Soybean acreage has increased rapidly over the last 20 years, from approximately 60,000 to more than 400,000 acres. There are several reasons for the rising interest in soybean production: (1) producers have learned they can grow the crop successfully, (2) markets have become more available, (3) effects of rotation, which improves the yield of the succeeding crop, (4) a recognition that soybeans are an excellent source of farm-grown protein, and (5) the availability of glyphosate tolerant varieties, which simplify weed control. Soybeans are basically a cash crop, but the acreage being used as a farm-grown protein source is increasing yearly.
Soybean (Figure 1.6-1) has two types of growth habit, determinant and indeterminate. Determinant types stop vegetative growth once reproductive development starts. Indeterminate types, on the other hand, continue vegetative growth after reproductive growth begins. The determinant character is found in varieties grown south of Pennsylvania. Varieties grown in Pennsylvania show the indeterminate character. This difference in growth habit is one of the main reasons row spacing and population recommendations differ between northern and southern growing areas.
As is true for other grain crops, soybean growth and development are influenced by temperature. Soybean, however, is very sensitive to photoperiod, or day length, and does not move from vegetative to reproductive growth until a critical day length is met. This requirement restricts a variety’s adaptability to a band about 150 miles north and south of its origin. Day length does not restrict the adaptability of varieties planted at the same latitude (east and west). Soybean varieties are placed in one of 12 maturity groups. These range from 00 (Canada) in the north to X in the south (Florida). Since Pennsylvania falls within the same latitudes as the major growing areas of the Midwest, our growers have access to the same varieties. These include varieties in maturity groups II, III, and early IV.
Soybean is classed as an oil seed crop, but the seed contains twice as much protein as oil (40 percent protein versus 20 percent oil).