**1. Introduction**

Brazil has become the second largest soybean producer after the USA. Projections to the year 2020 indicate the country's soybean production will increase to 105 million tons, largely due to Brazil's large expanse of potential arable land. This expansion of soybean production will occur mainly in the North and Northeast parts of the country that includes the states of Maranhão, Piauí, Tocantins and Bahia, a region collectively referred to as the MAPITOBA. Within this region, the Cerrados, an area of stable climate, consistent rainfall, and flat topog‐ raphy, is especially suited for soybean production [17].

Soybean was introduced into Brazil in the late nineteenth century. However,widespreadpro‐ duction did not occur until about 60 years ago in the southern part of the country. Cultiva‐ tion then spread from the southern to the middle and northern sections of the country. In recent years, production has spread into the Cerrado Region, an area in the middle and northern parts of Brazil's interior containing 204 million hectares of land. The northern states of Piaui and Maranhao contain about 21 million hectares of land [6]. The savannas of this region are characterized by having acid soils of low fertility, high average temperatures (25-26 ° C) and average rainfall of 1,200 mm from October to April. However, they are sub‐ ject to occurrences of dry spells [18].

The expansion of soybean into the low latitudes of northeast Brazil has been facilitated by cultivar development for adaptation to the soils and climate of this region. This process has allowed for a large expansion of soybean production into the Cerrado region of Brazil [1].

Cultivar Tropical was first released in 1980 for production in the low latitude regions of Bra‐ zil. This created a continual demand for more adapted cultivars [4]. Other cultivars released

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for northeastern Brazil between 1982 and 1998 are: Timbira; BR 10 (Teresina) and BR 11 (Carajás); BR-27 (Cariri) and BR-28 (Seridó); BR- 32 (Nova Tropical); BR/ EMGOPA 312 (Po‐ tiguar) and BR 35 (Rio Balsas); Embrapa 9 (Bays); Embrapa 30 (Vale do Rio Doce), Embrapa 31 (Mina), Embrapa 33 (Cariri RC) and the Embrapa 34 (Teresina RC); Embrapa 63 (Mira‐ dor); MA/BR 64 (Parnaíba) and MA/BR 65 (Sambaíba) and MA/BRS-164 (Patí) and MA/ BRS-165 (Seridó RCH).

Characteristics that have been selected for production in low latitude areas are: suitable ma‐ turity, adequate plant height, height to first pod of more than 12 cm ,a non-woody stem, erect plant stature, lodging resistance, pest resistance, good seed quality and high oil content and protein content, high yield, yield stability (robustness), uniform ripening, indehiscent pods, high capacity for N2 fixation, tolerance to aluminum and manganese toxicity, a long juvenile phase and photoperiod insensitivity. Across planting dates and environments, the growing season can vary from 75 to 200 days. Thus a wide range of cultivars have had to be developed for adaptation to many environmental conditions [22, 43]. The average height of the plant can range from 20 to 150 cm or more, according to the cultivar and environment, and the height to first pod may be characteristic of the cultivar. Environmental factors or cultural practices may also affect height to first pod [35], which prevents mechanical har‐ vesting and can cause harvest loss.
