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© 2008 Plant Management Network. The Economic Impacts of Disease Suppressive Rotations in Maine Potato Cropping Systems John M. Halloran, Robert P. Larkin, and C. Wayne Honeycutt, USDA-ARS New England Plant, Soil, & Water Laboratory, Orono, ME 04469 Corresponding author: John M. Halloran. john.halloran@ars.usda.gov Halloran, J. M., Larkin, R. P., and Honeycutt, C. W. 2008. The economic impacts of disease suppressive rotations in Maine potato cropping systems. Online. Crop Management doi:10.1094/CM-2008-0912-01-RS. Abstract Soil borne diseases can greatly reduce marketable yields and are a major concern to Maine potato (Solanum tuberosum L.) producers. Control of these organisms is difficult as they can persist in the soil for several years and symptoms typically worsen each year. Depending on the specific pathogen, different mechanisms for moderating soil borne disease can be employed, such as breaking the host-pathogen cycle, stimulating beneficial microbial activity, and direct inhibition of the pathogens. Current research at the USDA-ARS New England Plant, Soil, and Water Laboratory is investigating the effect of rotation crops and rotation sequences on the incidence and severity of soil-borne diseases in the following potato crop. These results (incidence, severity, and potato yield) are incorporated into an economic simulation model to determine the impact of rotations on profitability, level of income risk, and probability of economic loss for different rotation sequences and rotation lengths. The results show that several rotation sequences can increase profitability and reduce economic risk when compared to continuous potato or to the standard barley-potato rotations. Introduction Potato (Solanum tuberosum L.) producers in Maine have identified the need for profitable rotation crops and disease control as two of their most pressing issues. Research at the New England Plant, Soil, and Water Laboratory (USDA-ARS) is addressing these issues through rotation crop trials to identify rotation sequences that are disease suppressive and profitable. There are several recognized benefits to crop rotation. In general, they lead to increased and sustained productivity of the target crop(s). Productivity can be measured as increased per unit marketable output/unit of input effort. Increased productivity can be enhanced via several mechanisms: (i) improvement in soil quality; (ii) improved nutrient cycling; (iii) disruption in disease and pest cycles; and (iv) reduced environmental degradation. These mechanisms can have a direct impact on farm profitability. Increased marketable yields – if achieved at equal or lower per unit costs – lead to increased profitability. Or increased profitability can be achieved via cost reduction, such as reduced fertilizer or pesticide usage and the attendant application costs of machinery and labor. The use of rotation crops does not assure increased profitability. Even when crop rotation delivers some of the aforementioned benefits, producers may be unwilling or unable to institute them because of low or negative returns that the rotation crop may earn. For example, in the Maine potato system the typical rotation is two-years, primarily barley (Hordeum vulgare L.) under-seeded with clover (Trifolium spp.) followed by potatoes (Solanum tuberosum L). The producers earn little positive return from the barley crop, although there are agronomic benefits from its inclusion. However, it is still possible to derive economic benefits when rotation crops make little contribution to farm profitability. In most cases, the inclusion of rotation crops will reduce income variability and the likelihood of negative returns over the long term in addition to their beneficial agronomic and environmental benefits. The combination of these benefits can lead to a more sustainable operation. Heady (4) was the first economist to examine the impact of rotation crops
on income variability. He noted that the addition of rotation crops to a fixed
cropping pattern was analogous to diversifying an investment portfolio. When a
single crop is grown all income variability (dI²) is a
function of its yield and market prices, which are stochastic variable. The
addition of rotation crops can lead to reduced fluctuations in annual net
income, as well as potentially increasing net income. With crop rotation income
variability is a function of all crops’ prices yields or Since Heady’s article there has been a proliferation in the economic analysis of risk in agriculture [see (3) for a review]. Much of this work has been devoted to analyzing the economic risk associated with different cropping systems and strategies. These efforts have shown that producers are not indifferent to risk levels inherent in a particular cropping system or strategy. The selection of a strategy is tied to the producer’s risk preference (risk averse, risk neutral, or risking loving). In most studies, producers are found to be predominately risk averse. Thus, they are more likely to adopt a strategy that provides stability in net returns over a strategy that may a higher payoff, but also has a higher probability of incurring an economic loss. Previous Work on Crop Rotations The literature on rotation impacts is large and only a few studies are cited to demonstrate the potential impacts. Zentner et al. (9) found that the traditional mono-culture of cereal grain cropping systems in the Canadian prairies is being transformed to include small oil-seeds and pulses. These practices are leading to higher and more stable incomes in most soil-climatic regions. The adoption has also resulted in yield and substantial resources savings. In a review of several studies using ground covers as a rotation crop Snapp et al. (8) examined the costs and benefits of cover crops. They found the greatest disincentive to adoption was the lack of a marketable crop. A related issue was that the benefits of cover crops may develop only after a long time period, thus increasing short-term financial pressures. However, they also found that after adoption most producers were pleased with the outcome. Peters et al. (7) found that three year rotations significantly lowered levels of potato canker and black scurf caused by Rhizoctonia (Rhizoctonia solani Kǜhn) in plants versus those grown under a two year rotation. They also found similar results with other soil-borne diseases. Their results support the idea that soil ecosystems can be modified to improve disease suppression through rotation sequencing. A study conducted in Canada (1) measured productivity parameters and soil health in two year rotations in potato production. The experiment was carried out for a period of 11 years and showed that potato crop productivity and soil organic carbon were generally maintained in rotations that include Italian ryegrass, but declined under rotations with red clover and barley. Krupinsky et al. (5) conducted research to determine if crop diversification and crop sequencing can influence plant disease risk in cropping systems. They evaluated ten different crops and sequences on the presence of leaf diseases on barley and wheat (Triticum aestivum L). The concluded that certain crops lead to reduction in disease risk, but the crops and sequences varied with respect to barley or wheat. Bail-Coehlo looked at using rotation crops as a bio-control for root lesion nematode in potato cropping systems. The rotation crops were pearl millet and marigolds. They found that both rotations lead to significant reductions in disease damage. Furthermore, they found the rotations to be as effective as the standard rye with fumigation industry practice. Working in Maine potato cropping systems, Halloran et al. (2) determined that two-year crop rotations did significantly lower income variability as compared to a continuous potato rotation. In some cases, when high value crops were used as rotations, net income also increased relative to the continuous potato rotation. Investigating Rotation Effects on Soil-borne Diseases in Potato Eight 3-year cropping systems were evaluated for their effects on the development of Rhizoctonia (R. solani) disease of potato. The rotations consisted of soybean (Glycine Max L.)-canola (Brassica napus L.), soybean-barley, sweet corn (Zea mays L.)-canola, sweet corn-soybean, green bean (Phaseolus vulgares L.)-sweet corn, canola-sweet corn, barley-clover (Trifolium pretense L.), and consecutive potato (non-rotation control) plantings. All cropping systems included potato as a third crop. The experiment was conducted on field research plots established in 1998 in Presque Isle, ME, as a randomized complete block design with eight different rotation treatments each represented at two different entry points (either potato grown in the first year or potato rotation crop beginning in the third year) with four replications of each. Thus, not all rotational possibilities were present in each year (they were present in 2 out 3 years). Prior to the implementation of the study, the plots were in continuous potatoes except for the immediate year before commencement of trials, when the plots were seeded to oats. Disease incidence and severity were evaluated on potato crops in the third or fourth year of the rotations. All crops were managed using recommended production practices, including fertilizer rates, pesticide applications, and weed control measures for that particular crop. After harvest potatoes were washed, graded, and rated for incidence and severity of black scurf. Yield was evaluated as total weight of potatoes per 12.2-m row, and the weight of misshapen tubers was also determined. Using the results from the field trials with respect to disease incidence, Monte Carlo simulations were performed using Latin Hypercube sampling to estimate the expected mean net income and income variability for each crop rotation. Each simulation run contained 5000 iterations. In the simulation, crop yields and prices are treated as stochastic variables with distributions defined by historical state data. While historical potato yields are used to define the distribution, the final yield for each potato crop is adjusted downward reflecting the average percentage of misshapen tubers, which are unmarketable. Enterprise budgets were developed for crops. The budgets were assumed assuming a 400-acre operation to develop equipments lists. Correlation coefficients between the yield and price of potatoes with the yields and prices of the rotation crops were calculated using the historical data (Table 1). Table 1. Simple correlation coefficients between potatoes and rotation crop species.
Yield and Economic Impact of Disease Incidence & Severity All cropping sequences significantly reduced the incidence and severity of black scurf on potato tubers in 2000 relative to the continuous potato control (see Table 2). As reported by Larkin and Honeycutt (6), "Potato crops following barley, canola, or sweet corn provided the lowest levels of Rhizoctonia disease and best tuber quality, whereas potato crops following clover or soybean resulted in disease problems in some years." Soybean may be best used in the rotation when not immediately preceding potato. Table 2 Disease incidence, total yield and misshapen tubers of potato as affected by different crop rotations grown in the prior two years.*
* Means within columns followed by the same letter are not significantly different according to Fisher’s protected least significant difference at P = 0.05. Severity results were comparable to incidence and are not shown. All rotations also significantly reduced the proportion of misshapen tubers produced. Larkin and Honeycutt (6) also found that both rotation crop and cropping sequence were important in shaping the microbial population, soil borne disease, and tuber qualities. Although total yield was not significantly different among rotations in 2000, the soybean-canola and soybean-barley rotations produced the highest overall yields, averaging 9 to 12% higher than the continuous potato plots. Results for the 2001 year were similar, but less discriminating. Poor weather during the growing season led to a sharp reduction in yields. The significance of the results was consistent regardless whether total or marketable yields were used. The Monte Carlo simulation results for the eight rotation sequences are shown in Table 3. In the simulation, the average of misshapen potatoes for the two years was used to adjust marketable potato yields in order to correct the analyses to reflect market conditions The net revenue for the rotation sequence is calculated as the sum for each rotation of the gross revenue from each crop minus its costs of production. At the average disease levels it is clear that continuous potatoes would not be profitable nor sustainable. Given this result, it is clear why the industry uses, at a minimum, a two-year cropping sequence, such as barley-potato. In comparison to the continuous potato rotation all rotations showed more income stability as measured by the coefficient of variation. Income variability for the rotation sequence is a function of the correlation of income between the individual rotation crops. Thus, if the crops are not strongly positively correlated, as is the case, the rotation’s net income variability will decrease, vis-à-vis the continuous potato rotation sequence. The results also show that using high-value crops such as green beans and sweet corn in the rotation sequence lead to a reduction in income variability and increased net income. The barley-clover rotation has the least impact on these measures. While the rotation does reduce disease levels its economic impacts are negligible since barley is not a high value crop and the clover crop has no marketable output. Also note that while the higher valued crop rotations have less income variability, they are not equal. Thus, for a producer deciding between a canola-sweet corn-potato rotation (CV = 0.4) and a green bean-sweet corn-potato rotation (CV = 0.6) may not automatically choose the latter rotation as it has fifty percent higher income variability. Reduction in income variability is desirable because it makes planning easier, provides stability in cash flow, and may give the producer greater access to credit. Table 3. Monte Carlo simulation results: Mean net income and income variability of potato as affected by crop rotation.
Figure 1 shows a Cumulative Density Function for the canola-sweet corn-potato rotation. Among the 5,000 iterations of this rotation in the Monte Carlo simulation, only 0.68% of the runs results in zero or negative net income. The results reflect the income variability of each sequence and the presence of high value crops. In this case, the results are fairly clear cut. As expected, the continuous potato and the barley-clover rotations carry the highest risks of losing money over the entire rotation. This is due primarily high disease rates leading to misshapen tubers in the continuous potato sequence and to presence of clover in the second year of the rotation when no income is earned.
Discussion Our results show that rotations can reduce disease incidence and damage while also contributing to increased profitability and a reduction in income variability and risk. Our analyses assume that, for the most part, the rotation crops can be sold for competitive prices. While Maine producers have extensive experience in growing barley, increasing experience in growing soybean and canola, fewer have experience in the production of sweet corn and green bean. Though table stock producers are knowledgeable of fresh marketing channels, growers of processed and seed potatoes may have less familiarity. Thus, to achieve the net income levels found in our research will require growers to develop new contacts and marketing strategies. Furthermore, if significant acreage is devoted to these crops, price levels may drop. However, our positive findings may provide additional incentives to producers to adopt longer-term rotation sequences and incorporate higher valued crops. Research is still being conducted on the efficacy of using rotation crops to suppress soil borne diseases. More replications of the trials are being analyzed to determine if particular rotation sequences are more advantageous than others with respect to disease control and profitability. The question whether highly suppressive rotations can also lead to a reduction in inputs is also being addressed. Disclaimer Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture. Literature Cited 1. Carter, M. R., Kunelius, H. T., Sanderson, J. B., Kimpinski, J., Platt, H. W., Bolinder, M. A. 2003. Productivity paramerters and soil health dynamics under long-term 2-year potato rotations in Atlantic Canada. Soil Till. Res. 72:153-168. 2. Halloran, J. M., Griggin, T. S., and Honeycutt, C. W. 2005. An economic analysis of potato rotation crops for Maine potato cropping systems. Amer. J. Potato Res. 82:155-162. 3. Hardaker, J. B., Huirne, R. B. M., Anerson, J. R., and Lien, G. 2004. Coping With Risk in Agriculture, 2nd Edn. CABI, Oxfordshire, UK. 4. Heady, E. O. 1952. Diversification in resource allocation and minimization of income variability. J. Farm Econ. 54:182-496. 5. Krupinsky, J. M., Tanaka, D. L., Lares, M. T., and Merrill, S. D. 2004. Leaf spot diseases of barley and spring wheat as influenced by preceding crops. Agron. J. 96:259-266. 6. Larkin, R. P., and Honeycutt, C. W. 2006. Effects of different 3-year cropping systems on soil microbial communities and Rhizoctonia diseases of potato. Phyopathology 96:68-79. 7. Peters, R. D., Sturz, A. V., Carter, M. R., and Sanderson, J. B. 2003. Developing disease suppressive soils through crop rotation and tillage management practices. Soil Till. Res. 72:181-192. 8. Snapp, S.S., Swinton, S.M., Labarta, R., Mutch, D., Black, J. R., Leep, R., Nyiraneza, J., and O'Neil, K. 2005. Evaluating cover crops for for benefits, costs, and performance within cropping system niches. Agron. J. 97:322-332. 9. Zentner, R. P., Wall, D. D., Nagy, C. N., Smith, E. G., Young, D. L., Miller, P. R., Campbell, C., McConkey, B. G., Brandt, S. A., Lafond, G. P., Johnston, A. M., and Derksen, D. A. 2002. Economics of crop diversification and soil tillage opportunities in the Canadian prairies. Agron J. 94:216-230. |
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