Conservation Tillage and Weed Management

Tillage has long been an essential component of traditional agricultural systems. Broadly defined, tillage is the mechanical manipulation of the soil and plant residues to prepare a seedbed for crop planting. The benefits of tillage are many: it loosens soil, enhances the release of nutrients from the soil for crop growth, kills weeds, and regulates the circulation of water and air within the soil (Reicosky and Allmaras, 2003). In some cases, however, intensive tillage has been found to adversely affect soil structure and cause excessive breakdown of aggregates, leading to soil erosion in higher-rainfall areas. Intensive tillage can also have a negative impact on environmental quality by accelerating soil carbon loss and greenhouse gas emissions (Reicosky and Allmaras, 2003). Further, tillage operations account for more than 25 percent of agricultural production costs (Carter, 1996). With recent increases in fuel prices, tillage now accounts for a higher proportion of production costs than harvesting does (Edwards and Smith, 2005). Such concerns have fueled interest in finding tillage systems that minimize negative impacts to the environment while sustaining economic crop productivity. The tillage systems being developed and studied to address these concerns can broadly be termed conservation tillage (CT). In California, conventional tillage practices face additional challenges as population centers expand into farming areas and new residents raise serious concerns about the air quality effects of smog and dust (PM10, particulate matter 10 microns or less in diameter) emissions from farm machinery and vehicle use. Growers in California are looking at CT as a possible way to reduce their operating costs. Estimates from the Conservation Technology Information Center (CTIC, 1998) showed that by switching to CT, a U.S. grower can save as much as 225 labor hours and 1750 gallons of fuel per year on just 500 acres. Machinery would be used less, and that would mean an additional savings of an estimated $2500 in machinery wear. Conservation tillage is an umbrella term that encompasses many types of tillage and residue management systems (Reicosky and Allmaras, 2003). There are several definitions for CT. For example, Allmaras and Dowdy (1985) define it as “a combination of cultural practices that result in the protection of soil resources while crops are grown.” The Conservation Technology Information Center (CTIC) defines CT as any tillage and planting system that leaves at least 30 percent of the soil surface covered by residue after planting. California’s CT Workgroup characterizes it as a crop production system that deliberately reduces or eliminates primary intercrop tillage operations such as plowing, disking, ripping, or chiseling, and that manages surface residues so as to permit efficient planting, pest management, and harvesting. Several U.S. states have developed innovative tillage systems that conserve soil and residue and maintain crop productivity. However, findings in these states do not transfer directly to California because of differences in climatic and soil factors, dependence on irrigation and specific types of irrigation, and the overwhelming diversity of cropping systems in California. Mitchell et al. (2005) estimated that less than 2 percent of California’s cultivated crop land is under some form of CT, based on CTIC’s PUBLICATION 8200