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11
Sep
2012
What is that stubble really worth?
Author: Trenton Roberts, Research Assistant Professor, Crop, Soil & Environmental Sciences

 

 

While driving through the Delta the past two falls I wasn’t sure if I was in rice country or hay country. There have been more round bales in Eastern Arkansas than you can count. Anyone who has grown rice or corn knows that there is plenty of straw and stubble following harvest and with the recent drought there have been a lot of people looking to purchase hay/straw/stubble- anything that has some feed value.  The statement that needs to be made is “SELLER BEWARE!” The thought of making extra money from selling crop residues (straw and stubble) that makes ground preparation for the following crop a hassle is a hard bargain to pass up. That said, we need to examine the economic and agronomic value of the crop residues that is being sold.

The most often asked question has focused on the phosphorus and potassium value of the crop residues, but crop residues have value beyond just their phosphorus (P) and potassium (K) value that need to be discussed.  First, besides P and K, crop residues contain other plant essential nutrients (including nitrogen (N), sulfur (S) and zinc (Zn)) which are nutrients applied in many fertilization programs.  These nutrients are usually not included in estimating the fertilizer value of the crop residue.  Never-the-less, removal of the crop residues represents a removal of these nutrients that can influence the soils fertility status.  Among the most important of these other nutrients is N and carbon (C) because they are the primary constituents of soil organic matter which is vitally important to soil fertility.  Removal of crop residues will eventually reduce soil organic matter content and the soils ability to supply N and S to future crops, hold plant available water, improve soil structure, and store or retain nutrients like P and K.  These factors are all important to long-term soil fertility, but difficult to place a price tag on.  At a minimum, farmers might want to include the N content of crop residues in calculations that estimate its value.  Rice straw and corn stover usually contain about 1% nitrogen, which adds a significant amount of economic value to crop residues (see example below).  Other considerations that need to be accounted for in estimating the value of crop residue is its value as a soil cover that reduces soil erosion and its value to waterfowl habitat.  Again, these are benefits of crop residue that are difficult to place a dollar value on.

A high yielding rice or corn crop can easily produce 5 to 6 tons of “straw or stubble” (dry weight) per acre, but that doesn’t mean all of that biomass is baled and removed following harvest. Cutting height, moisture, and whether or not you mow (rice) following harvest all play a role in determining the final or baled straw yield.  Grain yields are usually related to aboveground stover yields.  Corn and rice often produce straw yields that are equal to slightly greater than their grain yields.  Straw yields can be estimated by multiplying your grain yields expressed in pounds per acre (56 lb corn and 45 lb rice per bushel) by a factor of 1.0 to 1.4 provides an estimate of straw yield based on common harvest index values that range from 0.4 to 0.5.  Harvest index is simply the harvested grain weight divided by the total aboveground (grain + straw) weight.  For example, a field that produces 200 bu/acre of corn or rice with an assumed harvest index of 0.5 has estimated residue yields of 9,000 lb rice straw (4.5 tons) and 11,200 lb corn stover (5.6 tons) per acre.  The efficiency of collecting and baling the crop residues varies, but for this example let’s assume 70% of the available crop residue is baled.  For the greatest accuracy, the actual final baled yields should be used.

The differences in crop grain and straw yields and the efficiency of collecting these residues that exists from field-to-field makes it difficult to place one value of these crop residues that can be used in all situations.  Thus, perhaps the best approach is to discuss the typical nutrient content of these crop residues.  Average N, P and K contents (per ton of dry matter) of four different crops are listed in the accompanying table.  While these are good averages to use, we must understand that that they are average values.  The actual nutrient and moisture content of these residues can most accurately be determined by having representative samples analyzed at a laboratory.  Samples can be taken before or after baling.  When sampling crop stubble (before harvest) take 15-20 (or more) handfuls of straw randomly per field and chop or cut the residue into 6 inch sections and mix thoroughly. After chopping and mixing, take a 1 gallon plastic bag and fill with a subsample. For situations where crop residues have already been baled, farmers can use the same sample probes that are used to sample round hay bales.   If a lab analysis is obtained, be sure that the form of phosphorus and potassium is expressed as P2O5 and K2O, respectively, which are the same units that fertilizer is sold (e.g., not in the elemental forms of P and K).

 

Crop Stubble

lbs N per ton of stubble

lbs K2O per ton stubble

lbs P2O5 per ton of stubble

Rice

20

24

6

Corn

20

25

6

Soybean

12

9

4

Peanut

34

41

7

 Although average values are provided, there are risks involved with their use to estimate the nutrient content of crop residue.  For growers who have employed aggressive crop fertilization programs aimed at building and maintaining soil fertility levels, crop residues may have greater-than-average nutrient contents. Thus, values based on average nutrient contents may underestimate the fertilizer value of the residues. For soils that already have low phosphorus and potassium fertility levels, the ‘added’ nutrient removal will likely have an immediate impact on next year’s crop yields and recommended fertilizer rates. In contrast, crop residues that remain in the field for several weeks following grain harvest and are exposed to significant rainfall, may contain lower than average K contents because, unlike the majority of the other nutrients, the K is easily leached from the crop residues and does not require that the residue be decomposed for the nutrient to be returned to the field.

Clearly, calculating the answer to the question of ‘What is the fertilizer value of baled crop residue?” is not as simple as it might first appear.  The final information needed is the price of the fertilizer nutrients.  At current market prices, nitrogen (sold as urea) is roughly $0.66 per lb of N, potash is roughly $0.56 per lb K2O and phosphorous is $0.65 per lb P2O5.  Right now barge traffic on the Mississippi is moving at a snail’s pace and sometimes it’s not moving at all. Although we do not know how fertilizer prices will change over the next year, it is possible that fertilizer prices could increase (or decrease) between now and next spring, especially if the water level in the Mississippi River remains low for the next several months!

In its simplest form, the fertilizer value of crop residues should be calculated on a per ton basis and eventually multiplied by the baled yield.  An example calculation is listed below.   In this example we show estimates for the fertilizer value of N, P, and K to highlight that there can be a significant difference in the estimated value depending on what nutrients are included in the calculation.

Example Calculation

(2,000 lbs of rice straw) x (20 lbs N per ton) x ($0.66 per lb N) = $13.20 per ton

(2,000 lbs of rice straw) x (24 lbs K2O per ton) x ($0.56 per lb K2O) = $13.44 per ton

(2,000 lbs of rice straw) x (6 lbs P2O5 per ton) x ($0.65 per lb P2O5) = $3.90 per ton

 

$13.20 (value of N per ton of rice straw)

$13.44 (value of K2O per ton of rice straw)

+ $3.90 (value of P2O5 per ton of rice straw)

$30.54 per ton of rice straw based on N, P, and K

 

Checklist for Estimating the Value of Crop Residues

  1. Determine current prices for the most common N, P, and K fertilizers which in Arkansas include urea, triple superphosphate, and muriate of potash.
  2. Determine the nutrient content of the crop residues by having representative samples analyzed or using average nutrient concentrations that have been published for the crop.
  3. Determine whether the value of your crop residue will include only phosphorus and potassium or include a value for N as well.
  4. Include appropriate fees for any resources (equipment use, fuel, labor, etc…)  that you may provide for the residue baling and removal.  Fertilizer nutrient application costs might also be considered.
  5. Estimate the quantity of straw produced that is available (per acre) for baling and a harvest efficiency for the baling and removal process. The straw yield should be corrected after the crop residues are baled.
  6. Calculate the total value of the crop residue in each field.

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