**3. Rice production and crop rotation**

Rice ranks third behind corn and wheat in total tons of grain produced in the world but it is the primary dietary staple for more people than any other cereal (Raun and Johnson, 1999). It is grown on every continent except Antarctica. By the 1990's rice was providing 35% to 59% of the total calories consumed by nearly 2.7 billion people in Asia (Neue, 1993). Peng et al. (1999) quoted that world rice production would need to be at least 600 million tons by 2025, an increase of 266 million tons above 1995 production just to maintain current nutrition levels. This increase will likely not be sufficient to alleviate current malnutrition in many of the rice dependent cultures (Neue, 1993). In areas where it is virtually the sole source of calories it is seldom grown in rotation with other crops. Anders, et al., (2004)

 Studying the net returns of various crop rotation schemes involving corn, Singer and Cox (1998) calculated a greater net return (\$250 US ha-1) with a corn-soybean rotation than a continuous corn (\$193 US ha-1) or a three year soybean-wheat/red clover rotation (\$133 US ha-1). A recent study reported though, that yield comparisons are not the appropriate basis for decision making on cropping systems but rather economics is most important (Stanger et al., 2008). This report showed that, with the exception of continuous corn grown with 224 kg N ha-1, a corn-soybean rotation was the most stochastically efficient cropping system

Though soybean is one of humankind's oldest crops, it did not really become of significance in the United States until the late 1940's. The species was introduced in Europe from China in the mid 18th century and into the new world in the early 19th century where it was used primarily as a hay crop. The combination of the destruction in China from World War II and the Cultural Revolution removed it as the world's primary supplier of soybeans and opened an opportunity for the United States to develop the crop as a major oil seed (North Carolina Soybean Producers Assn. , 2011). Currently the United States produces about 40% of the world's soybeans followed by Brazil and Argentina combining to produce 50%. Besides corn, soybean is being rotated with rice or cotton in the Mid South and Southeastern States (Anders et al., 2004: Stallcup, 2009). In the eastern Great Plains soybean is often rotated with wheat or grain sorghum (*Sorghum bicolor* L. Moench) as well as corn (Kelley et al., 2003). Kelley et al. (2003), found that in general soybean yields grown in rotations with wheat or grain sorghum produced a 16% greater seed yield than when grown in a monoculture (Table 6). One of these rotations was soybean double-cropped behind winter wheat which is frequently practiced in areas of the United States south of 39o N latitude. This practice does risk failure from drought either causing poor emergence or poor seed set. Above 39o N there is also the risk of early frost terminating growth and above 40o N the

**Rotation†** W-S/S W-Fal/S GS/S Cont. S **Yield Mg ha-1‡** 1.91 2.09 1.99 1.68

Table 6. 10 Year average 2nd year soybean yields in a two year rotation scheme at Columbus,

Prior to the extensive production of soybean for seed in the United States, and important rotation scheme in much of the New England, Mid-Atlantic and North Central states was corn-winter wheat-red clover. Frequently the red clover would be over seeded in late winter or early spring in the developing wheat crop. Many times timothy (*Phleum pretense* L.) a cool-season perennial grass would be seeded along with the red clover. The mixture

†W-S/S=Wheat-double crop soybean/soybean; W-F/S= Wheat-fallow/soybean; GS/S= Grain

across a range of N fertility treatments and other rotation schemes.

practice of double-crop soybean after wheat is not advisable.

sorghum/soybean; Cont. S= continuous soybean. **‡**All means are significantly different by LSD (P0.05).

KS from1980 to1998. (Kelley et al., 2003)

**5. Rotations for forage crops** 

**4.1 Other soybean rotations** 

stated that producers growing continuous rice will likely experience lower grain yields than those using a rice-soybean rotation.

A common rotation with rice in southern and eastern Asia is a rice-wheat rotation system that occupies an estimated 24 to 27 million hectares (Wassmann, et al., 2004). Lattimore (1994) reviewed the literature pertaining to rice-pasture rotations in southeastern Australia. Annual pastures based on subterranean clover (*Trifolium subterraneum* L.) are well adapted to this part of the world and the rice cropping system. It provides considerable fixed N to the rice crop thus reducing the need level of supplemental N fertilizer as well as breaking weed cycles. It helps sustain a complimentary animal agriculture to use crop residues and provides opportunities for improved farm income. With respect to disease control in rice, both false smut (*Ustilaginoidea virens* (Cooke) Takah) and kernel smut (*Neovossia horrid*a (Takah.) Padwick & A. Khan, syn. *Tilletia barclayana* (Bref.) Sacc. & P. Syd.) two serious fungal pests in rice production areas of the United States, appear to be best controlled when rice is grown in three year rotations with soybean and corn between rice crops (Brooks, 2011). Traditional rotations of rice-soybean, with winter wheat grown between the two summer annuals, were observed to have the highest levels of these diseases especially with high N- fertility levels.
