WEED MANAGEMENT IN NONIRRIGATED GLYPOHSATE-RESISTANT AND NON-RESISTANT SOYBEAN FOLLOWING DEEP AND SHALLOW FALL TILLAGE

Authors: Heatherly, Larry; SPURLOCK, STAN - MISSISSIPPI STATE UNIV.; Reddy, Krishna

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2003
Publication Date: 6/1/2004
Citation: Heatherly, L.G., Spurlock, S.R., Reddy, K.N. 2004. Weed management in nonirrigated glypohsate-resistant and non-resistant soybean following deep and shallow fall tillage. Agronomy Journal. 96:742-749.

Interpretive Summary: Nonirrigated soybean in the midsouthern USA occupies the majority of the area planted to soybean, and profit opportunity is low to moderate. Level of profit is affected by management inputs such as tillage and weed control. Tillage costs can be affected by the use of either minimum (shallow) or intense (deep) tillage that is usually applied in the fall. Weed management costs can be controlled by using either glyphosate-resistant or non-glyphosate-resistant varieties and manipulating herbicide inputs and application timing. In 3 years of nonirrigated studies conducted on Tunica clay soil near Stoneville, MS, using glyphosate-resistant varieties and glyphosate herbicide resulted in $9 to $214/acre greater profit than using non-glyphosate-resistant varieties with non-glyphosate herbicides. The larger differences were associated with better control of johnsongrass when glyphosate-resistant varieties and glyphosate were used. Use of postemergent-only weed management resulted in greater profits than did use of weed management that contained a preemergent component. Use of shallow tillage resulted in $20 to $22/acre lower cost than did use of deep tillage; however, profits were greater from deep tillage when little or untimely rainfall coincided with reproductive development of early-maturing (maturity group IV) varieties. When later-maturing maturity group V varieties were used, profits from deep tillage were similar to or lower than those from shallow tillage. Increased infestations with johnsongrass were associated with continued deep tillage. These results indicate that the investment in equipment for deep fall tillage for nonirrigated early soybean plantings in the midsouthern USA is justified only if early-maturing varieties are used and yield-limiting drought stress is common.

Technical Abstract: Management inputs that maximize economic return from the Early Soybean Production System (ESPS) have not been evaluated fully. The objective was to assess perennial weed control in and compare yields and economic returns from ESPS plantings of glyphosate-resistant (GR) and non-GR maturity group (MG) IV and MG V soybean cultivars grown under two levels of weed management without irrigation following shallow (ST) and deep (DT) fall tillage. Adjacent experiments receiving either ST or DT were conducted in 2000, 2001, and 2002 on Tunica clay soil near Stoneville, MS (lat. 33 deg 26'N). Weed management systems were 1) preemergent followed by postemergent broadleaf and grass weed control (PRE + POST), and 2) postemergent broadleaf and grass weed control (POST). Control of perennial redvine declined in the ST environment when non-GR cultivars were used. Control of perennial johnsongrass at the end of the study averaged < 30% when non-GR cultivars were used and > 94% when GR cultivars were used regardless of tillage treatment. Greater expense associated with use of PRE + POST compared to POST resulted in lower profits in 2001 and 2002 regardless of tillage treatment. In years when water-deficit stress coincided with MG IV reproductive development, average yields and profits from MG IV cultivars grown in the DT treatment were noticeably larger than those from the ST treatment. When later-maturing MG V cultivars were used, profits resulting from DT were similar to or lower than those resulting from ST.