Anything but simple: Using models and field studies to explore the effects of cropping system diversification in the upper Midwestern United States
Date
2021-12
Authors
Nichols, Virginia Anne
Major Professor
Advisor
Liebman, Matt Z
Archontoulis, Sotirios
Hatfield, Jerry
Castellano, Michael
Niemi, Jarad
Committee Member
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Abstract
Cropping systems are composed of a set of crops and their attendant management practices. Addition or removal of a crop from a cropping sequence results in both biophysical and managerial changes, creating complex interactions that can be difficult to study. Long-term field experiments provide a crucial resource for examining differences between cropping systems, and models (e.g., conceptual, process-based, statistical) can aid in dissecting complex problems and in distilling results into actionable gains in knowledge. In this dissertation, I used field studies and various models to look at two general categories of cropping sequence changes: (i) addition of a cover crop between cash crops (Chapters 2-4), and (ii) extension of rotation lengths through addition of cash crops (Chapters 5-6).
Over-wintering cover crops such as cereal rye (Secale cereale L.) present a potentially beneficial addition to cropping systems consisting of summer annuals, such as the maize (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation prevalent in the Midwestern United States (US). However, several questions regarding the impacts of this addition, particularly in the long-term, remain unanswered. My colleagues and I used a meta-analysis to synthesize the current state of knowledge regarding winter cover crops’ effects on weeds in maize-soybean rotations from studies conducted in the Midwestern US (Chapter 2). This analysis showed that, on average, cover cropping reduced weed biomass but had no effect on weed density. Additionally, using a process-based model, we found achieving sufficient cover crop biomass for significant weed suppression may be challenging within the managerial and climatic constraints of a Midwestern maize-soybean system. While we found no consistent effect of cover cropping on weed density, the majority of studies had been in place for less than three years. The most common Midwestern weed seeds persist in the soil for several years, suggesting the short-term nature of the studies may preclude detecting effects on weed densities. To address the lack of data on the effect of long-term cover crop on weeds, my colleagues and I used four rye cover crop experiments that had been in place at least 10 years (Chapter 3). We found long-term use of cover crops can significantly reduce weed seedbank densities compared to winter fallows. These reductions were driven by decreased numbers of waterhemp [Amaranthus tuberculatus (Moq.)] seeds, a problematic weed with resistance to multiple groups of herbicides, implying cover crops can be an effective complement to herbicide-based weed control. Through additional collaborations we used these same four experiments to look at how cover crops alter soil structures in ways that affect soil hydrological parameters (Chapter 4). Using non-linear statistical models, we found the cover crop’s effect on the soil structure depended on the site, and by proposing a causal model we provided a framework for identifying measurements to help understand site-specific effects. This work helps to identify ways in which crop models could incorporate cover crops’ effects on soil structure to better predict how cover crop use can contribute to cash crop yield resilience.
Extending crop rotations through inclusion of additional cash crops has agronomic implications that are not well understood. In Chapter 5, we used a meta-analytic approach to quantify the effect of diversifying cash crop rotation on weed biomass and density and found a more consistent reduction in weed density compared to weed biomass. We used a statistical model to rank covariates in their ability to predict weed suppressiveness of a given rotation and identified the variation in months between planting activities (planting-interval-variation-index; PIVI) as a useful metric for describing cropping systems with respect to weed dynamics. In addition to impacting weeds, crop yields are also affected by extending crop rotations. In maize, as the number of years between maize crops increases, maize yields likewise increase. While this phenomenon is well-known and documented, its driving mechanisms have remained elusive. In Chapter 6 we used multiple years of field data collected from a long-term experiment to investigate differences in soil and crop characteristics that occur when a simple maize-soybean rotation is extended to a more complex 4-year rotation. We found maize roots in the extended rotation had maximum rooting depths 8 cm deeper than maize roots in the simpler rotation. Maximum rooting depth is a critical parameter in several cropping systems models, so this data supports improvement of these models in their ability to reflect complex cropping sequences and their effects on keystone crops such as maize.
In summary, we identified benefits and challenges to increasing cropping system complexity in the Midwest. Inclusion of cover crops in the traditionally winter fallow periods of Midwestern cropping systems can reduce weed pressure but has variable effects on soil structure and more context-specific information is needed to understand the factors driving this variability. Utilizing sequences of crops with different planting dates may help reduce weed densities, and examining root form and function of maize in different rotation sequences may provide insight into the drivers of the rotation effect in maize. Adjusting maize root characteristics in response to crop history may be a promising avenue for including these phenomena in process-based models. Overall, the work presented in this dissertation utilized novel approaches to leverage models and field data to advance our knowledge of the complex interactions involved in cropping systems research, providing significant contributions to the pursuit of more sustainable agricultural systems in the upper Midwestern US.
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dissertation