**6. Summer fallow**

Summer fallow, the practice of growing a crop every other year and in most systems, controlling weed growth during the off-year, has been used almost exclusive in semi-arid regions for the production of wheat or other small grains. Its purpose is to accumulate sufficient limited rainfall during the fallow year to grow a crop the following year and to break crop disease cycles. Most of the world's drylands are in developing countries where water resources are usually limited by a lack of rainfall and potential irrigation (Ryan et al., 2008). Stewart et al. (2006) has stated that dry regions worldwide supply about 60% of human food stuffs. Areas where this is a common practice are the Mediterranean, semiarid regions of Africa, Asia, Australia, the Pacific Northwestern United States, and the western regions of the Canadian and United States Great Plains. Many of these areas practice a "clean" summer fallow where the land is kept weed free during the fallow year, usually by cultivation or herbicides. However, in northern Africa a weedy fallow is often employed where palatable weeds and volunteer crop plants are allowed to grow and then be grazed by livestock (Ryan et al., 2008). This allows for an animal agriculture to exist that provides needed calories and returns manure to the land to provide nutrients for the following crop.

In semiarid areas of the Pacific Northwestern United States summer fallow wheat production has been practiced nearly 130 years (Schillinger et al., 2007). In recent years though, studies have been conducted in semiarid regions of the United States and Canada to adapt a continuous cropping system to replace summer fallow. Water storage efficiency in most summer fallow-wheat rotations is usually less than 25% of the rainfall received during a 14 month period using conventional tillage (McGee et al., 1997). Research from Colorado and Nebraska demonstrated that precipitation storage efficiency could be improved to 40% to 60% through minimum or no-till which allows crop residue to remain on the soil surface and soil disturbance is held to a minimum or eliminated (Croissant et al., 2008). However, such practices added little or no increase in wheat yields and it was concluded that the resultant water savings could only be converted to profit by employing intensive cropping systems where fallow time is decreased and summer crops such as maize, grain sorghum, or annual forages and included in the rotation. Under no-till it was determined that it cost more to save the additional water than the value of the added grain yield. Lyon et al. (2004)

*altissima*), and Rhodes grass (*Chloris gayana (Kunth)*) (Verdramini et al., 2010). All of these grasses except atra paspalum, bahiagrass and Rhodes grass require vegetative propagation in order to be established. As with temperate pastures, the pasture must first be properly prepared by controlling weeds, insects, removing old or dead vegetative material and fertilizing to soil test recommendations. Burning is often used in preparing bermudagrass pastures for renovation (Stichler and Bade, 2005). Tillage operations are often performed during dry periods in the spring to further control weeds and old sward growth as well as better prepare the land for vegetative propagation with the onset of summer rains (Stichler and Bade, 2005;Verdramini et al., 2010). In both the subtropical and temperate climates warm season annual forages may be included into crop rotations on land used primarily for cultivated crops. This is to provide additional pasture, hay or silage for livestock operations. Species such as pearl millet (*Pennisetum glaucum (L.) R.Br.),* sudangrass *(Sorghum bicolor* subsp. *Drummondii* (Steud.) de Wet ex Davidse), and other forage sorghum

Summer fallow, the practice of growing a crop every other year and in most systems, controlling weed growth during the off-year, has been used almost exclusive in semi-arid regions for the production of wheat or other small grains. Its purpose is to accumulate sufficient limited rainfall during the fallow year to grow a crop the following year and to break crop disease cycles. Most of the world's drylands are in developing countries where water resources are usually limited by a lack of rainfall and potential irrigation (Ryan et al., 2008). Stewart et al. (2006) has stated that dry regions worldwide supply about 60% of human food stuffs. Areas where this is a common practice are the Mediterranean, semiarid regions of Africa, Asia, Australia, the Pacific Northwestern United States, and the western regions of the Canadian and United States Great Plains. Many of these areas practice a "clean" summer fallow where the land is kept weed free during the fallow year, usually by cultivation or herbicides. However, in northern Africa a weedy fallow is often employed where palatable weeds and volunteer crop plants are allowed to grow and then be grazed by livestock (Ryan et al., 2008). This allows for an animal agriculture to exist that provides needed calories and returns manure to the land to provide nutrients for the following crop. In semiarid areas of the Pacific Northwestern United States summer fallow wheat production has been practiced nearly 130 years (Schillinger et al., 2007). In recent years though, studies have been conducted in semiarid regions of the United States and Canada to adapt a continuous cropping system to replace summer fallow. Water storage efficiency in most summer fallow-wheat rotations is usually less than 25% of the rainfall received during a 14 month period using conventional tillage (McGee et al., 1997). Research from Colorado and Nebraska demonstrated that precipitation storage efficiency could be improved to 40% to 60% through minimum or no-till which allows crop residue to remain on the soil surface and soil disturbance is held to a minimum or eliminated (Croissant et al., 2008). However, such practices added little or no increase in wheat yields and it was concluded that the resultant water savings could only be converted to profit by employing intensive cropping systems where fallow time is decreased and summer crops such as maize, grain sorghum, or annual forages and included in the rotation. Under no-till it was determined that it cost more to save the additional water than the value of the added grain yield. Lyon et al. (2004)

and sorghum x sudangrass crosses often fill this role (Hancock, 2009).

**6. Summer fallow** 

reported that winter wheat yields in the central Great Plains were negatively affected by eliminating the 11 to 14 month summer fallow by spring planting a transitional crop before wheat in the fall. However, a spring planted forage crop that was harvested early had a minimum negative impact on wheat yields and that the value of the forage combined with the following wheat yields resulted in greater income than the traditional winter wheatfallow rotation. Research from the Horse Heaven Hills region of Washington looked at a continuous no-till hard red spring wheat system verses a winter wheat-fallow rotation and found the hard red spring no-till system did not match the winter wheat-fallow system of production in yield or income. However, the continuous no-till system did offer a benefit of providing ground cover that reduced wind erosion and air pollution by dust particles (Young et al., 2000).

Summer fallow is also practiced on fields used to produce castor bean (*Ricinus communis* L.) the previous year. This is not done for accumulating moisture but to rid the field of any volunteer plants. Castor bean contains a very deadly toxin, ricin, which in very small quantities can kill humans and livestock. Summer fallow in this case allows volunteer plants to be destroyed and the field cleaned for future feed and food crops.
