Estimating crop residue cover
For producers, it is very important to know how to measure crop residues as this provides an estimate of how well soil is protected from wind and water erosion. How much residue is enough? To meet the definition of conservation tillage (including no-till, strip-till, ridge-till, and mulch-till), at least 30 percent of the soil surface must be covered with residue after planting.
There are three main methods used to estimate residue, but the most reliable is the line-transect approach. For this method, use either a 100-foot tape measure or a rope with 100 knots tied at 1-foot intervals. Stretch the tape or rope at a 45-degree angle to the row direction, walk along the tape, and count the number of times a piece of residue at least 1/8” in diameter occurs under each foot mark or knot (Photograph 1). Calculate the percentage coverage by dividing the number of times that residue occurred by the total number of observations (100). You could also use a 50-foot rope with knots at 6-inch intervals, or a 50-foot tape and measure every 6 inches. The important thing is to make 100 observations at each site, and repeat this process at five sites per field in order to determine the field average.
The meter stick method is another approach for measuring residue levels in the field. Throw a meter stick onto a random area of the field, and measure the amount of residue under one side of the meter stick. If 45 cm of the meter stick has residue beneath it, the residue level is 45% for that observation method. It is best to avoid end-rows in any of the methods for residue estimation.
The third method is to compare your fields to photos that contain a known percentage of crop residue. The following K-State Research and Extension publications can be used for comparison purposes:
Corn residue: http://www.ksre.ksu.edu/bookstore/pubs/L784.pdf
Grain sorghum residue: http://www.ksre.ksu.edu/bookstore/pubs/L782.pdf
Soybean and sunflower residue: http://www.ksre.ksu.edu/bookstore/pubs/L783.pdf
Wheat residue: http://www.ksre.ksu.edu/bookstore/pubs/L781.pdf
If you estimate residue in the fall, how much will be left in the spring? One estimate is that approximately 10 percent of the residue will blow away or decompose over winter, but residue decomposition varies depending on temperature and moisture. Residue decomposes more quickly under warm, moist conditions. If you graze crop residues, some studies suggest that cattle will remove approximately 25 percent of the material, but that also depends on the location of water, mineral tubs, etc., and how long the cattle remain on a particular field.
For fields that will be planted to row crops next spring, the critical period for wind erosion in Kansas is November through spring until new vegetation is established.
Figure 1. Tape measure used for the line-transect method. Place tape measure at a 45-degree angle to the rows.
DeAnn Presley, Soil Management Specialist