices that are tailored to specific climate and crop combinations. In the southeastern U.S. summer cover crops can be grown through the fall until desiccation by frost. At this time a cash crop such as bulb onions (Allium cepa L.) can be grown using reduced tillage for over winter production. Field experiments were conducted on first year transitional (non-organic) land in 2006-07 and 2007-08. Cover crops of foxtail millet ‘German Strain R’ [Setaria italica (L.) Beauv.] and cowpea ‘Iron & Clay’ [Vigna unguiculata (L.) Walp.] were grown as monocrops (MIL, COW) and biculture mixtures and compared to a bare ground control (BG). Mixtures of cowpea and millet consisted of seeding rates aimed at producing 70%, 50%, and 30% millet per total biculture biomass (MIX-70, MIX-50, MIX-30). Cover crop residue treatments were evaluated for weed suppression and N contribution to no-till organic onion production. Supplemental N in the form of surface applied soybean meal [Glycine max (L.) Merrill] was applied to cover crop treatment subplots at three rates: 0, 105, and 210 kg N∙ha-1. Onion yields increased linearly from 0 to 210 kg N∙ha-1 rates of soybean meal. Cover crop treatments COW and BG had the greatest total marketable onion yield and were statistically equivalent both years. Losses in marketable yield were primarily due to mortality in MIL and MIX in 2006-07 and bolting across all cover crop treatments, which averaged 28% in 2006-07 and 72% in 2007-08. Onion mortality was over 50% in MIL and MIX treatments in 2006-07 and was attributed to physical properties of thick surface mulch. Onion mortality in 2007-08 was highest on MIL compared to all other cover crop treatments but remained below 20%. Weed interference during onion production was highest in COW and lowest in treatments with millet (MIL, MIX) in 2006-07. Nitrogen rates of 105 and 210 kg N∙ha-1 increased soil mineral N (NO3- and NH4+) on BG plots two weeks after surface application of soybean meal at the end of November each year but generally ceased to have an effect on soil mineral N by February or March. The 105 kg N∙ha-1 soybean meal rate appeared to be more than sufficient for use as a starter fertilizer. Split applications of soybean meal could be an important improvement in N management to avoid N leaching and better meet increased N uptake demand during bulb initiation and growth in the spring. Overall, this study shows that cowpea cover crops grown preceding an over-wintered no-till allium crop is feasible with appropriate management, and provides onion yield comparable to bare ground production.
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    Effects Of Both Above And Below Ground Biomass On Soil Chemical, Physical And Biological Properties On A Coastal Plain Soil In North Carolina
    (2010-03-04) West, Eric; Nancy Creamer, Committee Member; Carl Crozier, Committee Member; Ronnie Heiniger, Committee Chair; Chris Reberg-Horton, Committee Member
    The literature has shown the possibility to enhance many of the soil properties that improve production with additions of plant biomass to the soil in conjunction with long-term conservation tillage. Since it has been proven that reduced tillage is extremely beneficial to a soil’s health, the key question is if the incorporation of deep rooted cover crops and/or large quantities of above ground biomass over a two-year period would result in extensive improvements on a soil’s natural properties throughout the effective rooting depth, or is this too short of a time period for any beneficial effects to occur to the chemical, physical and biological properties of the soil? While SOC additions to the soil surface through decaying biomass can make measurable improvements to a soil in the humid, tropical southeastern U. S., the dynamics of total carbon (total SOC), total nitrogen (TKN), particulate organic matter (POM), potentially mineralizable nitrogen (PMN), CEC and bulk density (Db) in the 0-5, 5-10, and 10-18 cm depths in a production system that incorporates deep rooted cover crops and/or additions of organic matter from cover crops at rates >6 Mg/ha/yr (3 T/ac) are unknown. The objectives of this research were to determine what effect rye (Scale cereale), barley (Hordeum vulgare), alfalfa (Medicago sativa), wheat (Triticum aestivum), triticale (Triticale hexaploide Lart.), annual white sweetclover (Melilotus officinalis), blue lupine (Lupinus angustifolius), rye/hairy vetch (Scale cereale/Vicia villiosa) and alfalfa/rye (Medicago sativa/Scale cereale) would have on the following parameters: total SOC, POM, TKN, PMN, CEC, and Db after two (2) years of seeding. Significant spatial/temporal interactions and main effects were found in Db, PMN, POM and CEC while significant main effects were found in total C and total N. Significant treatment interactions were found in Db, total C, PMN, POM and CEC. Relative to treatment biomass, significant effects were seen between treatments and treatment*year interaction. Results on Db found spatial variability with depth but not with season and a minimal treatment effect depending on surface texture. Total C and total N decreased with depth but were independent spatially and temporally; however, treatment effect on total C was <5 months. Between Db and total C, the two parameters were inversely correlated. PMN either fluctuated between sampling times or declined with time. Notably, rye and rye/hairy vetch effected PMN in the 0-5 cm depth where other treatments showed no effect. POM declined with time regardless of depth implying a priming effect was occurring; however, rye/hairy vetch appeared to show an early reversal trend. As for CEC, there was no consistent trend. Additionally, CEC exhibited a moderate correlation to POM but not total C. Biomass measurements indicated no treatment consistently exceeded >6 Mg/ha/yr. Rye and rye/hairy vetch more often produced the most biomass and lupine achieved the overall maximum yield. Relationships between biomass and N parameters found a moderate, positive effect from the biomass inputs that appeared to compound with time. Overall, rye and rye/hairy vetch were the best cover crop treatments but two years of biomass inputs were not enough to prevent declines in POM. Conversely, PMN fluxed from applied N, and biomass with low C:N ratios oxidized quickly resulting in less carbon. Total C, total N and Db were not responsive to short term management indicating more effort is needed to define and/or develop a cover crop that will consistently reach the >6 Mg/ha/yr.