Establishing Nitrogen Credits Following a Sweet Corn Crop on Non-Irrigated Soils

Introduction

In 2020, Minnesota ranked second in the US for processing sweet corn, with about 100,000 acres planted. Most of the crop is grown under rainfed conditions on fine-textured soils in southern Minnesota, often in rotation with field corn and soybean. Relative to field corn, sweet corn has a relatively short growing season because the crop is harvested at an immature stage of growth and then processed for canned or frozen produce. Because sweet corn is harvested as an immature crop (Figure 1), significant nutrients, and in particular nitrogen, remain in the residue. In general, sweet corn stover can contain as much as 60-80 lbs of N/A with a C to N ratio of 30:1 to 35:1. With these relatively low C:N ratios and a harvest time many weeks to months before freeze-up, there is a high potential for nitrogen mineralization during the fall or following spring, and sweet corn may therefore provide a nitrogen credit to a subsequent crop. Currently, no N credit is given for a previous crop of sweet corn. One of the risks involved with a nitrogen credit from sweet corn residue is that there is also a potential for leaching losses over the fall and early spring. To circumvent this problem, a properly planted cover crop can help to minimize leaching, as well as providing additional benefits such as reduced erosion and, in some cases, disease suppression.

To evaluate the potential for a nitrogen credit from a previous sweet corn crop, a three-year study funded by AFREC was conducted at the Southern Research and Outreach Center in Waseca. The main objectives were to 1) determine the effects of a previous sweet corn crop relative to soybeans and field corn on nitrogen response by a subsequent field corn crop and 2) determine the effect of a winter rye cover crop following sweet corn on field corn nitrogen response and nitrogen carryover.

Methods

The first step in this study was to establish previous crops of sweet corn, soybean, and field corn. The previous crops (field corn, soybean, sweet corn, and sweet corn + rye) were established in large plots in 2017, 2018 and 2019. Each large plot was replicated four times and a different field was used each year that had followed a large bulk planting of field corn. Field corn N response following these previous crops was evaluated in 2018, 2019 and 2020. The nitrogen rates applied within each large plot ranged from 0 to 250 lbs N/A in 50 lb N/A increments. The nitrogen source was urea and was broadcast applied and incorporated just before planting the field corn in the spring.

The rye cover crop following sweet corn was planted with a no-till grain drill (Figure 2) at the end of August or early September following sweet corn harvest in mid-August, with a seeding rate of 90 lb/A. Rye growth in the fall and spring is shown in Figures 3 and 4. The rye was killed in the first or second week of May and then disked in 3-8 days later. Biomass and C/N ratio of the rye cover prior to killing was recorded (Table 1.) Note that in the third year the C/N ratio of the rye was higher than the previous two years and may have had an impact on corn nitrogen response as discussed below. Field corn was planted in mid-May of each year.

Results

Based on our results for Field 1, which was planted in rotation crops in 2017 and in field corn fertilized at a range of N rates in 2018:

Residual Soil Nitrate and Carryover
Averaged over the three years, the rye cover crop reduced residual nitrate in the top two feet of soil in both the fall and the following spring compared to no rye cover (Table 2). The highest residual nitrate in the fall just before the soil freeze up was with sweet corn as the previous crop (with no rye cover crop). In the spring just before planting, the highest residual nitrate was with soybean as the previous crop.

Previous crop effects on economically optimum nitrogen rate (EONR)
The effect of previous crops on the EONR is presented in Table 3. In all years, the EONR was lower for field corn following soybean and sweet corn than continuous field corn, and in two out of three years, the ENOR was lower following sweet corn with rye than continuous field corn. In the third year, the EONR was slightly higher following sweet corn and rye than continuous field corn. The reason for the higher EONR in the third year with the rye cover crop is likely due to a higher C/N ratio of the rye residue. The cover crop was not killed until May 15 that year, suggesting that letting the rye crop grow too long in the spring will reduce the potential for a nitrogen credit from a previous sweet corn crop. Overall, the nitrogen credit for sweet corn is slightly less than the nitrogen credit provided by a previous crop of soybean.

Yield at the EONR was also affected by the previous crop (Table 4). In all cases, yield with field corn as the previous crop resulted in lower yield at EONR than soybean, sweet corn, and sweet corn + rye. Overall nitrogen response by field corn as affected by previous crop is summarized in Figure 6. The results clearly show that, at all N rates, field corn yields were higher following sweet corn and soybean then following field corn. Adding more nitrogen helped increase yield in continuous field corn, but there was also a rotation effect that could not be overcome by adding more nitrogen fertilizer.

conclusions

Based on this three-year study, we can conclude that field corn following field corn had a much higher nitrogen requirement than field corn following soybean and field corn following sweet corn with or without rye as a cover crop. The nitrogen credit for sweet corn is about 15 lbs less than that for soybean. Currently the soybean nitrogen credit is about 35-40 lb N/A. Therefore, a safe nitrogen credit for sweet corn would be about 20-25 lb N/A. If rye is used as a cover crop following sweet corn, the rye should be terminated by before May 10 to ensure that a nitrogen credit can be taken. If rye is not terminated by May 10, then a nitrogen credit should not be taken for a previous sweet corn crop. Use of rye following sweet corn resulted in lower residual nitrate in the soil in the fall and the following spring and therefore would reduce the potential for nitrate leaching during the months after sweet corn harvest and prior to planting of field corn.