Annual vs. Perennial Crops in Commercial Agriculture

Modern agriculture is based heavily in annual monoculture farms. In these farms, each operation is specialized to only grow large amounts of one crop. These annual crops will only grow for one year and must be torn up and replaced by new ones after each harvest. 

Annual plants grow shallower root systems that don't last through the winter. This causes the soil to have lower nutrient capture and increases soil erosion. As a result, crops planted require more external chemicals to be added for them to produce sufficient yields. Due to the lack of depth in their root systems, these crops are also not able to capture all the nitrate from added fertilizer. The excess from nitrogen fertilizer that isn’t collected by the crops’ roots goes unused and dissolves into groundwater in a process called nitrate leaching. This fertilizer nitrate is a groundwater pollutant.

Nitrate (NO3) is the chemical that comes from decaying organic matter, legume plants, sewage, nitrate fertilizers, and nitrates in soil (NGWA). It is a chemical necessary in soil for plants to grow. However, when found in groundwater, concentrations over 3 ppm indicate water contamination, and over 10ppm is a significant health risk according tho the Environmental Protection Agency (EPA).

Nitrate leaching is the process by which nitrogen dissolves into soil. When there is an overabundance of nutrients such as nitrate in the soil, the nutrients will drain past the root level and dissolve into groundwater, becoming a pollutant.

Unlike annual plants, perennial plants develop deeper root systems that last through the winter. The plant is able to grow for multiple years and doesn't need to be torn up and replaced each year. Perennial plants increase the soil’s nutrient quality and reduce the nitrate leaching by capturing and using more nutrients with their larger root systems.

Intermediate Wheatgrass (IWG) is a perennial wheat-related grass which provides the edible grain known as Kernza® (Thinopyrum intermedium), trademarked by The Land Institute. Intermediate Wheatgrass is being cultivated by The Land Institute and at the University of Minnesota to be an alternative to annual grains. This perennial grain is in the process of being cultivated as a perennial alternative to common annual crops.

To analyze perennial agriculture’s ability to reduce groundwater nitrate levels and create a sufficient crop yield, I am analyzing data from research by assistant professor of agronomy and plant genetics at the University of Minnesota, Jacob Jungers. This research compares IWG with maize, an annual grain crop commonly grown in the Upper Midwest, and switchgrass, a perennial biomass crop.

First I directly compare the overall amount of seed (grain) yield, biomass yield, and nitrate leached from each plant over the three years of the study. I then run linear regressions on the data to determine the effect each crop and each nitrogen fertilizer input has on seed yield, biomass yield and the amount of nitrate leached in each plot. To do analyze and visualize the data I use the 'dplyr', 'ggplot2', 'car', and 'stats' packages.

Then I display a map showing groundwater nitrate levels across the Upper Midwest (Iowa, Minnesota, Wisconsin) and compare this to a map of agriculture land cover using the 'raster' package.

Question: How well does IWG perform in seed yield, biomass yield, and nitrate leached compared to annual grain crop, maize, and perennial biomass crop, switchgrass, based on the input of nitrogen fertilizer?

As expected, maize significantly outperforms IWG in seed yield. It is also notable that there is an extremely significant negative effect of year for each crop (p values < 4.71e-6) meaning a negative effect of time on overall grain yield. The interaction of year and maize is significantly negative (p = 0.0009) meaning seed yield is decreasing over time, but there was no significant interaction of year and any other crop. Based on Anova test, the interaction of nitrogen treatment and year is significant, however this interaction is only significant in the first year, and has no significance in 2014 or 2015.

Switchgrass produces the largest biomass yield, however IWG also produced a significantly positive yield, and maize had a significantly negative effect on its biomass yield (p values < 6.0e-6). There is a significant positive interaction of nitrogen fertilizer treatment and IWG (p values < 0.0008), however no significant interactions of treatment and either of the other crops.

Nitrogen treatment had an extremely significant effect on nitrate leached. In each treatment and over time, IWG leaches the least nitrate on average, even though it has a significantly positive effect on nitrate leached (p = 2.03e-9). Maize has a slightly significant positive effect (p = 0.0125), and switchgrass has no effect. Year had a significant negative effect on nitrate leaching over time with a positive effect in the first year (p <2e-16) and a slightly significant negative effect in the last year (p = 0.00143). In the 0 fertilizer treatments there is a significant negative effect of time, while the other treatments show a significantly positive effect in the second year, but no significant effect in the third year.

“Potential reductions in NO3-N leaching to groundwater by expansion of a perennial grain crop like IWG are only possible if the crop is economically viable to grow. In 2013, IWG grain yields in the low N fertilizer treatment were 26% of average Minnesota spring wheat yields (NASS, 2018)" -Jungers et al. 2019

Based on the data comparing IWG to maize and switchgrass, at all levels of fertilizer input, maize has a significantly higher grain yield, which is good for agriculture, but also a higher level of nitrate leaching into the soil. IWG on the other hand is able to create a stronger root system allowing for lower nitrate leaching, but is not able to produce an economically viable grain yield.

It is clear to see why further research into increasing perennial grain yield is necessary by looking at where significant change can be made and the potential for its effect on agriculture and and groundwater:

USDA land cover data across the Upper Midwest United States, which is roughly 52% (70.3 million acres) farm land across Iowa, Minnesota, and Wisconsin, shows an overwhelming amount of corn crops compared to any other annual or perennial crop. Maize and other corns' presence in agriculture across these states align with areas annually receiving between 20-100 kg/ha of nitrogen fertilizer annually (USGS). IWG's long-lasting root system's ability to capture more nitrate than annual maize, has the ability to reduce nitrate leaching in these areas using the most nitrogen across the Upper Midwest. By reducing the amount of nitrate leaching from their operations, these farms would lower the amount of nitrate dissolving into groundwater sources that are used for drinking.

The below maps model the groundwater nitrate concentration of the Upper Midwest in mg/L (equivalent to parts per million) based on "14 inputs of nitrate sources, transportation and attenuation factors" including the previous measurement of groundwater nitrate from farm fertilizer modeled by the US Geological Survey Ground Water Vulnerability Assessment (USGS, 2006). Concentrations over 1 mg/L nitrate indicate human activity (Dubrovsky et al. 2010), and the 10 mg/L threshold is the EPA's maximum contaminate level (MCL).

The 5m depth chart models nitrate levels within the depth range of crop roots while at 50m depth the nutrients have leached below recovery by crops and are present in groundwater supplies used for drinking (USGS). While these nitrate levels are not above an extremely unsafe level, with the previous data showed a decrease in seed yield over time. Because of this, I expect that agriculture is also seeing a steady increase of fertilizer use over time to maintain the same yields. Nitrate levels are already above half the EPA (MCL) across agricultural land and its presence in drinking water sources will only increase as the same agriculture practices continue.

"Compared to annual crops, research is showing that perennial crops like Kernza leach less nitrogen to groundwater, sequester more carbon, and prevent more erosion. Kernza has long roots that can access nitrate deeper in the soil, and its longer growing season helps keep soil and nutrients in place. Experiments have shown very low nitrate leaching below its rooting zone, meaning that Kernza takes up fertilizer before it reaches groundwater." - Green Lands Blue Water

Through agricultural use of perennial biomass and grain crops, it is possible to decrease nitrate presence in groundwater through their more efficient use of soil nutrients and lower levels of nitrate leaching. Perennial grains like Kernza® also live longer and are able to trap higher amounts of carbon dioxide and retain larger amounts of water than annual crops (Green Lands Blue Waters). Based on these results, research into cultivating an increased grain yield in perennial crops must be done to allow farms to take advantage of their environmental benefits, while still generating profitable yields.

I would like to thank Jacob Jungers for providing data from his research for me to apply to this project, and Erin Meier of Green Lands Blue Waters for sharing information and connecting me with resources to help complete my research.

I would also like to thank Professor Brown and TA Leslie for teaching me in class and helping me whenever things went wrong along the way.