Estimating Residue Cover

Crop residue management through conservation tillage is one of the best and most efficient methods farmers have to control soil erosion. Each year about 140 million tons of topsoil are eroded from Nebraska farmlands. Recent research shows that farmers can greatly reduce this loss by maintaining a residue cover of at least 20 to 30 percent after all tillage and planting operations. Leaving this amount of residue can reduce water caused erosion by 50 percent of what it would be from a cleanly tilled field, while leaving more residue will reduce soil losses even further. Conservation tillage and residue management can save soil, labor, fuel, and money- all of which are important to both individual producers and the farm economy. Conservation tillage is best defined by the amount of residue cover remaining after planting, rather than by the tillage implement or operations used. For example, two disking operations in dryland corn residue will usually leave about a 20 percent residue cover and could be classified as a conservation tillage system. However, just one disking in soybean residue cannot be classified as conservation tillage because too much of the fragile residue is destroyed. In addition to being the least expensive method of erosion control, conservation tillage is also the most efficient means because evenly distributed residue limits soil loss over the entire field. Rainfall, while essential for crop growth, dislodges soil particles upon impact and allows them to be washed away. Uniformly distributed crop residue shields the soil surface from raindrop impact, reducing soil particle detachment and eventual erosion. Residue also creates small dams which slow the rate of runoff, allowing more time for water to infiltrate into the soil. The slower runoff rate and reduced volume of runoff mean less soil will be removed from the field. Residue also helps trap snow to retain more moisture after spring thaws. All of these can help reduce irrigation requirements or save a dryland crop in a year having lower rainfall amounts. Residue can also protect the soil from the erosive forces of wind. However, standing residue may be more effective than flattened residue in reducing wind erosion. Thus, mass of residue in addition to percent cover may be needed to evaluate wind erosion control potential. Estimating Residue Cover Residue cover estimates can be useful in planning field operations to maintain erosion control. Measurements of residue cover may also be required to determine whether adequate residue remains to qualify for federal, state, or local conservation programs. Three methods can be used to estimate the percent of residue cover. Two of these are easily accomplished with field observations. The third is less accurate and requires generalizations and calculations that are helpful, but provide only approximations. The Line- Transect Method The line-transect method is an easy, reliable way to determine percent cover. This method involves stretching a 50- or 100-foot tape diagonally across the crop rows, and then checking at every foot mark to see if that point touches a piece of residue. When using a 100-foot tape, the percentage of residue cover is the number of times residue touches the points checked. If a 50-foot tape is used, double the figure to arrive at the proper percentage of residue cover. While checking, a good question to consider is: If a raindrop falls at this point, will it hit residue or bare soil? Care must be taken to avoid overestimating. Look straight down at the tape and take all readings on the same side of the tape. If there is any doubt whether residue touches the point, do not count it. Take at least three measurements at sites typical of that particular field, and average them to obtain the estimate of residue in the field. Do not take measurements in turn row areas