DR. Don Research Update
September 19, 2003
Longterm organic vs. conventional management trials find lower inputs offset lower yeilds in organic fields

By Don Lotter, Ph.D.

September 19, 2003: Some interesting patterns have emerged from another long-term organic vs. conventional management trial, published this year in Agronomy Journal. The research was carried out at the University of Minnesota and has been running since 1989. This research complements similar organic vs. conventional management comparative research on corn-soy done at The Rodale Institute Farming Systems Trial (FST), and several other universities.

The treatments were: 2-year and 4-year rotations, each with zero (ZI), organic (OI), low (LI), and high inputs (HI) management strategies. All of these combinations (i.e. 2-yr ZI, 4-yr ZI, 2-yr OI, etc.) were carried out on both a previously intensively farmed site as well as a previously non-intensively farmed site. The 2-year rotation was corn-soy, and the 4-year was corn-soy-oat-alfalfa. Organic fertilizer used on OI was steer and swine manure at recommended rates, and was comparable to the LI fertility levels. Via replication and staggered rotations, every crop in every treatment combination produced yield in every year. The 2-yr HI is used as a yield standard, since this strategy typifies conventional agriculture.

Averaged over 1993-1999, corn yields in the 4-yr OI treatment were 95% of the 2-yr HI treatment, while OI soybean yields were 82% relative to HI. The larger reduction in OI soybean yields compared to OI corn, when both are compared to HI, is a pattern that almost exactly replicates The Rodale Institute’s long-term corn-soy organic vs. conventional experiments.

The authors of the study attribute the yield differences between OI and HI to weed pressure in the OI, which is another recurring theme in organic vs. conventional comparative trials.

Also similar to The Rodale Institute’s results was a reduction in OI soy yields in the second half of the experiment. The Rodale FST has two organic treatments, a legume and a manure treatment, and the manure treatment was the one with declining yields. The Minnesota study uses manure on all of the OI plots, which suggests that there may be a problem with long-term use of manure on soybean crops.

Weeds in the 2-yr OI treatment seriously reduced yields in both corn and soybean, compared to the 4-yr rotation. The 2-yr rotation OI yields were reduced 40% and 21%, respectively, in the corn and soy, compared to the 4-yr, demonstrating the importance of rotations in organic systems.

As demonstrated in other studies, the net returns in the OI system were equivalent to the HI system, despite the lower yields, even without organic price premiums. This is due to the lower input costs of organic systems. Based on this, the authors conclude that organic crop systems are a viable management choice for farms.

Porter, P.M. et. al. Organic and Other Management Strategies with Two- and Four-Year Crop Rotations in Minnesota. Agronomy.Journal 95: 233-244. 2003

Farming Systems Trial: the first 15 years. The Rodale Institute.

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