Timing For A Harvest Aid In A Relay-Intercropped Soybean Production System

Light and moisture may be limiting for relay-intercropped soybean production. Therefore, wider wheat row spacings have been utilized to allow relay-intercropped soybeans to intercept more light. Relay-intercropped soybean production has been challenging in Northeast Missouri due to dry weather in late June. Therefore, the use of glyphosate as a wheat harvest aid may reduce the impact of wheat and weeds on relay-intercropped soybean grain yield. The objective of this study was to evaluate the impact of a harvest aid timing on wheat and relay-intercropped soybean grain yields.

‘Pioneer 25R37’ was planted at 110 lbs/a on 11 October 2002, 17 October 2003, and 8 November 2004 in 7.5 and 15 in. rows. Relay-intercropped soybean were planted in 15 in. rows with a John Deere 7200 split-row planter on 23 April 2003, 29 April 2004, and 25 April 2005 at 180,000 seeds/a. Glyphosate was applied at 0.75 lb ae/a plus 17 lb/100 gal ammonium sulfate on June 10, June 16, and June 24. All plots were harvested 7 July 2003, 28 June 2004, and 29 June 2005. Double-crop soybean were planted on the same day in 15 in. rows at 180,000 seeds/acre. Data were subjected to ANOVA and means separated using Fisher’s protected LSD (p<0.05).

Glyphosate applied on 10 June 2003 or 2005 reduced wheat grain yield 20 to 33 bu/a (Table 1) when compared to the untreated control; however, grain yield was not affected by the early harvest aid treatment in 2004 which was due to earlier maturing wheat during this season. Grain yield was not reduced when a harvest aid was applied on or after 16 June 2003, 2004, or 2005. Soybean grain yield increased when a harvest aid was used in 2003 while no effect of the harvest aid on soybean yield was observed in 2004 when compared to an untreated relay-intercropped control. Wheat grain yield in 7.5 in. rows was 21 bu/a greater than 15 in. wheat in 2003, similar to 15 in. wheat in 2004, and 6 bu/a greater than 15 in. wheat in 2005. Soybean grain yields in 2005 will be determined to evaluate the impact of the harvest aid on yield.

Table 1. The effect of glyphosate harvest aid timing on wheat grain yield in 2003, 2004, and 2005 in a double-crop and relayintercrop production system, and soybean yield in a double-crop, relay-intercrop, and full-season production system in 2003 and 2004.