Development, growth and yield of late-sown soybean in the southern Pampas

Soybean/wheat double cropping is becoming a key component of farming systems in the southern Pampas. In contrast to milder environments, where reduced seed number and shorter duration of early reproductive stages (between R1, one open flower at any node, and R5, 3 mm-long seed in a pod at one of the four uppermost nodes) account for most of the variation in the yield of late-sown soybean, it is proposed that seed weight and duration of later stages (R5 to R7, physiological maturity) are also important in a cooler environment. To evaluate this hypothesis, the yield and yield components of late-sown soybean, as if double-cropped after wheat, were measured in an environment where radiation and temperature fall sharply in late summer (37°S). We carried out a factorial experiment that combined seasons (dry, 1998 and wet, 1999), sowing dates (mid December and early January), cultivars (maturity group II and III), inter-row spacings (0.19 and 0.38 m) and water regimes (dryland and irrigated). This research established a new environmental boundary for soybean performance in soybean/wheat double cropping, moving into temperatures 2 to 4 °C below those in the cooler environments reported in the literature. Yield ranged from 3.7 t ha-1 in irrigated, early sown crops to 2.2 t ha-1 in late-sown dryland crops. Significant interactions between sowing date and other experimental sources of variation were detected that highlight the complex process involved in yield determination. Averaged over all sources of variation, delayed sowing reduced yield at a rate about 2% d-1 from mid December. Seed number accounted for 86, shoot biomass for 66 and harvest index for 56% of the variation in yield. Early-sown crops had heavier seed than their late sown counterparts, i.e. 136 vs. 118 mg seed-1. The decline in crop biomass, harvest index, seed number and individual seed weight with delayed sowing can be attributed to three main factors: (i) shorter season length, (ii) slow growth rate associated with short days, and low temperature and radiation, and (iii) a dramatic reduction in the relative duration of key phenostages, chiefly the period from R5 to R7 which accounted for 77% of the variation in yield, 56% of the variation in seed number and 62% of the variation in seed weight. Crop management practices for late sown soybean are discussed.