EVALUATING CONSTRAINTS AND OPPORTUNITIES IN MANAGING WEED POPULATIONS WITH COVER CROPS
Open Access
- Author:
- Mirsky, Steven Brian
- Graduate Program:
- Agronomy
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 07, 2008
- Committee Members:
- William Curran, Committee Chair/Co-Chair
David A Mortensen, Committee Chair/Co-Chair
Robert Paul Gallagher Iii, Committee Member
Mary Ellen Barbercheck, Committee Member
Katriona Shea, Committee Member - Keywords:
- cover crops
population dynamics
weed seedbanks
no till
sustainable agriculture - Abstract:
- Efficacy of weed management in cropping systems with reduced reliance on herbicidal weed control is relatively lower and more variable than conventional herbicide management programs. Yet lower herbicide-input systems rely more heavily on cultural controls that can differ in their ability to increase soil organic carbon, a goal that is frequently highlighted by organic and reduced pesticide producers. Reduction in tillage and an increase in cover crop use are both highlighted as practices that increase soil carbon. Therefore, a series of studies were conducted to assess the efficacy of low herbicide weed management practices that vary in cover cropping practices and soil disturbance frequency. The approach involved: 1) using soil disturbance and cover crop residue management to rapidly deplete weed seedbanks in high disturbance organic and low herbicide input systems; and 2) using novel cover crop residue management strategies to reduce or eliminate herbicides in no-tillage cropping systems. Field experiments were initiated to quantify the influence of five cover crop systems on weed seedbank flux across a range of initial weed seedbank densities. The influence of soil disturbance and cover cropping on the germinable weed seedbank of common lambsquarters (Chenopodium album L.), velvetleaf (Abutilon theophrasti L.), and foxtail spp. (Setaria spp.) was assessed. Cover crop systems ranged from a control treatment (summer fallow) that included tillage approximately every 30 days during the summer, to an oat (Avena sativa L.)/red clover (Trifolium pretense L.) cover crop mowed periodically to prevent weed seed production. Field studies were conducted in both Pennsylvania and Maine. Soil disturbance associated with cover cropping resulted in weed seed germination and establishment thereby reducing the density of germinable seed in the weed seedbank. Of the five systems studied, the summer fallow (SF) and yellow mustard (Brassica juncea (L.) Czern.)/buckwheat (Fagopyrum sagittatum Gilib.)/winter canola (Brassica napus L.) (B-BW-B) cover crop consistently decreased seedbanks of a diverse weed community. A quality of both systems is late season disturbance that preempts weed seed rain. If weeds are permitted to reach reproductive maturity in cash or cover crops, the “debits” to the seedbank resulting from early season disturbance will likely be overwhelmed. The magnitude of decline in these high disturbance systems reached complete depletion of the germinable seedbank for foxtail spp., 85% depletion of common lambsquarters, and 80% for velvetleaf. Additionally, the effect of initial weed infestation levels on the efficacy of weed management was tested. Seedbank decline was a positively related (linearly) to initial seedbank density. The slope of that relationship was smallest for the SF and B-BW-B, meaning those treatments provided the greatest level of weed suppression. Nested within the SF and B-BW-B cover crop systems from the cover crop systems experiment, and within a separate component study, the influence of soil disturbance and green manuring frequency on giant foxtail (Setaria faberi) and velvetleaf (Abutilon theophrasti) seed persistence, cumulative emergence, and mortality (fatal germination and seed death) was determined. In early spring of 2005 and 2006, after-ripened weed seed buried in the previous fall and exhumed, were either: 1) kept in mesh bags and buried in field receiving a high frequency of soil tillage that varied in green manure additions (mesh bag experiment), or 2) removed from the mesh bags and placed into wire mesh cages buried flush with the soil surface (seed cage experiment). Seed cage treatments reflected a gradient of soil tillage frequency and green manuring. Over a one year incubation period, the magnitude of decline for velvetleaf and giant foxtail was 72 and 80% in 2004, and 47 and 87% in 2005, respectively. Weed seed persistence decreased with tillage for both species. Green manuring did not influence weed seed persistence, however, reductions in giant foxtail cumulative emergence was observed. However, green manure did not influence weed seed persistence. Giant foxtail mortality was primarily due to fatal germination; whereas other factors appear to influence velvetleaf seed death. The second area of research tested the efficacy of mechanical control of cereal rye (Secale cereale) cover crop cultivars using a mechanical crimping apparatus (roller/crimper) across a range of planting and termination dates. Biomass accumulation and subsequent weed suppression were quantified. To increase grower adoption of cereal rye covers, a thermal-based phenological model using growing degree days (GDD) to predict cereal rye development was developed. A Julian date-based analysis of cover crop control identified differences between the two cultivars; however, when based on the Zadoks growth stage, they responded similarly to rolling/crimping. Whereas cultivar growth rates differ, the growth stage at which optimal suppression was achieved was remarkably consistent over years and sites. A thermal-based phenological model separating the effects of heat units accumulated in the fall (FallGDD) from those accumulated in the spring (SpringGDD) best predicted the phenological development of cereal rye growth stage and biomass accumulation. Cover crop biomass increased approximately 2000 kg ha-1 from earliest to latest summer and fall planting dates (August 25-Octorber 15) and for each 10-day incremental delay in spring termination date (May 1-June 1). Delay in cover crop termination had the greatest reduction in weed density; early and late emerging summer annuals were both suppressed when cover crop termination date was delayed. Yellow nutsedge was not influenced by cover crop type or the timing of cover crop management. Identifying susceptible growth stages for mechanical control coupled with simple thermal-based phenological models that predict cereal rye development provides a useful decision support framework for farmers interested in such practices. We speculate that the degree of synchrony between weed species emergence periodicity and cover crop biomass accumulation played an important role in defining the extent of weed suppression. The body of this work provides compelling evidence that cover crops can be used effectively to suppress weedy plant populations in no-till cash crops. Adoption of such practices will be aided by information provided here on the timeliness of establishment and termination of cover crops. This research underscores that species composition changes will result from such management selecting against small seeded weeds, while leaving larger seeded broadleaves and perennial weeds less affected. Such changes in population dynamics can be offset by changes in the sequence of crops to crop phases with increasing frequency of tillage and cover cropping. Another benefit of coupling rotational tillage practices with cover crop-based management is that negative effects of tillage may be offset by incorporation of 3000 kg ha-1 of cover crop biomass in the crop phases with soil disturbance.