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Can variable rate irrigation optimize nitrogen management and improve water quality?

Study title: Effect of variable rate irrigation and nitrogen fertilizer on crop productivity and water quality
Lead researcher: Dr. Vasudha Sharma
Years of study: 2020, 2021, and 2022
Location(s): Sand Plain Research Farm, Becker, MN and Rosholt Farm, Westport, MN

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1. What we did

Our overarching goal is to build capacity to provide government agencies, stakeholders and producers with scientific research-based irrigation and N management data, and information on fundamental relationships that help develop policy/rules, make better on-farm management decisions, and help advancing the sensor-based (proximal and remote sensing) irrigation and N management research. Since corn is one of the principle crops irrigated in Minnesota, our research is focused on continuous-corn cropping systems under sprinkler irrigation. To do so, field plot experiments are being conducted at the Sand Plain Research Farm (SPRF) in Becker, MN (45º 20’ N, 93º 51’ W) and Herman Rosholt farm (45° 42′ 49.07′′ N; 95° 10′ 29.39′′ W), in Westport, Minnesota (both sites are in vulnerable groundwater areas). Four (4) irrigation treatments and six (6) N rate treatments are being evaluated, and replicated four (4) times. The irrigation treatments are: full irrigation (FI), i.e., imposing no water stress on the crop, 75% of FI, 50% of FI and rainfed conditions. The N application rates are 0, 70, 140, 210, 280 and 350 lb/ac. Irrigation is applied using a GPS guided variable rate linear move irrigation system at both locations. Urea-N fertilizer is being applied to the plots in two splits, with 30% at V2, and 70% at V9 growth stages. The experimental design is a split-plot design with irrigation treatments as the main plot and N-rates as the sub-plot.

2. Why do this study

In Minnesota, the interest in deficit/limited and variable rate irrigation management, to address/reduce water quality and quantity problems, has been increasing amongst farmers, agricultural professionals and key stakeholders. However, it’s challenging to understand how these variables – how much a reduction in irrigation rate as compared to full irrigation, and what amount of nitrogen (N) fertilizer is optimum under that reduced irrigation rate – will affect  sustainable crop production. Consequently, when farmers use deficit irrigation amounts in combination with recommended N rates that are developed under well-watered conditions, plants cannot utilize all the N applied and thus the remaining can be lost in the environment.

Several researchers have investigated the combined effect of irrigation and N on crop production, nitrate leaching and water use efficiency, and found that N and water are codependent management factors that cannot be evaluated independently (Al-Kaisi and Yin, 2003; Pandey et al., 2000a; Pang et al. 1997). In these studies researchers found that crop water productivity (yield/water use) vary with varying rates of N and irrigation, and that under deficit irrigation N must be correspondingly adjusted to optimize economic crop production. However, most of this knowledge is based on research conducted in more arid regions where precipitation and the water balance of cropping systems are substantially different from those in Minnesota. To the best of our knowledge, very limited research has been done to investigate the N and irrigation interaction effect on nitrate leaching in corn cropping systems in Minnesota. One such researcher is Maharjan et al. (2014) who compared the effects of different N treatments for fully irrigated and minimum-irrigated corn in Becker, MN. They found greater yield-based nitrate leaching and lower grain yields in minimum-irrigated plots than fully irrigated plots. However, in their study, they used only one N rate (180 kg N/ha) under two irrigation levels with different types and timing of N fertilization.

Therefore, major objectives of this study are to: 1) evaluate variable irrigation and N rate interaction effects on corn yield, nitrate-N leaching, crop evapotranspiration, crop water productivity, and water- and N-use efficiency; 2) develop corn evapotranspiration crop coefficient (Kc) curves for efficient irrigation management, and, 3) develop proximal and UAV remote sensing-based non-destructive in-season corn water and N status diagnosis methods and in-season variable rate N and irrigation management strategies.

3. what we learned

In 2020, we did not find any significant effect of irrigation on grain yield, however the effect of nitrogen on grain yield was significant. There was no interaction effect of irrigation and nitrogen on grain yield. At all irrigation levels, no difference in grain yield was observed between 280 lb N/ac and 350 lb N/ac. A quadratic relationship was observed between grain yield and N application amounts for all irrigation levels at both sites. At Becker, there was no significant grain yield difference between 50%, 75% and 100% irrigation at any level of N application which suggests that limited irrigation can be effectively used to conserve water with minimal to no effect on grain yield in years like 2020 when precipitation is above or close to long term average. However, a significant grain yield difference was observed between rainfed treatment and all other irrigated treatments, but only at 210 lb N/ac. Similar results were obtained for the Westport site, except there was no difference between irrigated and rainfed treatment. This could be due to the fact that there was sufficient rainfall at the Westport site; that even rainfed treatment had enough moisture to avoid any crop water stress that could impact yield.

In 2021, at all levels of irrigation except rainfed treatment, grain yield significantly increased with an increase in nitrogen application up to 140 lb N/ac and then plateaued at higher N rates. At both sites, grain yield was not significantly different between 210, 280 and 350 lb N/ac treatments at all levels of irrigation. Comparing irrigation treatments, a significant difference existed in grain yield between irrigation treatments at all levels of nitrogen except 0 and 70 lb N/ac. At Rosholt farm, irrigation and nitrogen interaction effect was significant, however, at all N rates, there was no significant difference between 100%, 75% and 50% grain yield. This indicates that even in the drought year like 2021, reducing the irrigation amount up to 50% could have a potential in reducing water applications for irrigation without significantly impacting the grain yield. Since only one of the study sites indicated this trend, it can be suggested that soil type and climate has an effect on the yield response to irrigation. At Westport site, the total seasonal rainfall was higher than SPRF which might have resulted in mild water stress that did not impact grain yield even at lower irrigation rates.

4. what’s next

This is ongoing research. Based on 2 years of study results, we found that differences in treatment grain yields between 2020 and 2021 as well as between sites support the importance of assessing irrigation management decisions in regards to current climate conditions and soil types. Overall, we found that in wet years irrigation can be reduced to up to 50% without any significant reduction in grain yield. Our next step is to conclude this study in 2022 and work on developing corn crop coefficients under different irrigation and nitrogen management strategies that can be used for growers for irrigation management and other researchers for further studies.

5. Additional resources

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