Fields of Green: Fall Cover Crop Biomass Sampling

An unseasonably warm fall has made an excellent year for cover crop growth! The ILF team has been traveling across the state and has seen some beautiful, green cover crop fields. With more growing degree days this fall, it has been a good year for radishes in the southern portions of the state. Check out this growth on radishes at Crawfordsville!

If you have cover crops and are interested in measuring the amount of biomass growing in your fields, follow along with our methodology in this blog. Check out the end of the blog for a summary of how you can apply this research to your own farm.

As part of our National Conservation Innovation Grant/Cover Crop Mixtures demonstration project, we are interested in learning if more biomass can be generated by seeding a single species of cover crop in a plot versus a mixture of cover crop species. To see which treatment yields more biomass (pounds per acre), we collected biomass in the fall and spring from six research sites throughout the state.

To sample biomass, we start with a frame that we constructed out of PVC piping. Our frame measures 19.2 inches x 30 inches (about four square feet). We toss the frame randomly into our test plots. Wherever the frame lands, we sample the cover crop biomass to the soil surface within the frame. We use clippers to cut the cover crop biomass, and we do not include soil, cash crop residues, or weeds in the sample. We try to only capture cover crop biomass and then place it in a labeled paper bag that tells us which test plots the sample came from.

We walk to a different portion of the plot and repeat the process. We always use a different paper bag for each sample and make sure to close the bag after sampling so that sampled biomass cannot escape. We go through the same process in all of our cover crop plots.

We take our biomass samples back to a lab on campus at Iowa State University within the Department of Agricultural and Biosystems Engineering and immediately open the bags the allow them to start air drying. This can prevent sample degradation, like molding on wet samples, from occurring.

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We then sort our samples from each plot by species. For the mixture plots, sorting helps us separate the biomass generated by each cover crop species. For all plots, it ensures that crop residue and other items can be separated from the sample before it is dried and then weighed.

Sorted samples are then placed into their own paper bag and dried in a drying oven at low heat (104 degrees) for at least 48 hours to remove any remaining water. We weigh the samples to get the dry measurement of the biomass. For our demonstration project, the last stop for our samples is the ISU Soil Processing Lab in the Agronomy Department. The samples are analyzed for Total Carbon and Total Nitrogen of the plant.

On-farm Research: If you’re interested in measuring the biomass growing on your own farm, here’s a summary of steps you can follow to make it happen.

Step 1: Create your frame (as long as you know how many square feet are within your frame, the math will work).

Step 2: Take samples in your field and place the samples in separate paper bags. We recommend taking at least eight square feet of samples to get a representative average for the field.

Step 3: Dry the biomass (about 104 degrees for 48 hours). Regional ISU Research Farms may have facilities for you to dry your samples.

Step 4: Calculate total biomass. Weigh the sampled biomass with a scale with two decimal precision (ounce or gram). Also weigh the paper bag by itself.

 

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Average the total biomass from all of your samples to get an average biomass for the field. Convert results to appropriate lbs/acre using unit conversions.  1lb = 16 ounces = 453.592 grams, 1 acre = 43,560 sq. ft.

And finally, always expect surprises! We found a few purple top turnips mixed into our mix of oats, radish, and hairy vetch.

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Julie Whitson

 

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