Soybeans (Glycine max L. Merr.) are becoming important in cropping systems that increase competition for light among crops. One of the products of N2‐fixation is protein production. A field study was conducted to determine the effects of various degrees of shading (0,20,47, 63, 80, and 93%) on soybean No‐fixation, grain yield, and plant composition. The soil was a Flanagan series, a fine, montmorillonitic, mesic Aquic Argiudoll. Ambient illumination was 124.2 klux. Various degrees of shading were obtained by using different screens. Acetylene‐ethylene assay was used to determine N2‐fixation. Shading accelerated the rate of loss of total nodule N2‐fixing activity (TNA) as plants developed. The average TNA and dry weight of plant tops were highest at 20% shade (99.3 Klux) and decreased curvilinearly as shading increased, while specific nodule activity decreased linearly. Grain yields under 20, 47, 63, 80, and 93% shade were 90, 75, 48, 18, and 2% of unshaded plants. Seed percent protein and oil content of the seed were virtually unaffected between 20 and 80% shade. The highest percent protein and lowest percent oil occurred at 93% shade. Number of pods per plant percent leaf N, total stem N, TNA, and grain yield were highly correlated with shading and may be good selection indices for soybean shade tolerance. Soybean N2‐fixation was positively correlated with seed protein content (r = 0.77) but negatively correlated with seed protein (r = −0.44). Cropping practices should allow at least 80% ambient illumination measured at the height of 50 cm for substantial soybean No‐fixation.
A 2‐year field study was conducted to determine how total grain yield per unit of land area could be maximized by intercropping sorghum (Sorghum bicolor (L.) Moench) and soybeans (Glyycine max (L.) Merrill). These two species are being intercropped more frequently through‐out the world to increase protein yield per unit of area and utilize available moisture. Grain yields, yield components, and relative yields were analyzed. Two sorghum cultivars, grain type ‘DeKalb BR‐44’ (semi‐dwarf) and dual purpose ‘DeKalb FS‐16’ (tall), were intercropped with soybean cultivars ‘Calland’ (Group III) (constant plant density) at 16 sorghum population densities, ranging from 1.38 to 33 plants/m2. The soil was a Flanagan series, a fine, montmorillonitic, mesic Aquic Argiudoll. Intercropping changed the response patterns of sorghum yield components as plant population changed and reduced yields of FS‐16 and BR‐44 by 14 and 74%, respectively. Sorghum yield components were reduced more in mixtures of BR‐44 than in mixtures of FS‐16; with number of heads per plant being the most affected. Intercropping also reduced the compensatory alterations in sorghum yield components as population changed. In mixtures, tillering ability and population density were the main determinants of grain yield. Highest yields in monoculture were obtained at densities higher than 25 plants/m2 for FS‐16. and between 12 and 15 plants/m2 for BR‐44. Grain yields of both sorghum cultivars increased linearly with increases in plant population in the mixture. lntercropping soybeans with FS‐16 and BR‐44 reduced soybean yields by 75 and 17%, respectively; primarily due to reduced pod number. Relative yield curves indicated that the highest relative yield totals might be obtained by planting soybeans 5 cm apart between rows of sorghum planted at a density of 5 plants/m2 at a spacing of 74 ✕ 74 cm.
Sorghum (Sorghum bicolor (L.) Moench) and soybean (Glycine max (L.) Merrill) are being intercropped in the tropics so that crops more effectively utilize water; there is better weed control, and soil fertility is improved. A field experiment was conducted to determine the effects of sorghum‐soybean intercropping on soybean N2‐fixation and plant composition of both crops. The soil was a Flanagan series, a fine, montmorillonitic, mesic Aquic‐Argiudoll. Soybeans were intercropped with tall and semi‐ dwarf sorghum cultivars at 16 population densities from 1.38 to 33. plants/m2. Soybean N2‐fixation was estimated by acetylene reduction. Various plant parts were analyzed for present N. Nitrogen‐fixation by soybeans grown with tall sorghum was reduced 99% due to reduction in number of nodules per plant (77%), weight per nodule (50%), and specific nodule activity, SNA, (96%). Soybean dry matter and seed percent oil were also reduced by 87 and 8%, respectively. Soybean percent seed protein and leaf N were unaffected by intercropping. Soybeans grown with semi‐dwarf sorghum fixed 2.64 times more N than plants in monoculture but produced 40% less dry matter and 3% less oil in seed. Percent seed protein and leaf and stem N of soybeans were unaffected by intercropping. The increase in N2‐fixation was probably due to increases in number of nodules per plant (62%) and SNA (364%). Possible involvement of allelopathy, NO3‐ absorption, delayed senescence, and inter‐ specific mechanical support was considered. Protein yield of intercropped tall and semi‐dwarf sorghum was reduced by 15 and 71%, respectively. Only in the dwarf cultivar did intercropping increase percent seed protein by 15%. Sorghum grain oil was unaffected by intercropping.
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