Promoting Sustainable Potato Cropping Systems

1.) Objective 1 was accomplished through review of all of the current BMP guides for potato production in effect for the Pacific Northwest and then working collaboratively with various scientists, agronomists, and growers in the region to publish a BMP guide.

2.) The Ag Input spreadsheet in Objective 2 was worked on but then abandoned during the course of the project to avoid duplication with a computer program published by the University of Idaho’s Agricultural Economics and Rural Sociology department’s Crop Enterprise Budget Worksheet (CEBW) program (Patterson, 2006). Efforts were made to work with the economist annually refining this program to integrate the BMP ideas into the use of this program.

3.) Objective 3 was accomplished through one-on-one survey with 38 growers at the beginning and ending of the project (the initial number of growers was higher, but was reduced by the end of the project due to some of the growers leaving the potato industry due to financial, health, or other reasons). The growers were randomly selected from a list of regional potato producers found in University of Idaho’s Potato Conference data base. The growers were called by the project director in 2001 and asked to participate in the verbal survey. Among other questions, the growers were given a list of five BMPs covering a range of potato production disciplines. The five selected BMPs included: 1) applying fertilizer based on soil sampling and research-based recommendation tables, 2) planting seed pieces at a six-inch depth, 3) cultivating at or just prior to emergence, 4) measuring actual water use and cutting back irrigation during late bulking, and 5) applying protectant fungicides just prior to row closure and 7-14 days later with additional as-needed applications based on scouting and local pathogen pressure levels. The growers were asked if they were aware of the recommendations and if they had implemented these practices on their farm. These same growers were contacted in 2006, at the end of the project, and asked similar questions, as well if they were aware of this BMP project and how effective they felt that it had worked.

4.) Objective 4 was accomplished through fourteen potato field demonstrations/trials conducted at various locations across Idaho, Oregon, and Washington over four years (Table 1). Plot size was 40 ft. by 12 ft. (4 rows) with 10 to 12 in. in-row seed piece spacing.

Field selection criteria were based on an attempt to highlight “model growers” who best exemplified successful production and whose management coincided with research-based BMPs, with a maximum economic yield approach as a normal part of farm management. The cooperating scientists in each of the regions where trials were conducted were responsible for selecting the grower(s). Five replicates of two treatments, best management practices (BMP) management and maximum yield management (MYM), were established in randomized complete blocks (RCBD) in each field. This project is distinctive in that the treatments were not based on specific, controlled practices, but rather consisted of a comparison of two management approaches. There were many BMPs for growers to follow (Hopkins et al., 2006), but fertilizer and pesticide inputs were the focus of this project. In general, the cooperating “model growers” used the following practices with regard to fertilization and pesticide BMPs.

Pre-emergence fertilizer recommendations were based on reasonably intensive soil sampling and analysis with the amount of fertilizer applied derived from tables formulated from cooperative university and industry research (Lang et al., 1999; Stark et al., 2004). Typically, the amount of fertilizer recommended in these tables is for producing 90-95% of maximum yield and based on maximum economic yield (MEY). In-season fertilizer recommendations were based on petiole analysis, again with the fertilizer applied based on MEY research. Pesticide application philosophy was similar to the fertilization approach. Initial applications of herbicides, insecticides, fungicides, and fumigants were based on field history and expected pest/pathogen pressure. Additional applications were made only if scouting, sampling, and forecasting models indicated a need. The entire field was managed by the grower using a BMP approach, with little input from the cooperating scientists. As such, it was hoped that these fields would serve as a testament to the validity of BMPs.

A plot area within each field was set aside to compare BMP to MYM treatments. The BMP plots received the same treatments as the rest of the field. The MYM plots received additional fertilizer and pesticide inputs based on each field’s unique circumstances. The decision to apply fertilizer and pesticides was made by the local cooperating scientist in cooperation with the grower and the advising agronomist(s). If a particular input being considered for its potential to increase yields was decided against by the BMP grower because previous research data indicated the cost would likely exceed the benefit, the input was then applied only to the MYM plots. For example, the ID1 field had a substantial number of apparent weed escapes. The grower was contemplating applying an additional herbicide application, but the grower’s agronomist recognized that the emerged weed seedlings would probably be inhibited enough by the existing herbicide in the soil and, therefore, an additional application would not likely prove to be cost effective. The grower decided to follow this advice for the whole field. However, the near zero tolerance approach called for an herbicide application in the MYM plots. The grower’s decision proved to be justified, as the weeds in the areas outside the MYM plots were only slightly worse, with the effect on yield not significant.

Each entire field, including both BMP and MYM plots, typically received at least: 1) fertilizer applications based on university recommendations, 2) seed treatment for pathogen control, 3) two-way tank mix or separate applications of two herbicides, 4) insecticide as a seed treatment or applied at hilling, and 4) two fungicide applications. Those fields having a longer growing season (western Idaho and Washington and Oregon fields) tended to receive proportionally higher input rates by necessity. The additional inputs for these longer growing season areas typically included one additional insecticide application and several fungicide applications.

The added inputs for the MYM plots in each field are listed in Table 2. A majority of the fields received additional pre-emergence or water-run fertilizer in the MYM plots with an average rate of: 47 kg-N ha-1 (5 fields), 82 kg-P2O5 ha-1 (11 fields), 68 kg-K2O ha-1 (11 fields), 45 kg-S ha-1 (10 fields), 49 kg-Cl ha-1 (8 fields), 6 kg-Mn ha-1 (11 fields), 5 kg-Zn ha-1 (6 fields), 1 kg-B ha-1 (5 fields), 1 kg Fe ha-1 (2 fields), and 0.5 kg-Cu ha-1 (2 fields). In addition, all of the Idaho fields and two of the Washington fields had one or two additional foliar nutrient sprays (generally combined with fungicide sprays) in response to marginal petiole nutrient concentrations. The foliar nutrient sprays ranged from single element to complete nutrient regimes. The Oregon fields did not have foliar nutrient sprays, but had water-run fertilizers instead.

In addition, the MYM plots in all of the Idaho and one of the Washington fields received one to three additional foliar fungicide applications. Because the growing season in the Columbia Basin of Washington and Oregon is typically longer than it is in Idaho, the number of fungicide applications made throughout the season is usually greater. This is a good example of how BMPs may vary by region. The pathogen management BMP for fields with long growing seasons regularly requires frequent fungicide applications, whereas growers in short season areas may be able to avoid frequent fungicide applications, especially when humidity/rainfall and local pathogen pressure is low. As a result, the BMP growers in south central- and eastern-Idaho applied fungicide only one to three times, giving an opportunity for additional applications on the MYM plots. Alternatively, the BMP growers in Oregon and Washington applied fungicide more frequently and this resulted in a lack of opportunity to apply additional fungicide in the MYM plots. In contrast to fungicide applications, the differences between BMP and MYM treatments for the other pesticides were minimal. Only one field had additional herbicide, one had supplementary nematicide, and two had additional insecticides in the MYM plots.

For the MYM plots, water-run and foliar applications of fertilizers and pesticides were applied with compressed gas back-pack sprayers calibrated to deliver label rates with < 5% variation between nozzles. Foliar sprays were applied with manufacturer recommended spreader/stickers to enhance foliar absorption. Water-run treatments were simulated by application immediately prior to overhead irrigation and in the early morning to avoid the drying of product on leaves. Dry fertilizer was applied evenly across each plot with a spinning broadcast hand spreader and then incorporated as a part of the normal field operations (bedding or hilling). Soil samples, to determine pre-emergence fertilizer rates, were taken randomly throughout the field (~15 cores per field) to a 12 in. depth. The fourth fully emerged petiole was collected from the top of ~40 plants across each treatment to determine in-season nutritional needs. Soil samples were submitted to a commercial laboratory for drying and analysis using approved methods.

Tubers were harvested from 20 ft. of the center two rows of each plot, by hand or machine, to determine yield within one to seven days of the grower’s harvest date. Tuber fresh weights were determined gravimetrically. Marketable yields were determined by combining all US No. 1 and US No. 2 size grades. US No. 1 and, in some cases, US No. 2 tubers were separated into various size categories. Break points for size categories included: 4, 6, 8, 10, 12, and/or 14 oz. Cull yields were determined by combining all undersized (< 4 oz.), malformed, and tubers with external physiological, pest, or pathogen related defects. A random sampling of each of the tuber size categories over 4 oz. was saved for specific gravity analysis and internal defect evaluation (minimum of eight tubers per plot). A composite of tubers was used to determine tuber specific gravity using the weight-in-air/weight-in-water method (Kleinschmidt et al., 1984). Internal defects were determined by inspecting a sub-sample of at least eight tubers from each plot and cutting each tuber in half along the longitudinal axis.

Gross crop values were estimated by using five year average regional processing potato prices of $4.95 cwt-1 for marketable potatoes and $1.75 cwt-1 for cull potatoes. This simplified, least common denominator approach in determining crop value (as compared to using price incentives for size, specific gravity, etc.) was used because of the diversity of market scenarios faced by the growers across regions and time during the project. Although the main emphasis presented here is the non-incentive adjusted pricing, incentive adjusted scenarios were also applied to the yield data and presented in part below.

Net crop value was estimated by subtracting the cost of the added MYM inputs from the gross crop value. The cost of added inputs was estimated by determining the average prices of the inputs and custom application charges at the time of application. Field response was determined by subtracting the yields and crop values of the BMP from the MYM plots.

Differences across treatments were determined by ANOVA, with the PROC GLM procedure in SAS software (SAS Institute 1990) with significance indicated at P < 0.05 unless otherwise noted. Statistical analysis was conducted on yield and gross/net return parameters by combining individual field data with treatment and field location as dependent variables. Yield and gross/net return parameters for individual fields were analyzed separately when the treatment by field location interaction was significant.

5-6.) Objectives 5 and 6 were achieved by conducting several public field days located at some of the model grower’s fields, as well as at other grower meetings and with various extension and popular press publications. In addition, growers expressing interest in adopting BMPs were provided one-on-one assistance in evaluating their operation.

Literature Cited in Materials and Methods:

Hopkins, B.G., D.A. Horneck, M.J. Pavek, P.P. Patterson, J.S. Miller, P.J.S. Hutchinson, N.L. Olsen, J.W. Ellsworth, J.M. Alvarez, S.L. Hafez, and H. Neibling. 2006. Best Management Practices for Sustainable Potato Production in the Pacific Northwest. University of Idaho, PNW Publ. (submitted).

Lang, N.S., R.G. Stevens, R.E. Thornton, W.L. Pan, and S. Victory. 1999. Potato Nutrient Management Guide for Central Washington. Wash. State BULL 1871.

Kleinschmidt, G.D., G.E. Kleinkopf, D.T. Westermann, and J.C. Zalewski. 1984. Specific Gravity of Potatoes. Univ. Idaho CIS 609, Moscow, ID.

Patterson, P. 2006. Crop Enterprise Budget Worksheet. Univ. Idaho Ag. Econ. Rural Soc. Dept., Moscow, ID.

SAS Institute. 1990. SAS/Stat user’s guide. Version 6. SAS Inst., Cary, NC.

Stark, J.C., D.T. Westermann, and B.G. Hopkins. 2004. Nutrient Management Guidelines for Russet Burbank Potatoes. Univ. Idaho BUL 840, Moscow, ID.