Optimization of corn production following legume green manures
This grant funds continuation of a study initiated in 2007 and focuses on corn response to previous legume green manures and added nitrogen to optimize yield. With SARE funding (FNC06-626), we initiated a two-year study of annual legumes in summer 2007 with one follow-up corn response year, 2008. In this project, we repeated the legume phase in 2009, repeated the corn phase and will conduct the corn phase in 2010. At the end of the project we will have 3 rotational or cause-effect cycles. For this project, we also added a nitrogen response trial for corn following red clover to determine optimum N rate and the “nitrogen credit”.
Details of legume testing and measuring the impact on corn yield with and without added N fertilizer can be found in previous reports. To determine the optimum N rate for corn following red clover, we applied nitrogen fertilizer at planting at rates of 0, 40, 80, 120, 160 and 200 lb N/acre as ammonium nitrate in a randomized and replicated trial. At harvest, data was analyzed to generate a best-fit response curve from which the yield maximizing N rate (Nmax) and the economically optimum nitrogen rate (EONR) was determined. We used the nitrogen fertilizer to corn price ratio method employed by all major land grant universities in the upper mid-west to evaluate EONR. The EONR is the rate at which the last unit of nitrogen provides a return equivalent to the value of yield increase. Beyond EONR, the cost of added N exceeds the value of corn yield returned. Using the price ratio adjusts N rate for current market conditions.
Legume performance data is presented in Table 1 (see Table 1 attached). The 2009 growing season was abnormally cool and wet in Wisconsin. September was above normal temperature wise, but cool, wet conditions in October accompanied by a lack of sunshine limited legume performance. In general, legume performance demonstrated trends similar to 2007 and 2008. Red clover was the most productive and the summer-seeded legumes produced significantly less biomass and nitrogen. Red clover yield was lower than previous years, presumably because of the growing conditions, but this may be an artifact of sampling. Prolonged rain delayed sampling and the material was significantly matured (as indicated by quality measurements) meaning some of the material may have senesced. Visually, I would have rated both the clover in the trial and the rest of the field as the best stand I’ve ever produced.
2009 corn yields are reported in table 2 (see Table 2 attached). In 2009, we saw a significant yield response to added N, 52.2 bu./a on average for an increase of 55.2%. This response was predicted by the presidedress soil nitrate test (PSNT). Averaged across previous legume (treatment effect not significant), soil nitrate levels were 6.9 ppm. University of Wisconsin-Extension recommendations do not assign a nitrate credit when test values are less than 10ppm. Presumably, abnormally cool, wet conditions limited mineralization of legume N, resulting in low PSNT values and the significant corn yield response to added N.
Unlike 2008, we experienced a significant previous legume x N interaction in 2009. Corn following red clover responded significantly more than corn following the other legumes, 79.0 bu./a, a 77% increase. We assume this is not simply a lack of N because this treatment was the most productive in terms of N contribution. Rather, this could be a yield enhancement because these yields were significantly higher than the others aggregated at either N level. Averaged over N rate, corn after clover averaged 153.2 vs. 114.8, or 33.4% greater. A variety of factors may be responsible for this effect and we will pay close attention to this comparison in 2010, including field observations during the growing season.
To determine the effectiveness of red clover N for corn, or conversely to estimate a clover “N credit”, we conducted the fertilizer response trial. We calculated the corn yield response to added N following red clover, both actual yield and relative yield. From relative yield, both the Nmax and EONR rates were calculated, using a range of nitrogen: corn price ratios. In 2009, the response to N was significant and this site could be classified as “highly responsive”. Yield was maximized (Nmax) at 147 lb. N/acre and the EONR for the 2009 growing season (0.10 nitrogen: corn price ratio) was 116 lb. N/acre, similar to the recommended N rate without a previous legume. PSNT values predicted the response to the full rate of N fertilizer, despite significant amounts of legume N measured in fall 2008. Cool, wet conditions presumably limited mineralization and created the highly responsive site.
The magnitude of the response can be gauged by looking at the relative yield curve at the 0 N rate. In 2009, only 63% of the relative yield was obtained at the 0 rate. In the aggregate Wisconsin data set (multiple site-years under various environments), 88% of relative yield is obtained without added N, and the data confirms the current UWEX red clover N credit of 80 lb./acre. The 2009 results indicate the need to monitor early season growing conditions when making the decision to add supplemental N and determining the rate.
WORK PLAN FOR 2010
We will repeat what we did in 2009, except that the legume phase has been completed.
I conduct an annual cover crop workshop for the Michael Fields Agricultural Institute as a “Farmer Faculty” member on our farm. In 2009, 13 individual attended and viewed the legumes in the trial. Data was given to select UW-Extension educators at their request and has been incorporated into the master UW-Extension cover crop database, compiled by Kevin Shelley, Regional Agronomist with the UW Nutrient and Pest Management Program and myself. Our intent is to revise older publications with current data to promote the practice of cover cropping Statewide.
In 2009, I hosted a field visit as part of the SARE sponsored Cover Crop Professional Development Workshop, October 8, 2009 at the Michael Fields Agricultural Institute, East Troy.
This workshop targeted conservation agency personnel. Thirty-four participants visited the plots and learned of the benefits of cover cropping.
In 2010, I will again host the MFAI cover crop workshop. In addition, Dr. Peg Reedy, UW-Extension Agricultural Agent in Walworth County and I are setting up a summer field day to highlight this work. We envision this as part of a “conservation practice” tour in Walworth County.
We have a collection of digital images documenting the project that will be submitted with the final report.
- Table 1. Performance of Legume Cover Crops at East Troy, 2007-2009
- Table 2. Corn Yield following Cover Crops with and without Added Nitrogen, 2008-09