The soybean seed is at its optimum quality when it reaches physiological maturity in the field. The subsequent handling (transport from the field to the reception centre or to elevators, conditioning, seed packaging, storage, processing, etc.) results in a gradual, sometimes rapid, reduction of its quality.

Sometimes it is not clear whether losses are expressed on a wet or dry weight basis and the use of accumulated percentages is dangerous. Reports often do not make clear how accumulated losses have been derived. If losses in each of harvesting, threshing, transport, storage and processing are 10 percent, the total loss may not be 50 percent; if the losses were a percentage of the quantity left at each stage then the total loss is only 41 percent (Wright, 1995). Another feature of the common methods of assessment is that they are concerned only with physical weight loss rather than loss of quality, nutritional value, seed viability or commercial value (Boxall, 1986). Different aspects and types of quality loss were reviewed by FAO (1984). In Africa, where demand exceeds production, quality issues are less relevant than in food surplus situations (Wright, 1995). Nutritional loss, in terms of nutritional value per unit weight, is often a major factor. Many pest species are selective feeders; bruchids feed on the cotyledons (Haines, 1991); rats, mites, Ephestia and Plodia larvae feed on the grain germ, thereby removing much of the protein and vitamins. Weevils feeding on the endosperm remove much of the carbohydrates in the grain. In all these cases weight loss will give a distorted view of the nutritional loss. Loss of seed viability occurs in storage (Howe, 1973) and can be determined only by germination tests. Commercial losses may occur from farmer to country level. They include reduction in value of a sub-standard product, loss of goodwill, costs of legal action and the cost of pest prevention. (Wright, 1995). Although physical weight loss may be the simplest parameter to measure, it is not necessarily the most realistic. Any measure of loss must encompass the economic and social factors relevant to the loss sufferer.

Losses tend to be an amalgam of many, individual small, losses at different stages of the postharvest system. The postharvest sector is often considered in isolation although it is part of a continuous process from planting to consumption. For example, many pests begin their infestation in the field and poor storage of seed grain will result in reduced germination in the following season.

Even it is not really a postharvest loss, soybean yield reduction due to diseases and pests is considered an important loss since the reduction of the potential productivity does not allow to obtain such amount of soybeans as that obtained from a disease-and pest-free field.

Agricultural pests are present in most soybean fields. Table 23 shows the most common diseases and their corresponding causal organism. Depending on the extent of the infestation, soybean yield is affected. Several reports on soybean disease and pest losses exist in the literature. Table 24 shows a summary of these reports.

One of the countries with the highest losses in soybean yield due to insect pests is India. The national average soybean yield is about 0.8 tonne/ha (Kundu and Srivastava, 1992) and a productivity enhancement to 1.25 tonne/ha by 2000 and 1.5 tonne/ha by 2010 is expected (Paroda, 1999). The average world soybean yield is over 2 tonne/ha.

The magnitude of losses caused by soybean diseases has been estimated to be at least US$ 250 million per year in the United States alone (Liu, 1997). Brown leaf spot, frogeye leaf spot, brown stem rot, stem canker, purple seed stain and pod and stem blight are major soybean fungal diseases. Bacterial blight, pustule, wildfire and wilt are major soybean diseases caused by bacterial. Major viral diseases include soybean mosaic, yellow mosaic, bud blight and bean pod mottle. Soybean cyst nematode and root knot nematode are the main species of nematodes that attack soybeans. The estimated soybean yield losses due to diseases were 10.9 million tonnes in 1996, 11.9 million tonnes in 1997 and 14.0 million tonnes in 1998 (Wrather and Stienstra, 2000).

In 1994, soybean total yield losses caused by Heterodera glycines were greater than any other disease in the top 10 soybean producing countries (Wrather et al., 1997). Next in order of importance were stem canker, brown spot and charcoal rot. The total yield loss due to disease in these countries was 14.99 million metric tonnes, valued at US$ 3.31 billion. Methods used to estimate soybean disease loss included field surveys, plant disease diagnostic clinic samples, variety trial data, information from field workers and university extension staff, research plots, grower demonstrations and private crop consultant reports.

It has been estimated that soybean yield losses in the world due to diseases fluctuate between 10 percent and 15 percent (Ploper, 1997). In 1987, disease losses were estimated at 10.3 million tonnes, which amounted to 10.4 percent (Sinclair and Backman, 1989).

In Argentina, estimates by INTA (federal agency for agricultural research and extension) indicated that the most prevalent diseases reduce soybean yields annually by 400 000 tonnes, causing losses over US$ 90 million (INTA, 1993). However, considering other diseases, including those caused by nematodes, it is likely that losses in the country are much higher than those figures (Ploper, 1997). Among the most important soybean diseases in Argentina are Sclerotinia stem rot (Sclerotinia sclerotorium), root and stem rot (Phytophtora megasperma f. sp. glycinea) and charcoal rot (Macrophomina phaseolina) (Ivancovich et al., 1993). At present, Fusarium solani is considered one of the most important soybean diseases (Ivancovich et al., 1997).

In Bolivia, soybean is affected by at least 17 diseases. The average soybean yield loss is estimated at 8 to 10 percent, with annual losses valued at US$9 million (Wrather et al., 1997).

In Brazil, annual yield losses due to diseases are estimated at US$1 000 million (Wrather et al., 1997). The decrease in damage by A. gemmatalis, has been imputed to a greater effectiveness of natural enemies with the implementation of IPM systems, which resulted in drastic reduction in insecticide and use of more selective products (Panizzi, 1997).

In Paraguay, stem canker was the most devastating disease recorded. During 1991 to 1992 and 1992 to 1993, several fields had yield losses ranging from 50 percent to almost total loss (Wrather et al., 1997).

In Mexico, whitefly (Bemisia Argentifolii) was established in the Sonoran agricultural areas by 1993 (Yepiz-Plascencia et al., 1998); however, its economic impact was not evident until 1994. Average soybean reduction yields about 50 percent were reported in Sonora State; however, some soybean fields at the Yaqui Valley had almost total loss (Castillo, 2000). Since 1994, soybean has not been commercially produced in this region. Besides the whitefly problem, the lack of water availability for soybean cultivation during the summer has contributed to the disappearance of this important crop in the Northwest of Mexico.

Eight species of agromyzids, commonly called bean flies, infest soybean in tropical Asia, Africa and Oceania, but not in North or South America. Three of the species, Ophiomyia phaseoli (Tryon), O. centrosematis (de Maijere) and Melanagromyza sojae (Zehntner) cause significantly yield reduction (Talekar, 1997).

There are also other soybean diseases causes by seed-borne viruses such as peanut stripe, soybean mosaic and tobacco ring spot viruses (Sinclair, 1997).

There is a lack of reliable data not only on soybean harvesting losses, but also on each stage of the postharvest system. Losses during mechanical harvesting of soybeans were evaluated by Reis et al. (1989) and presented in Section 2.2. (Tables 14 and 15). The harvesting losses estimated by a soybean grower in Culiacan Valley, Sinaloa, Mexico using the equipment described in Figure 8 was 150 kg per hectare, which represents about 7 percent (Godoy, 2000). A similar percentage (£ 6 percent) was reported by Kowalczuk (1996) when soybeans were harvested with the Z056 "Bizon Super" grain combine harvester equipped with a floating cutting bar. Scott and Aldrich (1970) reported harvest losses between 5.4 and 12.2 percent depending on the height at which the combine cutterbar was operated. The lower loss corresponded to a height of cut of 3.5 inches and the higher to 6.5 inches. Based on these numbers, we could say that harvesting losses using combine harvester are between 4 and 7 percent. The appropriate adjustment to the combine cutterbar (height) is very important to keep losses in this range. Manual harvesting would be the ideal form of harvesting soybeans, since all soybeans are gathered and seed damage is null. Unfortunately, the huge soybean fields have to be harvested in a period of about 15 days. It is impossible to harvest by hand such amount of soybeans in only 2 weeks, so that the losses caused by the equipment (damage, broken, spills, etc) have to be undergone.