The Effect of Spring Seeded Annual Medic, Genus Medicago, on Weed Management and Soil Quality in Corn Production

The Effect of Spring Seeded Annual Medic, Genus Medicago, on Weed Management and Soil Quality in Corn Production

Summary

Production management systems that incorporate smother crops to control weeds and/or supply plant nutrients may reduce the transport of agrichemicals to surface and ground waters. Alternatives to agrichemicals will be adopted by producers when they effectively replace those chemicals without reducing crop yield, disrupting management practices, and diminishing farm profitability. This project investigated if annual medics, used as a living smother crop, would control weeds, not adversely effect corn productivity, supply N to corn, increase soil quality, and be similar in cost to synthetic chemical application.

Field experiments were conducted in South Dakota and Iowa using three species of annual medic (barrel medic, Medicago trunculata; burr medic, Medicago polymorpha; and snail medic, Medicago scutellata). The first of several experiments evaluated the effect of medic planting date and rate on corn production and weed control.

Medics were planted at two planting dates (about two weeks before average planting date for the area and at corn planting) and two broadcast planting rates (15 and 30 lb seed/acre). The crop year in 1995 was cool and wet, and medic grew aggressively except in drainage areas. The 1996 growing season was warmer and drier and medic biomass was about 70 percent less than in 1995. Medic planted early had more biomass and ground cover than medic planted at the same time as corn but all medic senesced by mid- to late-July. Medic reduced total weed biomass (predominant grass weeds were yellow and green foxtail, predominant broadleaf weed was common lambsquarters) in both 1995 and 1996. Corn yield was very poor in 1995 at both Brookings and Sioux Center sites and was reduced by medic. In 1996, the barrel medic broadcast at high rates reduced corn yield by about 30 percent compared to the weed-free control. However, when medic was banded into the interrow areas and alachlor was banded in the row, yields were similar to areas where only herbicide was applied.

In a second experiment, the effect of medic on soil quality and N cycling were evaluated. Medic increased water infiltration from about 1.5 inches per minute (bare soil) to about 6 inches per minute (medic planted at 30 pounds per acre). Nitrogen mineralization measured from mid-July to mid-August increased by about 50 percent in medic plots compared to areas without medic. However, nitrogen credit from medic the following year was minimal. Corn yield was not influenced in this study.

In economic analyses using enterprise budgets, results were similar (within 5 percent) when comparing the cost to produce a bushel of corn (assuming no N credit for the medic) using broadcast methods for medic alone or alachlor alone, or a band combination of medic (interrow) and alachlor (row). It is important to note that banding would reduce herbicide application by 50 percent compared to a broadcast application.

The information gained in this research indicates that annual medics may have a very important niche in achieving more sustainable, environmentally benign crop production systems. Early in the growing season, medic was competitive with corn for soil N. However, after the medic senesced, this N was subsequently mineralized and made available for plant uptake. When using medic, enough N must be present in the soil so corn does not become N deficient. In addition, using medic in conjunction with other agrichemicals, especially in sensitive areas such as acres coming from CRP, would benefit the soil water profile, reduce soil erosion, and reduce N losses in agronomic systems.

North Central Region SARE 1998 Annual Report.

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