Cover crops are used by many farmers, but very few know how much nitrogen (N) or dry matter they are getting from their cover crops. There are some methods in the literature for estimating cover crop contributions. We are evaluating these methods in on-farm WSARE-funded trials in the Northern Willamette Valley to find the most practical and accurate method for use on farms. The vast majority of research in this area has been done with single species cover crops. Since farmers in Oregon often use cover crop mixtures, we are evaluating these methods on mixtures chosen by the farmer collaborators. The three methods described below provide different approaches you can use to evaluate N and dry matter contributions from your cover crops. At the end of the project we will report how they worked for us.
OSU’s publication Using Cover Crops in Oregon, publication EM 8704 available through the Extension Service Publication and Media Catalog, describes how to measure the fresh weight of a cover crop to get estimates of N-contribution. This method is based on years of research with single species cover crops in the Willamette Valley. The % N in several legume species was tested and conversion factors where developed to simplify estimates the N contribution of a cover crop based on fresh weight of the cover crop. To use this method, cut the leaves and stems from representative 16 ft2 sections in your field. This should be done when the canopy is dry but not wilting. Weigh the fresh cover crop and multiply by the conversion factor for your species (table 1) to estimate total N, they recommend dividing the answer by 2 to estimate the lbs N/acre that will be available to the subsequent crop. One challenge using this method is that cover crop canopies are often wet during the spring. The method relies on rough estimates of %dry matter and %N in cover crops that can vary under different moisture regimes and at different growth stages.
| Cover Crop | N Factor |
|---|---|
| Austrian winter pea | 8 |
| Crimson clover | 5 |
| Fava bell bean |
9 |
| Hairy or lana vetch | 9 |
| Karridale subclover | 6 |
| Kenland red clover | 10 |
Table 1. N factor for various legumes (EM 8704)
Marianne Sarrantonio’s Northeast Cover Crop Handbook (1994) describes an entirely different approach to estimating cover crop N and dry matter contributions based on canopy height and % ground cover. This method uses look up tables found in the book that we have not reprinted here. To measure % ground cover one needs a strong piece of cord about 27’ long. Every 6” for 24 ½’ draw a thin band around the string with a permanent marker (making 50 bands). Tie each end of the string to a stake and stretch the string tightly across the cover crop. Then walk next to the string and look straight down to count the number of points directly above or below any part of a cover crop plant. Repeat this once in an area and add the two numbers together to get the estimate of % ground cover, and record an estimate of average canopy height. This procedure should be repeated in at least 3-5 representative parts of the field. Next, use the look up tables published in the Northeast Cover Crop Handbook (pp. 31-34) to find an estimate of the dry matter contribution. Finally, use the table on pg. 41 of the book to estimate the %N in the dry matter to estimate total N contribution. Remember to divide this value by 2 to estimate plant available N.
A third method, also described in the Northeast Cover Crop Handbook uses laboratory forage analysis for total %N and % dry matter analysis. If the lab only reports results as % crude protein, divide this value by 6.25 to estimate total %N. In our project we are cutting the cover crop from representative 4ft2 quadrants in the cover crop stand. Be sure to take at least 3-5 samples from a field. Record the fresh weight of the 4ft2 sample, remove a representative subset of the sample and record the fresh weight. Immediately send the samples to the laboratory for analysis. The following formulas will help you convert the lab results into estimates of lbs dry matter/acre and total N/acre contributed by the cover crop.
• Total lbs dry matter/A = 4ft2 quadrant fresh weight x % dry matter x 10890
• Total lbs N/A = Total lbs dry matter/A x %N
• Estimated plant available N = (Total lbs N/A) / 2
The final method we are using in the WSARE trials involves separating each species from the 4ft2 quadrant to test them separately. This will allow us to get more accurate estimates of the proportion of each plant in the mixture. We will compare the results of the three simpler methods to this more labor intensive method to find the most accurate and practical method for estimating cover crop N and dry matter contribution from multi-species cover crops on farms. In the meantime, you can choose the method that best suits your situation. Be aware that all these methods can only provide estimates of cover crop N and dry matter contributions, but they are likely to be more accurate than guesswork.