Conservation Tillage and Weed Management

Abstract

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 acceler-ating 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 practic-es 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 combina-tion 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 produc-tion 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 diver-sity 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 UNIVERSITY OF CALIFORNIA Division of Agriculture and Natural Resources http://anrcatalog.ucdavis.edu definition. By that view, the acreage under CT in California is very low in comparison to that in several other states. It is interesting to note, however, that with the advent of chemical herbicides, the concept of eliminating both tillage and cultivation from crop production had its first evaluation in a California orchard, in 1944, using a practice called " chemical fallow " (Owens, 2001). As herbicide-tolerant crops (HTCs)—mainly cotton (Gossypium sp.) and corn (Zea mays L.)—have increased, so has interest in CT systems among California grow-ers. Along with the availability of HTCs, several other factors including increased fuel prices, access to better CT, global positioning system (GPS) technology, and environ-mental air quality issues have had the combined effect of increasing interest in CT systems in California. Conservation management plans (CMPs), now required by the San Joaquin Valley Air Pollution Control District (SJVAPCD), can include HTCs such as Roundup Ready crop varieties and the reduction or elimination of tillage as accept-able practices for dust reduction. The SJVAPCD (2004) suggests that the reduction in the number of passes and tillage that accompanies these practices can reduce soil and water losses and mitigate dust problems. Similarly, there is increased interest in testing CT systems in other non-HTC varieties such as tomatoes, wheat, oats, and dry beans in California. Reduced tillage, however, often brings with it changes in weed species and popu-lations, and therefore in weed-management needs, and this is a major concern for the growers who may want to adopt CT systems (Buhler et al., 1994). Phillips and Young (1973; as cited in Owens, 2001) stated that the vital factor for success of no-till row crop production is weed control, and that this depends largely on the proper use of suitable herbicides. For this reason, our focus in this publication is on the weed management issues in CT and we will suggest some techniques for the successful implementation of CT systems in California. T ILLAGE AN d WEE d MANAGEMENT