Glyphosate-resistant corn was grown in 38- and 76-cm row spacings at two locations in 2001 to examine the effect of weed competition and row spacing on soil moisture. Volumetric soil moisture was measured to a depth of 0.9 m in 18-cm increments. Glyphosate was applied when average weed canopy heights reached 5, 10, 15, 23, and 30 cm. Season-long weed interference reduced soil moisture compared with the weed free controls. At Clarksville, MI, where common lambsquarters was the dominant weed species, weed interference reduced soil moisture in the 0- to 18-cm soil depth from late June through early August and at the 54- to 72- and 72- to 90-cm depths from mid-July through the end of the season. At East Lansing, MI, where giant foxtail was the dominant weed species, weed interference reduced soil moisture at the 18- to 36-, 36- to 54-, and 54- to 72-cm soil depths from mid-June to the end of the season. Season-long weed competition reduced yields more than 90% at each location. Weeds that emerged after the 5-cm glyphosate timing reduced soil moisture and grain yield at both locations. Delaying glyphosate applications until weeds reached 23 cm or more in height reduced corn yield at both locations and soil moisture at East Lansing. Grain yields in the 10- and 15-cm glyphosate-timing treatments were equal to the weed-free corn, even though soil moisture was less during pollination and grain fill. Row spacing did not affect grain yield but did affect soil moisture. Soil moisture was greater in the 76-cm row spacing, suggesting that corn in the 38-cm row spacing may have been able to access soil moisture more effectively.
Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #3 CHEAL; giant foxtail, Setaria faberi Herrm. # SETFA; corn, Zea mays L.
Additional index words: Narrow-row corn, glyphosate-resistant, weed interference, weed competition, time domain reflectometry, soil water.
Abbreviations: DAP, days after planting; FC, field capacity; PRE, preemergence; PWP, permanent wilting point.
