What Should You Resi-do? Nutrient Management Considerations When Managing Heavy Crop Residues

Amy Shober, Extension Nutrient Management and Environmental Quality Specialist; ashober@udel.edu and Richard Taylor, Extension Agronomy Specialist; rtaylor@udel.edu

Grain crop yields are on the rise in Delaware due, in part, to advances in crop genetics, expansion of irrigation, and better crop management. Not to mention, that both 2014 and 2015 were extremely good years due to rainfall patterns that aligned well with crop needs. Grain yields of 300 bu/A for corn, 80 bu/A for winter wheat, 50 bu/A for double crop soybean and 80 bu/A for full season beans are no longer out of the ordinary. One result of increased grain yield is an increase in crop residues that must be managed by the grower. The increase in cover crop acreage has also led to an increase in the need for residue management. Over the last few weeks, we have fielded several questions related to residue management and nutrient management, mainly related to nitrogen (N) management. In this article, we will examine some options for residue and nutrient management to address issues related to heavy residue generated by high yielding grain crops or high biomass cover crops.

Nitrogen management when planting into heavy crop residue can be challenging. Most crop residues have a high carbon to nitrogen (C:N) ratio, which means they contain much more carbon than nitrogen. The higher the C:N ratio, the greater the chance that available soil N (or N applied in fertilizers for that matter) will be immobilized by soil microbes as they work to break down these carbon rich crop residues. Literature values place the C:N ratio of corn stover at 57:1 and wheat straw at 80:1. In contrast, leguminous cover crops and cereal rye (in the vegetative growth stage) have an estimated C:N ratio of 17:1 and 25:1, respectively. As a rule of thumb, residues with a C:N ratio >30:1 will promote immobilization of N, which residues and amendments with a C:N ratio <20:1 will actually act as a source of N during decomposition. A C:N ratio between 20-30 usually indicates that equal amounts of N are immobilized and released during decomposition of the material.

Tillage operations also play a role in N dynamics under heavy residue. Residues left on the soil surface as part of a no-till operation may result in immobilization of N during decomposition, but the effects are likely to be limited to the top 2 inches of the soil; soil N present in the topsoil below this surface zone should remain accessible to plants during the growing season.

The potential for immobilization is increased if residues are worked into the soil during tillage operations. Tillage results in more soil to residue contact, thereby increasing the chance that microbes in the top 6-8 inches of the soil will steal soil N from your crops. Removal of residue is an option in some cases that can reduce the potential for N immobilization, but may result in losses of other nutrients (like potassium) and organic matter.

Scenario #1: Planting corn or wheat behind 300 bu/A corn, do you need to apply extra N?
The amount of residue left in the field following harvest of 300 bu/A corn can be estimated based on harvest index (HI). Values for HI for corn range from 50-60%, meaning that grain accounts for 50-60% of the total biomass. However, recent measurements of HI during irrigated corn trials at the UD Warrington Irrigation Research Farm averaged 65%. Using this later value and a test weight for corn of 56 lb/bu, we estimate that 300 bu/A corn yields 16,800 lb/A of grain and 9,700 lb/A of corn stover.

The book value for C:N ratio of corn stover is 57:1, yet we measured a range of 51:1 to 127:1 for C:N ratio of stover at Warrington. Also, estimates of N in stover range from 0.8 to 1.1%. Using a C:N ratio of 57:1 and an N content of 1%, our 9,700 lb of stover contains 97 lb of N, for a total of 5,530 lb of C. To attain a C:N ratio of 25:1, you would need 221 lb N less what is already in the stover (97 lb), for a total of 125 lb/A of additional N. This is quite a bit of extra N to prevent immobilization.

In a no-till system where starter N is placed 2×2, the N applied in starter should provide N to the corn plant and not the bacteria trying to consume the stover C. But if the stover is worked in, then additional N is needed. We suggest applying approximately 50 lb N/A in the starter (but if you also add K, don’t go above the 75 lb/A of N plus K2O on sandy soils). Additional N can be broadcast with pre-emerge herbicide.

Planting winter wheat behind corn? Consider taking a fall soil nitrate test to determine if N rates are adequate to produce the early season biomass of commodity small grains needed to achieve good yields in the following summer. Collect a composite soil sample to a depth of 8 inches following corn harvest and before planting your small grain. If you plan to work in the corn residue, we suggest waiting until closer to or just after planting as some of the available N may be immobilized after you work the ground. The sample can be submitted to a reputable soil testing laboratory for nitrate analysis. If concentrations of soil nitrate <8 ppm (wheat) or <11 ppm (barley), apply 30 lb/A of N in fall to the crop. Alternatively, you can use a quick test kit to determine the amount of nitrate in the soil. Thresholds for the quick test are <10 ppm for wheat and <13 ppm for barley. University of Maryland researchers showed that yield response to fall nitrate was unlikely to result in a yield response when soil test nitrate levels exceed these thresholds. So, if your soil test reveals nitrate concentrations that are higher than these thresholds, you can save money by skipping the fall application.

Scenario #2: Planting corn behind a cover crop
Small grains, like wheat, barley, triticale, and cereal rye are becoming more popular choices for use as a cover crop to prevent erosion and scavenge N left behind from the previous cash crop. Maximum ecosystem services are attained by terminating the cover crop as close to cash crop planting as possible. However, the C:N ratio increases as the plant matures to increase straw strength. As the plant matures, N translocates to the grain and the roots. Termination of cover crops at or past boot stage will result in a higher C:N ratio, which will reduce decomposition rate and potentially immobilize N for subsequent crop. Leaving wheat straw on the surface (no-till) or harvesting wheat straw will reduce the potential for N immobilization. However, if straw is incorporated during tillage operations prior to planting corn, you could face another scenario where additional N is needed to prevent immobilization and yield reductions due to N deficiency.

Book values for C:N ratio of wheat straw fall around 80:1, yet C:N ratios when harvesting at boot stage are expected to be lower. Yet, most of the N in the cover crop biomass is likely to be released only very slowly over the growing season. We recommend terminating the cover crop during the vegetation growth stage if you plan to recover N from the cover crop for the subsequent crop. If you allow your cover crop to remain in the field later so the residue can help with weed control in no-till systems, you will likely need to apply some additional N to avoid early season N deficiency in the corn.

Many growers will terminate cover crops earlier in the spring (mid-March to early-April). Early termination will result in a more favorable C:N ratio in the biomass, which will result in release of N when the residues break down. However, breakdown of crop residues will be slow early in the growing season due to cooler temperature, which results in low microbial activity. Therefore, you are not likely to see the benefits of N from the cover crop early in the season. For corn planted in late April to early May, application of some N at planting is recommended. University of Delaware recommends no more than 25% of total recommended N (1 lb N per bu expected yield) be applied at or just prior to planting. For a corn crop with a realistic yield goal of 250 bu/A, we would recommend no more than 63 lb/A N be applied early in the season. A pre-sidedress nitrate test (PSNT) can be run to adjust sidedress or fertigation N rates to account for N mineralization from the cover crop.

Scenario #3: Planting double crop soybean or lima bean behind winter wheat.
Planting soybean into heavy wheat straw residue (no-till, reduced till, or conventional till) provides less challenge that planting corn after a small grain. Because soybeans are leguminous plants, immobilization of N by wheat straw is less of a concern when following a wheat crop with soybeans. If anything, the lack of available soil N will help to encourage nodulation of soybean roots. Don’t forget to use a good inoculant on your soybean seed.

Lima beans are a different story. Despite the fact that lima beans are a legume, we still can see N deficiency in this crop. Following high yielding wheat with lima bean can present a challenge. With a C:N ratio of 80:1, planting limas into heavy wheat/barley residue may require application of additional N. Again, harvesting wheat straw can reduce the potential for N immobilization and make it easier to plant the subsequent crop. However, harvesting wheat straw removes a significant amount of K. We estimate that an additional 20 lb K2O is removed from the soil per ton of wheat straw baled and removed. This K should be replaced with inorganic fertilizers.

Planting into heavy residue presents a nutrient management challenge, yet the presence of residue on the surface helps reduce the risk of erosion and suppress weeds. Growers should consider the C:N ratio of residues, residue management, tillage, and crop N requirements when managing crops grown into heavy residue. Applications of additional N may be warranted to prevent N deficiency to crops that could affect end-of-season yields.

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