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 production alternative in environments where doublecropped soybean production after wheat harvest is unreliable. However, moisture availability, planting 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 growers, we compared the influences of water regimes, planting patterns, and wheat cultivars on soybean growth and yield. In 1988 and 1989, five wheat cultivars were planted in solid and skip-row patterns near Manhattan and Rossville, KS, and ‘Resnik’ soybean was intercropped in wheat at the late boot stage. All plots received equal irrigation prior to wheat harvest and different amounts (limited vs. full) during the remaining growing season. Soybean yields at Rossville were not affected by 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 was planted in 8 in. rows in a solid and one-out-of-three skip-row pattern. 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 conventionally tilled, sole cropped (SC) planting. When intercropped in solid wheat stands (SI), soybean yielded 52 and 37% of soybean in skip-row intercropped (SRI) and SC patterns, respectively, in 1989, and 80 and 64% of SRI and SC yields, respectively, in 1990. Solid intercropped soybean received 36 and 64% of the total measured photosynthetically active radiation that reached SC and SRI soybean canopies, respectively. Seventy six percent of un harvested soybean plots (reduced or eliminated populations) were SI. Soybean intercropped into short wheat cultivars had higher (14 to 20%) yields than soybean in taller wheat cultivars. Wheat cultivar maturity had no consistent effect on intercropped soybean yield. Of the soybean plots abandoned because of reduced populations, 76% were intercropped into tall or medium-late maturing wheat cultivars. If relay intercropping soybean into winter wheat is to be implemented on soils that are droughty or have low moisture-holding capacity, supplemental irrigation must be available. Using an SRI planting pattern with wheat cultivars of shorter stature will increase chances for soybean survival and profitable yields. Research QuestionRelay-intercropping soybean into winter wheat at the boot stage may be a productive alternative to doublecropping soybean after wheat harvest for small grain/double crop farmers. However, soybean must survive and flourish for the relay-intercropping system, and the producer, to be successful. This study was conducted to compare effects of wheat cultivars, planting patterns, and limited vs. fully irrigated environments on yields of relay-intercropped wheat and soybean. Literature SummaryPrevious studies have shown that intercropped soybean yields are significantly less than those of sole cropped (SC), but greater than those of doublecropped soybean. A study with two wheat cultivars having similar growth characteristics showed that wheat cultivar had no effect on soybean yield. Wheat will compete vigorously for light, moisture, nutrients and other growth factors from boot until dough stage. In Kansas, where soil moisture can be limiting, soybean planted into a skip-row pattern in wheat tended to have greater yields than soybean planted into solid-planted wheat. Study DescriptionLocations: Manhattan and Rossville, KS Soil: Eudora silt loam Growing season (April through September) moisture: Precipitation: 1989: Manhattan-19.4 in., Rossville-26.5 in.; 1990: Manhattan-21.6 in., Rossville-20.0 in. Full Irrigation: 1989: Manhattan-16.5 in., Rossville-14.8 in.; 1990: Manhattan-10.0 in., Rossville-8.5 in. Limited Irrigation: 1989: Manhattan-8.4 in., Rossville-9.1 in.; 1990: Manhattan-4.0 in., Rossville-6.5 in. Treatments: Water regimes: Full and limited irrigation Wheat cultivars: AgriPro ‘Abilene’, AgriPro ‘Mesa’, AgriPro ‘Victory’, ‘TAM 200’ and ‘Triumph 64’ Soybean cultivar: ‘Resnik’ (maturity group III) Planting patterns: Wheat planted in 8 in. rows in solid (SS) and skip-row (SRS) SC in the fall of 1988 and 1989. Soybean intercropped in half of the SS and SRS (now intercropped into solid wheat [SI] and solid-row intercropped [SRI]) plots, and planted as a SC at boot stage of wheat in 24 in. rows Herbicides: Sethoxydim (8 oz/acre); acifluorfen (8 oz/acre); and bentazon (12 oz/acre) Applied QuestionsDid wheat plant characteristics or planting pattern influence the success of relay intercropped soybean? Relay-intercropped soybean yields were lower than those of SC soybean and lower in SI than in SRI plots. Competition for photosynthetically active radiation (Fig. 1) was less for soybean seedlings intercropped in 16 in. SRI than in SI seedings. The increased early-season competition between wheat and soybean inversely influenced final seed yield (Fig. 1). More than 75% of failed plots in this study were SI because of the greater competitive ability of the wheat compared with the seedling soybean. The longer the period of competition from wheat, the greater the cultivar influence on intercropped soybean. Solid-seeded, taller, later maturing wheat tended to reduce soybean populations and final soybean grain yields with 54 d of competition in 1989. Trends for wheat cultivar influence were significant for only one cultivar in SRI plots. Over 75% of failed soybean plots were in the medium or tall wheat cultivars, Abilene, Victory, or Triumph 64. These medium and tall cultivars experienced greater stem elongation than Mesa and TAM 200 (short cultivars) after soybean was intercropped, and competed to the point that soybean seedling populations were reduced and in some cases eliminated. Can limited irrigation relay-intercropped soybean be a success in Kansas? Though intercropped soybean germinated and became established in all plots, all plots were irrigated to maintain a reasonable soybean population. The period from soybean planting until wheat harvest is traditionally a high rainfall period. Rainfall occurred during this period in our study, but was of insufficient amounts and untimely, so plots were irrigated both years. The “rescue” irrigations were quite timely for the limited-irrigation soybean plots, as evidenced by soybean at two of the four sites producing no yield advantage from full irrigation. On droughty soils such as those in this study, the ability to irrigate, even if not practiced, is imperative for stand survival prior to wheat harvest. RecommendationSoybean should not be relay-intercropped into standing wheat on droughty soils, unless irrigation capabilities exist. The use of a SRS planting pattern will greatly enhance the probability of soybean seedling survival and productivity, and profitability of the system. We would not recommend intercropping soybean into solid-seeded wheat at the boot stage unless the wheat stand was thin and the wheat cultivar was short. Fig. 1Effects of planting pattern on the amount of photosynthetically active radiation at intercropped soybean canopy levels from soybean emergence through milk stage, and final soybean seed yields.