James E. Board

Soybean Yield Formation: What Controls It and How It Can Be Improved

Soybean [Glycine max (L.) Merr.; family leguminosae, sub family Papilionoideae; tribe Phaseoleae] is the most important oilseed crop grown in the world (56% of world oil seed production) (US Soybean Export Council, 2008). Major producers are the US (33% of world production), followed closely by Brazil (28%) and Argentina (21%). Remaining producers are China, India, and a few other countries. Currently, soybean is grown on about 90.5 million hectares throughout the world with total production of nearly 220 million metric tons (US Soybean Export Council, 2008). At current prices, total value of the world’s soybean crop is about

Growth Dynamic Factors Explaining Yield Improvement in New Versus Old Soybean Cultivars

100 billion. Soybean is used as human food in East Asia, but is predominately crushed into meal and oil in the US, Argentina, and Brazil; and then used for human food (as cooking oil, margarine, etc.) or livestock feed (Wilcox, 2004). These uses are derived from the crop’s high oil (18%) and protein (38%) content. Soybean meal is a preferred livestock feed because of its high protein content (50%) and low fiber content. Soybean oil is mainly used by food processors in baked and fried food products or bottled into cooking oil. Other uses are biodiesel products and industrial uses. Global demand for soybean has been increasing over the last several years because of rapid economic growth in the developing world and depreciation of the US dollar (US Soybean Export Council, 2008). In response to this demand, world production has been increasing through a combination of increased production area and greater yield. Among major producers, most of this increase in Argentina and Brazil has come from increased production area, whereas in the US it has come from increased yield (US Soybean Export Council, 2008). However, over the last 10 years US soybean yields have been increasing by only 66 kg ha-1 yr-1 compared to 396 kg ha-1 yr-1 for corn (USDA, 2007). An even greater problem is the disparity in yield between the three main producing countries [US, Argentina, and Brazil (2,800 kg ha-1)] and that in the remainder of the world (1,510 kg ha-1) (US Soybean Export Council, 2008). Because of the limited potential for increasing production area, it is very important that yield be accelerated in order to meet increasing global demand. Our objective is to describe the basic processes affecting yield formation in soybean and to apply this information to development of management and genetic strategies for increasing soybean yield. First, we will outline potential yield gains possible with management modifications in soybean. Secondly, the main abiotic and biotic stresses will be detailed describing their modes of action on yield.

Contribution of Remobilized Total Dry Matter to Soybean Yield

Reasons for genetic yield improvement in soybean (Glycine max [L.] Merr.) during cultivar development are not clearly understood. Since greater yield in new vs. old cultivars is largely regulated by final vegetative total dry matter (VTDM[R7]) and/or production efficiencies for node, pod, and seed yield components ([yield component no.]/ VTDM[R7] [no. g−1]), our objective was to clarify the importance of these two factors for yield improvement in 18 public southern cultivars released between 1953 and 1999. The study was done near Baton Rouge, Louisiana, during 2007 and 2008, with a validation study in 2009. Yield increases were affected by increased production efficiencies for nodes, reproductive nodes (node bearing at least one viable pod), pods, and seeds (R2 = 0.42–0.47), but not VTDM(R7). Increased yield component production efficiencies resulted in greater harvest index (HI; R2 = 0.55–0.73), which in turn, resulted in greater yield (R2 = 0.42). We concluded that a major factor contributing to yield increases between old and new cultivars was increased yield component production efficiency.

Cluster and Principle Component Analysis of Soybean Grown at Various Row Spacings, Planting Dates and Plant Populations

It is recognized that soybean remobilizes carbohydrates and protein compounds from vegetative to reproductive organs during the last half of the seed filling period. However, reports quantifying how much of final yield is constituted by these remobilized materials under field conditions are lacking. The objective of the current study was to determine the contribution of remobilized total dry matter (TDM) to final yield under field conditions. Using five field studies conducted from 1987 through 1995 near Baton Rouge, LA (30°N latitude), % remobilized yield was determined by using crop growth rate during the final half of seed filling (CGR[R6.2-R7]) to calculate how much of the final 50% of seed weight was contributed by remobilized TDM. The % remobilized yield was then calculated as 100 x (remobilized seed TDM/yield). Results indicated that % remobilized yield varied from 15% to 30% and tended to increase as growing conditions improved and plant size increased. Percent remobilized yield also increased when source/sink ratio declined during the rapid seed filling period (R6-R7). In conclusion, a substantial portion of final yield is derived from remobilized TDM.

Row Spacing Effects on Light Extinction Coefficients of Corn, Sorghum, Soybean, and Sunflower

Abstract Increased light interception (LI), along with concomitant increases in crop growth rate (CGR), is the main factor explaining how cultural factors such as row spacing, plant population, and planting date affect soybean yield. Leaf area index (LAI), LI, and CGR are interrelated in a “virtuous spiral” where increased LAI leads to greater LI resulting in a higher CGR and more total dry matter per area (TDM). This increases LAI, thus accelerating the entire physiological process to a higher level. A greater understanding of this complex growth dynamic process could be achieved through use of cluster analysis and principle components analysis (PCA). Cluster analysis involves grouping of similar objects in such way that objects in same cluster are similar to each other and dissimilar to objects in other cluster. PCA is a technique used to reduce a large set of variables to a few meaningful ones. Seasonal relative leaf area index (RLAI), relative light interception (RLI), and relative total dry matter (RTDM) response curves were determined from the data by a stepwise regression analysis in which these parameters were regressed against relative days after emergence (RDAE). Greatest levels of RLAI, RLI and RTDM were observed in soybean planted early on narrow row spacings and recorded greater plant population. In contrast, lower levels of these parameters occurred on plants with wide row spacings at late planting dates. For farmers, these results are useful in terms of adopting certain cultural practices which can help in the management of stress in soybean.