Cropping Sequence and Rotation: Impact on Potato Production and Soil Condition

Cropping sequence and length of rotation play an important role in potato production. Too often, growers are forced to base their cropping system decisions on short-term economic survival. Our market-based economy rewards economic efficiency and has helped push the trend to larger farms with lower production costs. Improving economic efficiency, with large, specialized potato equipment, means farming more potato acres. If the farm size can‟t be increased, then the rotation must be shortened to get more potato acres. But shorter rotations can drive up input costs and reduce yield and tuber quality and, what appeared to be a good economic decision, often turns out to be a cropping system that is not sustainable. It is a mistake to assume that yield, tuber quality, and input costs will remain static as rotations are shortened. Many factors need to be included in the cost of production estimates that all farms should be calculating as they consider length of rotation and cropping sequences. These factors include yield and tuber quality loss due to increased pest damage and decreased soil health, as well as increased operating costs in response to these negative impacts. It is also important to consider the risk of development of pesticide resistance in poorly designed cropping systems. Risk of developing pesticide resistance is much greater as the years between potato crops decreases and if pesticides of the same mode of action are used in the potato and the rotational crops. Truly sustainable cropping systems must balance agronomics and economics over both the short and the long term. PESTICIDE RESISTANCE Pesticide resistance is defined as the ability of a pest population to survive a pesticide treatment to which the original population was susceptible. Weeds, disease pathogens, and insects are all pests capable of developing resistance. Pesticide-resistant organisms may occur naturally within a population in very low numbers. However, repeated use of pesticides with the same mode of action (MOA) (the sequence of events in which a pesticide kills a pest organism) allows the initially small percentage of resistant organisms in the population to survive and reproduce while susceptible organisms are killed. As a result, the percentage of the pest population that is resistant to the MOA being used repeatedly becomes so large that the MOA is no longer effective on that pest population (Figure 1). Presented in two parts at the Idaho Seed Potato Conference, January 20, 2004, and the Idaho Potato Conference, January 22, 2004. There are several mechanisms by which a pest organism may be pesticide resistant. (1) The site within the organism where the pesticide acts (site of action) may be altered so that the pesticide can no longer interfere with the organism. (2) A resistant organism may metabolize or detoxify the pesticide more quickly than a susceptible organism; and (3) A resistant organism may be able to sequester the pesticide away from the site of action or block uptake or translocation so the pesticide never arrives at the site of action. Pest populations may develop multiple-resistance or cross-resistance. Multiple-resistance occurs when a population is resistant to pesticides with different MOAs. Populations with cross-resistance are resistant to different pesticides that all have the same MOA. Figure 1. Resistant weed selection from repeated use of herbicides of the same mode of action. A similar effect occurs with disease pathogens, insects, and nematodes. Development of pesticide resistance can be prevented or delayed by use of several good management strategies. One such strategy is alternating pesticides with different MOAs, both within and across seasons. Increasing the length of rotation also may help prevent the development of pesticide resistance by providing more chances to use MOAs different than what may be absolutely necessary to use in potatoes. Herbicides such as EPTC (Eptam®), metribuzin (Sencor®), pendimethalin (Prowl®), rimsulfuron (Matrix®), or s-metolachlor (Dual Magnum®/Dual II Magnum®) are commonly and effectively used in potato fields in Idaho. Commonly used nematicides include Vapam®, Telone®, Mocap®, Temik®, and Vydate®. Strobilurin fungicides (e.g. Quadris®, Headline®) are commonly and effectively used in potato fields in Idaho for control of early blight. Increasing the frequency of potatoes grown on a particular field increases the likelihood of application of these pesticides and, as a result, increases the risk of development of pesticide resistance. The negative impacts of poor pesticide resistance management strategies can affect not only the fields in question, but resistant populations can spread to neighboring fields and even regions. It is common knowledge that growing potatoes in the same location year after year is unwise for many reasons. One such reason is the likelihood of building up pest populations for which potatoes are a primary host and/or are more difficult to control in potatoes. Similarly, it is usually unwise to select rotational crops that do not provide at least one year of a break in a pest‟s life cycle. Rotational crops often provide an opportunity to more effectively and/or inexpensively control certain pests. For example, inserting a grass crop (such as wheat, barley, or corn) between broadleaf crops (such as potatoes, sugar beets, and/or alfalfa) can allow for the use of a phenoxy herbicide, such as 2,4-D or 2,4-DB, which has the advantage of being less costly and having a different mode of action than most herbicides used in broadleaf crops. Other specific cropping sequence issues will be discussed in the sections below.