CURTIS B. ADAMS, CHRISTOPHER W. ROGERS, JULIET M. MARSHALL, PATRICK HATZENBUEHLER, OLGA S. WALSH, GARRETT THURGOOD, BISWANATH DARI, AND GRANT LOOMIS (USDA-ARS and UNIVERSITY OF IDAHO)
While grain is the primary product of wheat production, straw and other residues can be used as a secondary by-product. Following grain harvest, wheat growers can either retain or remove residue from their fields. In the Pacific Northwest and elsewhere, wheat residue is frequently harvested and sold to increase farm income. This option can be especially attractive when independent contractors provide full services-residue cutting, bailing, removal, and payment-with essentially nothing required of growers. But there are benefits to keeping residue in the system, including soil erosion reduction, maintenance of soil organic matter, and preservation of soil moisture. An additional benefit is preservation and recycling of mineral nutrients contained in residue, but there is currently little information available on the amount or economic value of residual nutrients. Therefore, we conducted research to quantify nutrient (N, P, K, Ca, Mg, S, Fe, Zn, Mn, Cu) content of wheat residue across a wide range of production environments and wheat varieties (representing five market classes), and we estimated the economic value of residual nutrients in terms of fertilizer replacement costs. Table 1 gives average N, P, K, and S content (total uptake, given in lbs/ac) of wheat residue at four yield levels (40, 80, 120, and 160 bu/ac). These data show that wheat residue has high levels of K, but also contains substantial amounts of N and other nutrients
(micronutrients were not included here). If residue were to be harvested containing these nutrients, 1able 1 also gives historical (2001 to 2019) low, average, and high fertilizer prices (U.S. $/lb) required to replace them. Fertilizer prices were the greatest overall for N, but costs for K fertilizer notably had the widest range, with the historic high price for K being near the historic high for N (87% of N).
Table 1. Average wheat residue nutrient content values, corresponding to a range of grain yields (top). Also, historic low, average, and high (from 2001 to 2019) market fertilizer prices for individual nutrients (bottom), should fertilizer need to be applied to replace nutrients removed through residue harvest.
| Residue Nutrient Content | ||||
|---|---|---|---|---|
| Grain Yield (bu/ac) | N (lb/ac) | P (lb/ac) | K (lb/ac) | S (lb/ac) |
| 40 | 15.5 | 0.94 | 81.6 | 4.91 |
| 80 | 26.8 | 1.63 | 141 | 8.50 |
| 120 | 38.1 | 2.31 | 201 | 12.1 |
| 160 | 49.4 | 3.00 | 260 | 15.7 |
| Fertilizer Nutrient Prices | ||||
| Cost Category | N ($/lb) | P ($/lb) | K ($/lb) | S ($/lb) |
| Low | 0.30 | 0.08 | 0.12 | 0.12 |
| Average | 0.45 | 0.16 | 0.29 | 0.18 |
| High | 0.68 | 0.25 | 0.59 | 0.25 |
Table 2 gives estimated residue macronutrient (N, P, K, S) replacement costs (U.S. $/ac)-the product of residue nutrient content and fertilizer prices. These estimates show that the monetary value of macronutrients contained in residue alone is substantial. For example, a dryland wheat crop that produced 80 bu/ac would contain an average of about $54/ac in macronutrient value in the residue, with about 74% of this value coming from K and 22% from N. Under a scenario of high fertilizer costs and higher yields (120 to 160 bu/ac), wheat residue could contain between approximately $148 and $192/ac in macronutrients. It is important to note that these estimates do not include micronutrients, which can have high costs, nor do they include the application cost of fertilizers to replace nutrients harvested in residue. In making residue management decisions, wheat producers should consider the tradeoff between immediate revenue gains from residue sale and the multiple benefits of residue retention, including savings on future nutrient costs. Persistent removal of nutrients from agronomic systems through residue harvest affects soil nutrient availability in the short- and long-term, and the timeframe and magnitude of these changes will depend on the cropping system and soil.
Table 2. Estimates of fertilizer replacement cost for select macro nutrients (N, P, K, S) contained in wheat residue, if residue is harvested and removed from the field. Estimates are given for a range of grain yields. The primary values were calculated using average residue nutrient contents and average historic fertilizer prices. The ranges below (in parentheses) were calculated using historic low and high fertilizer prices.
| Residue Nutrient Replacement Costs | |||||
|---|---|---|---|---|---|
| Grain Yield (bu/ac) | N ($/ac) | P ($/ac) | K ($/ac) | S ($/ac) | Total ($/ac) |
| 40 | 7.01 | 0.15 | 23.31 | 0.89 | 31.37 |
| (4.63, 10.52) | (0.08, 0.23) | (9.99, 48.09) | (0.58, 1.23) | (15.27, 60.08) | |
| 80 | 12.14 | 0.27 | 40.34 | 1.54 | 54.29 |
| (8.01, 18.21) | (0.13, 041) | (17.29, 83.25) | (1.00, 2.12) | (26.44, 103.98) | |
| 120 | 17.27 | 0.38 | 57.38 | 2.19 | 77.22 |
| (11.40, 25.90) | (0.19, 0.58) | (24.59, 118.40) | (143, 3.02) | (37.60, 147.89) | |
| 160 | 22.39 | 0.49 | 74.41 | 2.85 | 100.14 |
| (14.78,33.59) | (0.25, 0.75) | (31.89, 153.55) | (1.85, 3.91) | (48.76, 191.80) |
Taken from the 2024 WSU Lind Field Day Abstracts.