RELAY-INTERCROPPED SOYBEAN IN DIFFERENT WATER REGIMES, PLANTING PATTERNS, AND WINTER-WHEAT CULTIVARS

Relay-intercropping soybean [Glycine max (L.) Merr.] into winter wheat [Triticum aestivum L.) at the boot stage may be a profitable producti on alternative in environments where doublecropped soybean production after wheat harvest is unreliable. However, moisture availability, pla nting pattern, and growth characteristics of different wheat cultivars may influence soybean survival and final yield. To better define the conditions under which relay-intercropping would be acceptable to grow ers, we compared the influences of water regimes, planting patterns, a nd wheat cultivars on soybean growth and yield. In 1988 and 1989, five wheat cultivars were planted in solid and skip-row patterns near Manh attan and Rossville, KS, and 'Resnik' soybean was intercropped in whea t at the late boot stage. All plots received equal irrigation prior to wheat harvest and different amounts (limited vs. full) during the rem aining growing season. Soybean yields at Rossville were not affected b y irrigation regime, but at Manhattan, fully irrigated soybean yields were 27% greater than soybean under limited irrigation. The intercrop competition period averaged 16 d longer in 1989 than in 1990. Wheat wa s planted in 8 in. rows in a solid and one-out-of-three skip-row patte rn. Soybean in all planting patterns was in 24 in. rows - between the 8 in. wheat rows, in the middle of the 16 in. skip, or in a convention ally tilled, sole cropped (SC) planting. When intercropped in solid wh eat stands (SI), soybean yielded 52 and 37% of soybean in skip-row int ercropped (SRI) and SC patterns, respectively, in 1989, and 80 and 64% of SRI and SC yields, respectively, in 1990. Solid intercropped soybe an received 36 and 64% of the total measured photosynthetically active radiation that reached SC and SRI soybean canopies, respectively. Sev enty six percent of unharvested soybean plots (reduced or eliminated p opulations) were SI. Soybean intercropped into short wheat cultivars h ad higher (14 to 20%) yields than soybean in taller wheat cultivars. W heat cultivar maturity had no consistent effect on intercropped soybea n yield. Of the soybean plots abandoned because of reduced populations , 76% were intercropped into tall or medium-late maturing wheat cultiv ars. If relay intercropping soybean into winter wheat is to be impleme nted on soils that are droughty or have low moisture-holding capacity, supplemental irrigation must be available. Using an SRI planting patt ern with wheat cultivars of shorter stature will increase chances for soybean survival and profitable yields.