Date of Award


Degree Name

Doctor of Philosophy


Environmental Resources & Policy

First Advisor

Sadeghpour, Amir


Winter cereal cover crops (WCCCs) could provide extra profit by being harvested as forage or for biofuel purposes, could benefit soil, and the following cash crops, and are considered an effective practice in reducing the nitrate-N (NO3-N) leaching especially in corn (Zea mays L.) and soybean (Glycine max L.) fields. The extend at which WCCCs and their residue management (e.g. harvesting vs. terminating at different times) improve farm profit, influence the following cash crop, especially corn is less studied. Also, literature is scant on the residue management effects on NO3-N leaching potential and its tradeoff with soil nitrous oxide (N2O) emissions especially in Alfisols with claypans. Two trials (chapter 1-2) were conducted to evaluate the time of harvest of winter wheat (Triticum aestivum L.) or winter cereal rye (WCR; Secale cereale L.) to determine the best time of harvest for maximizing profit through improving biomass production at high quality. In chapter 1, a five site-yr trial was conducted in Colorado (CO) and Illinois (IL) to evaluate the effect of harvest date on WCR forage yield, quality, and its economic performance. From March to April, WCR dry matter (DM) yield increased exponentially in CO and linearly in IL. The DM yield at DOY 112-116 in CO was 6.9, 5.0, and 5.2 Mg ha-1 in 2018, 2019, and 2020, respectively compared to 4.7 and 2.7 Mg ha-1 in IL in 2019 and 2020. Delayed harvesting increased acid detergent fiber (ADF) and neutral detergent fiber (NDF) concentrations and decreased crude protein (CP), total digestible nutrients (TDN), and relative feed quality (RFQ). Yield-quality trade-off showed that forage yield increased rapidly but forage quality declined after DOY 105-108. Economic analysis, including cost of nutrient removal and 10% corn yield penalty following WCR production revealed harvesting WCR biomass as forage was economically feasible in four out of five site-yrs at hay price over 132 $ Mg-1. Eliminating corn yield penalty indicated profitability in four site-yrs at hay price of ≥110 $ Mg-1 and removing nutrient removal costs made all site-yrs profitable at hay price of ≥110 $ Mg-1. It was concluded that harvesting WCR biomass can be a profitable and effective strategy for sustainable intensification that can offer environmental stewardship and economic benefit. In chapter 2, a four-year trial was conducted in the 2017-2018, 2018-2029, 2019-2020, and 2020-2021 growing seasons to evaluate the effect of harvesting time (late-March to mid-May considering the growth stage) on winter wheat biomass yield, quality, and farm profit in single season corn vs. wheat-corn rotation. A delay in harvest of wheat resulted in increased DM biomass and lower CP and RFQ. The RFQ that was suitable for dairy production occurred at GDD of 1849 in which the DM biomass was 6.2 Mg ha-1 leading to $1526.46 ha-1 income. The RFQ for heifer production was 126 at 2013 GDD in which the DM biomass was 6.8 Mg ha-1 leading to $1290.85 ha-1 income. These results suggested that wheat-corn rotation could provide extra income while covering the soil year-round. A series of trials were conducted to evaluate the effects of cover crop (CC) and nitrogen (N) management on (i) corn growth, (ii) grain yield and yield components, (iii) the economic optimum N rate (EONR) for corn and farm profit, (iv) N removal, and balances, (v) N use metrics, (vi) soil NO3-N and ammonium-N (NH4-N), along with (vii) N2O emissions and factors associated with it. In chapter 3, an experiment was conducted as a randomized complete block design with split plot arrangement and four replicates to study winter wheat cover crop management practices on corn growth, production, N requirement, soil N, and farm profit. The main plots were four CC treatments: no CC (control), early terminated wheat CC (four weeks to corn planting; ET), late terminated wheat CC (just prior to corn planting; LT), and harvested wheat CC (residue removal; RR), and the subplots were six N fertilizer application rates (0-280 kg N ha-1) for 2018 and 2019 and seven N fertilizer application rates (0-336 kg N ha-1) for 2020 and 2021. Wheat cover crop management influenced corn grain yield where fallow was consistently high yielding while RR decreased corn grain yield drastically due to its negative effects on the corn plant population. All cover crop treatments immobilized N as shown by lower corn grain yields at zero-N control compared to the fallow treatment. The EONR generally ranged from 151.4 kg ha-1 to 206.4 kg ha-1 in fallow, 192.8 kg ha-1 to 275.8 kg ha-1 in ET, 225 kg ha-1 to 325 kg ha-1 in LT, and 175.3 kg ha-1 to 257.5 kg ha-1 in RR. At the EONR, corn grain yields ranged from 12.2 Mg ha-1 to 13.7 Mg ha-1 in the fallow treatment, 9.7 Mg ha-1 to 13.0 Mg ha-1 in the ET, 9.51 Mg ha-1 to 13.3 Mg ha-1 in the LT, and 8.2 Mg ha-1 to 10.5 Mg ha-1 in the RR treatment. Adding N beyond EONR resulted in a drastic increase in end of season soil N which could be subject to leaching emphasizing targeting EONR is critical for avoiding high N leaching and that if N is applied at rates beyond EONR, then cover cropping becomes even a more critical practice to avoid N losses. In chapter 4 and 5, we evaluated whether splitting N fertilization along with the two (no-cover crop vs. early termination; ET) (chapter 4) or four above-mentioned cover crops treatments (chapter 5) could improve corn production and farm profit through improved N use efficiency (NUE). Therefore, for chapter 4, a two-yr field trail was implemented at the Agronomy Research Center in Carbondale, IL in 2018 and 2019 to evaluate whether split N application to corn changes N use efficiency (NUE) in no-cover crop vs. following an early terminated (ET) wheat cover crop. A four-replicated randomized completed block design with split plot arrangements were used. Main treatments were a no cover crop (control) vs. ET and subplots were five N timing applications to succeeding corn: (1) 168 kg N ha-1 at planting; (2) 56 kg N ha-1 at planting + 112 kg N ha-1 at sidedress; (3) 112 kg N ha-1 at planting + 56 kg N ha-1 at sidedress (4) 168 kg N ha-1 at sidedress, and (5) zero kg N ha-1 (control). Corn yield was higher in 2018 than 2019 reflecting more timely precipitation in that year. Grain yield declined by 12.6% following the wheat cover crop compared to no cover crop control indicating corn yield penalty when wheat was planted prior to corn. In 2018, a year with timely and sufficient rainfall, there were no differences among N application timing while in 2019, delaying the N addition improved NUE and corn grain yield due to excessive rainfall early in the season reflecting on N losses. Overall, our findings elucidate necessity of revisiting guidelines for current N management practices in Midwestern United States and incorporating cover crop component into MRTN prediction tool. For chapter 5, a four-year trial conducted with a split plot arrangement and four replicates. Main plots were four cover crop management [no cover crop control (fallow); ET, late termination (LT), and residue removal at late termination (RR) and five N fertilizer application timings (all at planting, most at planting + sidedress; half-half; less at planting and more at sidedress; and all sidedress). Our results indicated that RR resulted in corn population and grain yield reduction compared to other treatments. Fallow was consistently high-yielding and 112-56 N management during the first two years for fallow worked the best (10.1 Mg ha-1). In 2020 and 2021, both applying all N upfront or sidedressing yielded similar for fallow giving growers options with N timing. For both ET and LT, in all years, delaying the N addition to sidedress timing resulted in high yields (9.1 – 11.7 Mg ha-1). Some N addition upfront plus sidedressing the rest (56-168) resulted in the highest yield in ET in 2021 (11.6 Mg ha-1). For RR, split application of N (56-112 or 56-168) was consistently most productive in all years (8.7 Mg ha-1) suggesting that there is an advantage to sidedressing than upfront N application in cover crop systems. The high productive N management practices generally resulted in higher NUE (24.0 – 38.6 kg grain kg N-1) and lower N balance (20.6 – 50.2 kg ha-1 for 2018-2019, and 74 – 106.4 kg ha-1 for 2020-2021) which are critical to achieve not only for farm profit but also minimizing environmental footprints. Except for N0, N balance was positive in all treatments in all years indicating the inefficiency of fertilizer N that was corroborated by low NUE and PFP data. We concluded that to optimize corn production and reducing nutrient loss, split N addition or sidedressing N is most suitable especially in cover cropping systems. For chapter six, a four-times replicated randomized complete block design trial was conducted to evaluate the effects of winter wheat cover crop management practices (ET, LT, and RR) vs. a no-cover crop control (fallow) on corn grain yield, N removal and balances, soil N dynamics, soil volumetric water content (VWC) and temperature dynamics, N2O-N emissions, yield-scaled N2O-N emissions, and factors that drive N2O-N and corn grain yield in 2019-2020 and 2020-2021 growing seasons in a silt loam soil with clay and fragipans. Our results indicated that corn grain yield decreased by both ET and RR as compared to the fallow and LT. Soil temperature was similar among all treatments, but soil VWC was higher in LT and ET than fallow and RR. The LT treatment always had lower soil NO3-N than the other treatments in both years. In 2021, the ET also had less soil nitrate-N than fallow and RR. Averaged over the two years, cumulative soil N2O-N was higher in LT (14.85 kg ha-1) and ET (12.85 kg ha-1) than RR (11.10 kg ha-1) and fallow (7.65 kg ha-1) indicating while these treatments are effective in reducing NO3-N leaching, they could increase soil N2O-N emissions. Principal component analysis indicated that higher N2O-N emissions in LT and ET was related to higher VWC suggesting at optimal N management scenarios, other factors than soil N drive N2O-N emissions. In this study, fallow had the least yield-scaled N2O-N emissions followed by RR. The yield-scaled emissions were similar between ET and LT. These results indicate the importance of evaluating N2O-N emissions in cereal cover crops prior to corn for informing best management practice for winter cereal cover crop adoption. Future studies should focus on manipulating cover crop management to capture residual N without creating microclimates with high VWC to avoid increase of N2O-N emissions. While a lot is known about CC effects on the following cash crop, less is known about rotational benefits of late terminated (planting green) wheat and nitrogen (N) management on the following WCR and soybean in rotation. Therefore, for chapter 7, a trial was conducted with a split plot arrangement in a randomized complete block design set up. The main plots were two cover crop treatments (a no cover crop control vs. LT) and the subplots were three N rates [0 (N0), 224 (N224), and 336 (N336) kg N ha-1). Each treatment was replicated four times and rye and soybean was planted in all of the plots in rotation. Our results indicated wheat, when terminated late, can uptake 50-80 kg N ha-1 and result in belowground:aboveground ratio of 0.18 in which belowground had much higher C:N than the aboveground biomass. The soil NO3-N was affected by wheat presence and often reduced due to wheat N uptake and also N immobilization negatively affecting the following corn especially at both N0 and N224. Nitrogen fertilization at 336 kg N ha-1 resulted in high end of season N, reduced NUE, increased N balance, and thus, potential for N loss especially in the fallow treatment. The end of season N was lower and NUE was higher in LT which was coincided with reduced rye N uptake in LT suggesting wheat effect lingers longer than just during the corn season and could potentially reduce N loss potential during the fallow period following corn harvest. Soybean yields were higher in LT than the fallow which could be due to (i) higher rye biomass in fallow or (ii) positive legacy effect of wheat in rotation. Improved soybean yields could offset some of the economic loss during the corn phase and push growers in the Midwestern USA to be willing to adopt cover cropping to minimize N loss while protecting soil and stay profitable. Our results from chapter 3-7, indicate a need to change in cover crop management strategy to make it more user friendly with lower costs. In general, in the Midwestern USA, growers are reluctant to plant WCR especially prior to corn due to N immobilization and establishment issues. Precision planting of WCR or “Skipping the corn row” (STCR) can minimize some issues associated with WCR ahead of corn while reducing cover crop seed costs. The objective of this study was to compare the effectiveness of “STCR” vs. normal planting of WCR at full seeding rate (NP) on WCR biomass, nutrient uptake, and composition in three site-yrs (ARC2019, ARC2020, BRC2020). Our results indicated no differences in cover crop dry matter (DM) biomass production between the STCR (2.40 Mg ha-1) and NP (2.41 Mg ha-1) supported by similar normalized difference vegetative index (NDVI) and plant height for both treatments. Phosphorus, potassium (K), calcium (Ca), and magnesium (Mg) accumulation in aboveground biomass was only influenced by site-yr and both STCR and NP removed similar amount of P, K, Ca, and Mg indicating STCR could be as effective as NP in accumulating nutrients. Aboveground carbon (C) content (1086.26 kg h-1 average over the two treatments) was similar between the two treatments and only influenced by site-yr differences. Lignin, lignin:N, and C:N ratios were higher in STCR than NP in one out of three site-years (ARC2019) indicating greater chance of N immobilization when WCR was planted later than usual. Implementing STCR saved 8.4 $ ha-1 for growers and could incentivize growers to adopt this practice. Future research should evaluate corn response to STCR compared with NP and assess if soil quality declines by STCR practice over time.

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