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"Hunter, Mitchell"
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Soil Water Holding Capacity Mitigates Downside Risk and Volatility in US Rainfed Maize: Time to Invest in Soil Organic Matter?
Yield stability is fundamental to global food security in the face of climate change, and better strategies are needed for buffering crop yields against increased weather variability. Regional- scale analyses of yield stability can support robust inferences about buffering strategies for widely-grown staple crops, but have not been accomplished. We present a novel analytical approach, synthesizing 2000-2014 data on weather and soil factors to quantify their impact on county-level maize yield stability in four US states that vary widely in these factors (Illinois, Michigan, Minnesota and Pennsylvania). Yield stability is quantified as both 'downside risk' (minimum yield potential, MYP) and 'volatility' (temporal yield variability). We show that excessive heat and drought decreased mean yields and yield stability, while higher precipitation increased stability. Soil water holding capacity strongly affected yield volatility in all four states, either directly (Minnesota and Pennsylvania) or indirectly, via its effects on MYP (Illinois and Michigan). We infer that factors contributing to soil water holding capacity can help buffer maize yields against variable weather. Given that soil water holding capacity responds (within limits) to agronomic management, our analysis highlights broadly relevant management strategies for buffering crop yields against climate variability, and informs region-specific strategies.
Journal Article
Weed Suppression in Cover Crop Monocultures and Mixtures
Interest in planting mixtures of cover crop species has grown in recent years as farmers seek to increase the breadth of ecosystem services cover crops provide. As part of a multidisciplinary project, we quantified the degree to which monocultures and mixtures of cover crops suppress weeds during the fall-to-spring cover crop growing period. Weed-suppressive cover crop stands can limit weed seed rain from summer- and winter-annual species, reducing weed population growth and ultimately weed pressure in future cash crop stands. We established monocultures and mixtures of two legumes (medium red clover and Austrian winter pea), two grasses (cereal rye and oats), and two brassicas (forage radish and canola) in a long fall growing window following winter wheat harvest and in a shorter window following silage corn harvest. In fall of the long window, grass cover crops and mixtures were the most weed suppressive, whereas legume cover crops were the least weed suppressive. All mixtures also effectively suppressed weeds. This was likely primarily due to the presence of fast-growing grass species, which were effective even when they were seeded at only 20% of their monoculture rate. In spring, weed biomass was low in all treatments due to winter kill of summer-annual weeds and low germination of winter annuals. In the short window following silage corn, biomass accumulation by cover crops and weeds in the fall was more than an order of magnitude lower than in the longer window. However, there was substantial weed seed production in the spring in all treatments not containing cereal rye (monoculture or mixture). Our results suggest that cover crop mixtures require only low seeding rates of aggressive grass species to provide weed suppression. This creates an opportunity for other species to deliver additional ecosystem services, though careful species selection may be required to maintain mixture diversity and avoid dominance of winter-hardy cover crop grasses in the spring.
Journal Article
Effects of defoliation and row spacing on intermediate wheatgrass I: Grain production
Increasing intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] grain yield and maintaining yield over the life of a stand will be critical to the economic viability of Kernza (The Land Institute) grain production. Research on perennial grasses has shown that seed yield can be enhanced by (a) mechanically defoliating the stand for hay production and (b) increasing row spacing. We evaluated the interacting effects of row spacing and defoliation across the 4‐yr life of an intermediate wheatgrass (IWG) stand in St. Paul, MN. We measured grain yield, harvest index, lodging, and yield components including grain mass and number of tillers, spikes, and grains. Data was analyzed with linear mixed models and partial least squares path analysis. Overall, grain yield declined substantially over time, from a mean of 880 kg ha−1 in 2015 to 276 kg ha−1 in 2018. Wider row spacings tended to increase grain yield. Defoliation increased grain yield in the first 2 yr, but may have decreased stand vigor in later years. Neither management practice fundamentally mitigated yield decline. The main cause of yield decline was the reduction in grain number per high‐yielding spike, which dropped by roughly half after the first year. The proportion of spikes that were high yielding also declined over time. Increasing competition among reproductive units likely contributed to yield decline, but there is also evidence that resource allocation to reproduction declined over time. Future research in IWG breeding and management should focus on maintaining high grain number, reducing intra‐stand competition, and increasing resource allocation to reproduction.
Journal Article
Agriculture in 2050
The prevailing discourse on the future of agriculture is dominated by an imbalanced narrative that calls for food production to increase dramatically—potentially doubling by 2050—without specifying commensurate environmental goals. We aim to rebalance this narrative by laying out quantitative and compelling midcentury targets for both production and the environment. Our analysis shows that an increase of approximately 25%–70% above current production levels may be sufficient to meet 2050 crop demand. At the same time, nutrient losses and greenhouse gas emissions from agriculture must drop dramatically to restore and maintain ecosystem functioning. Specifying quantitative targets will clarify the scope of the challenges that agriculture must face in the coming decades, focus research and policy on achieving specific outcomes, and ensure that sustainable intensification efforts lead to measurable environmental improvements. We propose new directions for research and policy to help meet both sustainability and production goals.
Journal Article
Effects of defoliation and row spacing on intermediate wheatgrass II: Forage yield and economics
Management systems that produce both grain and biomass coproducts could enhance the profitability of the novel perennial grain crop Kernza intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] (IWG). Harvesting IWG for grain typically results in a straw harvest; in addition, vegetative biomass can be cut in spring, fall, or both for hay production. We evaluated the interacting effects of defoliation and row spacing on yield, forage quality, and economic return across the 3‐yr life of a conventionally managed IWG stand in St. Paul, MN. We measured straw and hay yield and forage quality and then used recent hay auction results to model forage price and total potential value. We then used estimated production costs to calculate potential net return from straw production alone and with additional hay harvests. Overall, straw was more valuable than hay, despite being of much lower quality, since yields were 3–4 times greater. Straw potential value was similar to the cost of producing both straw and grain, greatly reducing the financial risk in Kernza grain production. Hay production was almost always profitable. Straw and hay yield and value were greater in 15‐ and 30‐cm rows than in 61‐cm rows. Defoliating in both spring and fall led to lower hay and straw yields in the third year. Our results indicate that the best strategy for achieving consistent high net return to biomass production is to plant in 15‐ or 30‐cm rows and only cut hay in the fall.
Journal Article
A regionally-adapted implementation of conservation agriculture delivers rapid improvements to soil properties associated with crop yield stability
Climate models predict increasing weather variability, with negative consequences for crop production. Conservation agriculture (CA) may enhance climate resilience by generating certain soil improvements. However, the rate at which these improvements accrue is unclear, and some evidence suggests CA can lower yields relative to conventional systems unless all three CA elements are implemented: reduced tillage, sustained soil cover, and crop rotational diversity. These cost-benefit issues are important considerations for potential adopters of CA. Given that CA can be implemented across a wide variety of regions and cropping systems, more detailed and mechanistic understanding is required on whether and how regionally-adapted CA can improve soil properties while minimizing potential negative crop yield impacts. Across four US states, we assessed short-term impacts of regionally-adapted CA systems on soil properties and explored linkages with maize and soybean yield stability. Structural equation modeling revealed increases in soil organic matter generated by cover cropping increased soil cation exchange capacity, which improved soybean yield stability. Cover cropping also enhanced maize minimum yield potential. Our results demonstrate individual CA elements can deliver rapid improvements in soil properties associated with crop yield stability, suggesting that regionally-adapted CA may play an important role in developing high-yielding, climate-resilient agricultural systems.
Journal Article
Achieving Diverse Cover Crop Mixtures: Effects of Planting Date and Seeding Rate
Cover crop mixtures retain higher diversity when allowed sufficient growth in fall. Cereal rye dominates mixtures in spring, particularly when fall planting is delayed. Grasses overperform in cover crop mixtures compared to their growth in monoculture. Brassicas underperform in cover crop mixtures compared to their growth in monoculture. Legumes’ growth in cover crop mixtures varies depending on species and planting time. Cover crop mixtures may provide greater diversity of benefits than monocultures. To develop management principles to establish diverse cover crop mixtures, we conducted a 3‐yr study in which monocultures and mixtures of six cover crop species (cereal rye [Secale cereale L.], oat [Avena sativa L.], common medium red clover [Trifolium pratense L.], Austrian winter pea [Pisum sativum L.], forage radish [Raphanus sativus L.], and winter canola [Brassica napus L.]) were planted in a wheat (Triticum aestivum L.)–maize (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation after wheat (AW) and after maize (AM). Post‐emergence stand counts and aboveground biomass in fall and spring were measured by species for all cover crop treatments. All species planted manifested in monocultures and mixtures in fall, though oat dominated and red clover, canola, and radish underperformed in mixtures. Cereal rye had the highest spring biomass in all mixtures, especially AM. Pea spring biomass was disproportionally greater in relation to seeding rate in the six‐species mixture (6 Spp.) than in monoculture when planted AW. A four‐species mixture (4 Spp.) planted AW retained the highest diversity after overwintering in two of the three planting years. Our study demonstrated that (i) cover crop mixtures retain higher diversity when allowed sufficient growth in fall; (ii) cereal rye dominates mixtures in spring, particularly when fall planting is delayed; (iii) grasses overperform in mixtures compared to their growth in monocultures; (iv) brassicas underperform in mixtures vs. monocultures; and (v) legume growth in mixtures depends on species and planting time.
Journal Article
Intermediate wheatgrass as a dual use crop for grain and grazing
Introduction: Intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] (IWG) is a novel perennial grain crop with the potential for dual use (DU) in a system that includes the harvest of summer grain and straw as well as the grazing of crop regrowth. This could diversify grower income streams but impacts on productivity and profitability of DU systems need evaluation.Methods: A 4-year on-farm trial was conducted in Minnesota, USA comparing yields and net revenue of a grain+straw production system (GP) vs. a DU system. For both the GP and DU systems, the grain and straw yields from the summer harvest were evaluated, the subsequent IWG regrowth was measured in the fall and again in spring to quantify forage production and nutritive value, and the economic value of grain, straw, and forage were calculated. In the DU system, the herbage intake and forage utilization were also studied.Results and discussion: The GP system produced 42% more grain and 41% more straw than the DU system in year 2 but both systems produced similar grain and straw yields in year 3. The DU system produced greater grain yields than the GP in year 4. Across systems, the forage yield peaked in year 3. Both agronomic systems generally displayed similar forage yields of comparable nutritive value. Crude protein (CP) in fall and spring forage averaged 140 to 150 g kg-1 whereas CP was 30 g kg-1 in the summer straw, comparable to common annual small grains. The relative feed value of IWG forage in the fall was 100 and 127 in spring compared with 80 in the summer. The sale of higher year 2 grain yields in the GP system led to this system earning a net return to the enterprise of $721 ha-1 yr-1 with the DU system producing $609 ha-1 yr-1. In conclusion, grazing IWG can take advantage of on-farm forage resources to generate revenue but waiting to begin grazing until after the second-year grain harvest may reduce the risk of grain and straw yield losses to enhance net returns to the enterprise.
Journal Article
Growing degree days and cover crop type explain weed biomass in winter cover crops
Cover crops are increasingly being adopted to provide multiple ecosystem services, including weed suppression. Understanding what drives weed biomass in cover crops can help growers make the appropriate management decisions to effectively limit weed pressure. In this paper, we use a unique dataset of 1764 measurements from seven cover crop research experiments in Pennsylvania (USA) to predict, for the first time, weed biomass in winter cover crops in the fall and spring. We assessed the following predictors: cover crop biomass in the fall and spring, fall and spring growing degree days between planting and cover crop termination, cover crop type (grass, brassica, legume monocultures, and mixtures), system management (organic, conventional), and tillage before cover crop seeding (no-till, tillage). We used random forests to develop the predictive models and identify the most important variables explaining weed biomass in cover crops. Growing degree days, cover crop type, and cover crop biomass were the most important predictor variables in both the fall (
r
2
= 0.65) and spring (
r
2
= 0.47). In the fall, weed biomass increased as accumulated growing degree days increased, which was mainly related to early planting dates. Fall weed biomass was greater in legume and brassica monocultures compared to grass monocultures and mixtures. Cover crop and weed biomass were positively correlated in the fall, as early planting of cover crops led to high cover crop biomass but also to high weed biomass. In contrast, high spring cover crop biomass suppressed weeds, especially as spring growing degree days increased. Grass and brassica monocultures and mixtures were more weed-suppressive than legumes. This study is the first to be able to predict weed biomass in winter cover crops using a random forest approach. Results show that weed suppression by winter cover crops can be enhanced with optimal cover crop species selection and seeding time.
Journal Article