Prepared by Justin Meissen1, Tallgrass Prairie Center, University of Northern Iowa
Published online 15 December 2020
Integrating reconstructed tallgrass prairie stands within and around crop fields holds great promise in reducing nutrient loss and increasing other ecological benefits. Past work shows that prairie strips reduce up to 90% of N and P surface runoff, reduce nitrates in shallow groundwater (Zhou et al. 2014), and fuel the denitrification process (Iqbal et al. 2014). Tallgrass prairie in and around farm fields can also enhance other ecological benefits like soil quality improvement, wildlife and pollinator habitat, and flood resilience (Schulte et al. 2017; Kordbacheh et al. 2020).
As the ecological benefits of tallgrass prairie become better understood, there are more initiatives to reconstruct prairie or prairie-like vegetation on agricultural landscapes. In addition to the broad adoption of USDA’s Pollinator Habitat Initiative (CP-42) which established over 80,000 ha of native vegetation in Iowa from 2014-2018, the new Farm Bill establishes prairie strips as a Conservation Practice (CP-43) for the Conservation Reserve Program (CRP). This practice is flexible and easy to integrate into conventionally farmed fields (Farm Service Agency 2019). Given its relative adaptability and provision of multiple ecosystem benefits, the Prairie Strips practice has great potential to be widely adopted and highly impactful in agricultural landscapes.
Given the potential for many new adopters, it is important to ensure farmers succeed with their first experience planting tallgrass prairie. However, farmer success with this practice can be uneven due to a lack of training in planting and establishing prairie vegetation, and because current recommendations are often not based on limited applicable scientific research. The comparatively high initial cost of planting tallgrass prairie further increases the need for planting guidelines that help ensure predictably successful outcomes. In order to inform such guidelines, we need a better understanding of the methods that can achieve success in a cost-effective way.
In current practice, prairies are planted using an array of methods that focus on efficient seed sowing. Many practitioners prefer using seed drills, which ensure seed to soil contact and are familiar in conventional farm operations. Specialized native seed drill rentals are often available from county or regional wildlife organizations, making them relatively easy to access as a seeding method. Other practitioners prefer to use broadcast seeders which scatter seeds on the surface, and are often used in natural areas restoration using bulk harvested seed. Past research comparing these methods has shown broadcast seeding to produce better outcomes for some broad groups of species (forbs) and drill seeding to produce better outcomes for others (C4 grasses) (Larson et al. 2017). However, it is still unknown which method results in more cost-effective stands and why different species may establish better using different seeding methods.
Seed traits may help determine how species establish under varying planting conditions. Specifically, small seed size may increase a species’ reliance on light for optimal germination and establishment. One study showed that small-seeded species germinated better with increasing levels of light (Jankowska-Blaszczuk & Daws 2007). Another found that small-seeded species only established well in the highest light conditions (bare soil compared to mowed or grazed areas) (Kahmen & Poschlod 2008). In the case of drill seeding (typically ~ 6 mm depth), small seeds may not receive the necessary light to germinate well. Light penetrates only 4 to 10 mm into the soil, and at the low end of that range when soils are dark and of small particle size (e.g., clay or silt loams) (Baskin & Baskin 2014). If seed size influences a species’ ability to establish at different depths, practitioners may be able to increase cost effectiveness by calibrating or modifying planting equipment to seed small species separately and on the soil surface.
We assessed the effect of seeding depth used in planter equipment in a series of field trials conducted in recently retired farmland. Our objectives were to 1) evaluate plant establishment and cost-effectiveness for prairie plantings that were either surface- or drill- seeded, and 2) determine whether seed size provides a mechanism to explain differences in seeding method performance.
The study site is located at the Prairie on Farms Research and Demonstration Site in Cedar Falls, IA (42° 51´ N, 92° 48´ W) in Black Hawk County (Fig. 1). The soil underlying the study site is primarily poorly drained Clyde clay loam (NRCS-Natural Resources Conservation Service 2016). Topographically, the study site is located on a low rolling hill, but slopes do not exceed 5% grade. Land use prior to this experiment was agricultural, with corn and soybeans consistently grown in rotation at the site.
We prepared the study site using tillage after crop production. In the summer of 2018, the farm operator grew corn throughout the site. The farm operator used a combine without a chopping header to harvest in the fall of 2018, leaving heavy residue throughout the site. To create a suitable seedbed before planting in the spring of 2019, we used four passes of disc cultivation, followed by one pass with a harrow in fall 2018. The prepared seedbed was firm, with clods less than 6.4 mm in diameter.
To assess cost-effectiveness and ecological performance of different seeding methods, we installed a pilot experiment with a completely randomized design consisting of four replicates in May 2019 (Fig. 2). We established a 35 ✕ 90 m study area consisting of eight 8 ✕ 30 m plots and a small informal demonstration area south of the plots. We randomly assigned a seeding method, surface seeding or drill seeding, to each plot (n = 8). We manipulated seeding methods at two levels: 1) surface-seeded and 2) drill-seeded.