Are alternative agricultural management practices right for water runoff?
By W. R. Horwath, A. Ristow, and Z. Kabir

	Will Horwath, SAFS principal investigator, and researcher Sam Prentice (r) install storm-runoff monitoring equipment in a UCD test plot.  (Photo by Z. Kabir/A. Ristow)

One of the main goals of our sustainable agriculture efforts at UC Davis is to promote “best management practices” (BMPs) to California growers by altering existing practices and implementing new ones to mitigate and improve runoff water quality. The Sustainable Agriculture Farming Systems (SAFS) project team has 16-years experience working with alternative agricultural practices in furrow-irrigated row crop production in the Central Valley. Our long-term research plots have been invaluable for quantifying interactions between farm systems (e.g., organic vs. conventional) and agronomic management practices and decisions. A unique aspect of our research is to collaborate with area growers to design and evaluate our research. The extension of research plot work to grower fields provides a mechanism to validate and extend our results to the larger field scale. Conservation tillage (CT) and winter cover cropping (CC) are alternative management practices that are proposed to reduce runoff and minimize nutrient and sediment losses. These practices have been widely adopted in the U.S. in the East and Midwest. The fact that CT and CC are working in other geographic regions does not mean it will work equally well in California’s Mediterranean climate, which is one of the reasons the adoption of these practices in California has been slow. Insufficient research has been done on the integration of CT and CC practices into existing practices, and on their effectiveness at mitigating pollutants and runoff loads. California growers cite this as the primary reason for their low adoption rates: lack of successful local examples.

Conservation Tillage in California

Large-scale adoption of CT practices in the U.S. began in the Midwest. This was done primarily to reduce soil losses from water and air erosion, and retain water in the soil profile in drier areas during fallow periods. Over the past four decades, CT in the Midwestern U.S. has increased by 300 percent. In contrast, by 2004, less than five percent of California Central Valley growers were practicing CT.

UC Cooperative Extension defines CT as a farming practice that leaves more than 30 percent of the crop residue on the soil surface or reduces tillage passes by at least 40 percent. Typical tillage systems used in California incorporate most crop residue into the soil to facilitate planting and furrow irrigation. There are many reasons why CT is not being adopted in California. Compared to Midwestern agriculture, California has a diverse system of irrigated crops. The development of CT practices requires developing individual management practices for a wide variety of crops grown in California. This is a major challenge to California growers compared to the Midwest where the CT learning curve is less steep because only a few crops are grown. In California, will deep tillage be necessary to allow water penetration and leaching of salts? Will maintaining residue on the soil surface interfere with furrow irrigation? Will the reduction in in-season cultivation of weeds increase herbicide use? Will changes in residue management require reassessment of fertilizer recommendations? Will runoff water contain higher dissolved organic carbon or herbicide levels under CT and CC? These questions require more research.

Economically, CT provides benefits by reducing labor, lowering fuel consumption, and decreasing capital investment in machinery. Agronomically, CT increases organic matter, enhances water infiltration and improves soil tilth. CT can minimize environmental impacts by reducing wind erosion and improve water infiltration. The challenges of implementation seem daunting to California growers; our research will assess the potential economic, agronomic, and environmental benefits of CT.

Cover Cropping in California

As with CT, the implementation of cover crops has lagged behind other parts of the country. CC has been adopted in less than five percent of the row crop area in California. This is primarily due to issues of delayed field entry in the spring, additional production costs, build-up of noxious weeds, and reassessment of fertilizer recommendations. All these factors can negatively affect the economic viability of farm operations. Increased irrigation water use due to enhanced infiltration may translate into decreased water use efficiency as in systems using CT. It is hoped that California growers will begin to adopt CC practices as more research continues to demonstrate its benefits, which include increasing soil fertility, interrupting pest cycles and decreasing winter water runoff.

Research Needed

The adoption of CT and CC in the Central Valley requires research into the integration of these practices into California’s distinctive climate. Questions remain about how to maintain the state’s phenomenal yields, which have required considerable inputs such as intensive tillage, fertilization, and furrow irrigation. Increased water infiltration during summer irrigation as a result of the positive influence of CT and CC practices on soil properties will reduce irrigation water use efficiencies. This may require additional irrigation water in the future and thus compete with population growth and environmental needs. Our research is addressing these issues by examining the effectiveness of CC species for fertility and runoff abatement and determining tillage methods to reduce soil disturbance and enhance profitability. Specific crop rotations using both traditional crops and new crops may be required to optimize the adoption of CT and CC. The interfacing of subsurface drip irrigation with CT and CC practices has not been examined and requires further development. These are some of the questions and challenges UC Davis investigators are addressing on research and grower-collaborator fields in their efforts to establish BMPs that reduce inputs and minimize winter runoff while enhancing farm profitability.

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