Comparison of Pest Management Interactions in Spring Wheat-Cover Crop and Spring Wheat-Fallow Cropping Systems

Final Report for SW97-056

Project Type: Research and Education
Funds awarded in 1997: $150,964.00
Projected End Date: 12/31/2001
Matching Non-Federal Funds: $76,908.00
Region: Western
State: Montana
Principal Investigator:
Andrew Lenssen
Montana State University
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Project Information


[Note to online version: The report for this project includes tables that could not be included here. The regional SARE office will mail a hard copy of the entire report at your request. Just contact Western SARE at (435) 797-2257 or]

The ramifications of crop diversification and pest interactions in spring wheat production systems were assessed by determining spatial associations of pest populations and crop response. Utilizing global positioning systems and geographic information systems for three years, insect, disease, and weed populations, nitrate and water in soil, and wheat grain yield and protein were mapped in twenty-two spring wheat fields. Fields of spring wheat produced in diversified rotations had lower levels of wheat stem sawfly, foliar disease, soil water, wheat tiller densities, beneficial and pest arthropods, and wheat grain yield and test weight than spring wheat grown in traditional cereal rotations. Russian thistle and total weed infestations, Fusarium crown rot, soil nitrate, drought stress, and grain protein were higher in wheat in diversified rotations than wheat produced in conventional cereal only rotations. The 1998, 1999, and 2000 crop years were characterized by low precipitation at our research sites, resulting in severe drought stress in spring wheat grown in intensified cropping systems.


Research results and discussion:

Over three field seasons, twenty-two fields on three farms were surveyed on a 3/4 acre grid system for arthropod, weed, and diseases of spring wheat grown in traditional or diverse rotations (Table 1).

During the growing season, foliar leafspot diseases (tanspot and/or Septoria), weed cover, and drought stress were estimated with numerical rating systems. Additionally, preplant soil water and nitrate, and crop stand and tiller numbers were determined at the same sampling points. Field areas mapped ranged from 27.5 to 60 acres; most fields were 40 acres. Crop yields were determined with continuous grain yield sensors mounted on the cooperators' combines. Grain samples were taken every one minute during harvest for protein analysis.

Fields of spring wheat produced in diversified rotations had lower levels of wheat stem sawfly, foliar disease, soil water, beneficial and pest arthropods than spring wheat grown in traditional cereal rotations (Table 2). Russian thistle and total weed infestations, Fusarium crown rot, and soil nitrate were higher in wheat in diversified rotations than wheat produced in conventional cereal only rotations. The primary weeds were Russian thistle and kochia. Wild oats were rarely present.

Spring wheat grown in traditional rotations averaged 40% greater grain yields than wheat grown in diversified rotations (Table 3). Although stand densities were similar between rotation types, wheat grown in the traditional rotations had more reproductive tillers per unit area. Wheat grown in diversified rotations had higher drought stress ratings and concomitant higher grain protein concentrations in all three years, averaging 1.5 points higher in crops grown in diversified rotations. Test weights were substantially reduced in wheat grown in diversified rotations. In large part, these differences were due to the dry growing seasons. Precipitation during all three growing seasons was insufficient to replace that soil water used by alternative crops during the previous year.

Correlations among pests were significant in many instances. Weed density ratings were positively correlated with foliar leafspot ratings for three fields, perhaps due to increased relative humidity within a denser canopy. In 2000, 3 of 8 fields studied showed kochia density positively correlated with numbers of arthropod pests and percentage of vommon root rot-infected plants. In 5 of the 11 fields where arthropods were sampled in spring wheat grown in diverse rotations, beneficials were positively correlated with insect pests. However, beneficial arthropods were positively correlated with pest insects in only 2 of 9 wheat fields grown in traditional rotations. Although other pest:pest correlations were significant in single fields, potential relationships among pests generally were not evident when examined by Pearson correlation coefficients.

Correlations of pests with edaphic factors were significant in some instances, with differences occurring between rotation types. Weed densities were negatively correlated with preplant soil nitrate in 5 of 12 of the comparisons for spring wheat grown in diverse rotations. Only 1 of 10 spring wheat fields grown in traditional wheat rotations had a significant negative correlation of weed density with preplant soil nitrate. Common root rot was significantly and positively correlated with preplant soil nitrate in 3 of 24 fields. Common root rot was negatively correlated with soil water for 2 fields. Wheat stem sawfly infestations were positively correlated with stand or reproductive tiller densities for 8 of 22 fields. Numerous other correlations were significant between beneficial or pest insects with soil water or nitrate, however, trends were not consistent within or across crop rotations, or years.

In 1998, spring wheat yield was negatively correlated with weed density ratings for all three fields grown in traditional cereal rotations. However, correlations of yield with weed density ratings were nonsignificant for all three fields of spring wheat grown in diverse rotations. In 1999, spring wheat yield was negatively correlated with Fusarium crown rot in three of the four spring wheat fields grown in traditional cereal rotations. However, correlations of yield with Fusarium, crown rot were nonsignificant for all four fields of spring wheat grown in diverse rotations. Correlations of spring wheat yield with other pests were significant in some cases, however, trends were not consistent within or across crop rotations, or years. Spring wheat yield was positively correlated with preplant soil water concentration for 8 of the 14 fields sampled in 1998 and 1999.

Geographically referenced maps of wheat produced in wheat-barley-fallow-wheat (traditional) and wheat-barley-chickpea-wheat (diverse) are presented in Figures 1 and 2, respectively, to demonstrate the large differences found in wheat production between the two rotations during this period of drought. Scales for yield, protein, and test weight are identical on this paired, two-map set.

Common root rot was negatively correlated with available soil water for a single field. Total weed density was negatively correlated with soil nitrate in four of the eight fields studied in 2000. Total weed density was negatively correlated with wheat stand or tiller density in 5 of the 8 fields studied, showing that competition from the crop is important in managing weed populations.

Spatial auto correlations were significant for numerous factors, including grain yield, protein, test weight, soil water and nitrate, weeds, insects, and several pest ratings, but not all factors were significantly correlated spatially in all fields. Wheat stem sawfly infestations generally were higher on field margins. A complete set of maps for all measured parameters from each of the 22 field years in the study will be available on a web site by fall 2001.

Research conclusions:

We have completed the 3 years of this study and have developed a good knowledge base on the impacts of replacing summer fallow with pulse or oilseed crops in wheat production systems on pest distributions and interactions, soil water and nitrate, and their combined impacts on spring wheat yield and quality during years with below-average precipitation. Our information will be useful for precision farming and for producers to make better informed decisions on their choices of cropping systems and variable rate applications of inputs, hopefully allowing for better environmental stewardship and profitability.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

Grey, Lenssen, and Blodgett have presented results from this trial at numerous Extension and farmer meetings from 1998 to the present (2001), and they will continue presenting findings from this study at Extension, regional, and national meetings for the next several years. Lenssen presented a poster on this study at the Annual Meeting of the American Society of Agronomy in Minneapolis, MN. Lenssen is scheduled to give two presentations at the NRCS Sustainable Agriculture meeting and field tour in June 2001. Grey is presenting results from this study at the national American Phytopath. Soc. meetings in Utah in August 2001. Additionally, a webpage providing results from this study, including nearly 400 maps, should be on line by December 2001. We plan on publishing two refereed journal articles in 2002.

Project Outcomes

Project outcomes:

Economic analyses showed that gross income from spring wheat was greatly reduced for crops grown in the diversified rotations. In three of the on-farm comparisons, spring wheat produced in traditional cereal rotations was sold at a premium as certified seed, while the corresponding wheat produced in the diversified rotations could not be sold for seed due to low test weights.

Farmer Adoption

Grain producers in Montana and the northern Great Plains had been replacing summer fallow with pulse and oilseed crops because crop options, profitability, and sustainability of the traditional wheat summer fallow system had appeared to change with the 1996 'Freedom to Farm' legislation. However, most producers, including our cooperators, increased summer fallow in 2000 due to the third consecutive year of drought in Montana. Recropping spring wheat has not been profitable for most growers during this period due to low yields. Additionally, producers have thought contract prices for most alternate crops, including pulses and oilseeds, were too low to be profitable compared to the current Loan Deficiency Program. When Montana returns to a more normal rainfall pattern, we believe that producers will once again increase production of alternate crops replacing summer fallow, particularly if nitrogen fertilizer prices remain high. We have documented that replacing summer fallow with pulses or oilseeds reduced available soil water for the following wheat crop, seriously reducing yield and quality of spring wheat.

Reactions from Farmers and Ranchers

The producers we are collaborating with are still excited about the project. Robert Boettcher worked hard to obtain additional funding for a fourth year of this study on Rob An Farms for 2001. All the producers collaborators went out of their way in contributing time and energy to this project during both growing seasons and at the critical period of harvest. Our results have already impacted dryland farming systems in Montana. One cooperator stated in February 1998 that 'there will never be another fallow acre on this farm again.' That cooperator now states that 25-30% of the farm must be in summer fallow to ensure an adequate level of wheat and income production the following year. Another comment was that recropping wheat during a drought not only caused poor, uneconomic yields for the current year, but also resulted in substantially reduced production as much as three years later. The cooperators with organic production systems originally were not sure how maps of pests or crop yield and protein could aid their operation, because they do not apply fertilizers or herbicides. However, they were impressed with the map presented in Figure 3. The western two-thirds of this field was cropped to lentil and green manured in 1994, while the eastern one-third of the field was cropped to barley that year. The field has been uniformly managed every year since then, but we were able to see a large increase in the wheat protein concentration in the western two-thirds of the field in 2000, which the cooperators and researchers attribute to the green manure grown 6 years earlier. This confirms that green manuring a pulse can provide long-term benefits to cereal production. Breaking from the past, few Montana and other Great Plains producers will have 50% of their acreage in summer fallow in the future. The obvious value of retaining some summer fallow acreage in dryland cropping systems for weed and water management, and enhancing yield stability of wheat, has been duly noted by producers during the current drought.

Producer Involvement

The producer cooperators spent considerable time and energy on this study. They were crucial to developing, conducting and reviewing this study. The Peterson and Grass farms grew pulse and oilseed crops for cash sales instead of as green manures. We changed our experimental protocol accordingly.


Areas needing additional study

Future Recommendations & New Hypotheses

Our study demonstrates that field-scale plots can provide an understanding of the complex interactions of pests and crops that is not possible in traditional small-plot experiments. Russian thistle and kochia emerged earlier and were more competitive in spring wheat following pulses than in spring wheat following summer fallow. Producers commonly wait to spray herbicides until just prior to canopy closure in traditional cereal cropping systems. When summer fallow has been replaced by chickpea, pea, or lentil, however, herbicidal control of weeds is necessary at a much earlier stage of wheat growth to prevent substantial water, nitrogen, and yield loss. Producers should monitor their spring wheat fields earlier after planting to make more timely weed management decisions.

Another important observation is that preplant soil nitrate values were not substantially different between cropping systems. This could be due to our (Montana) typical lack of moisture after harvest, and subsequent low temperatures during winter, that precludes sufficient microbial activity for nitrification. Additional research is necessary to better understand nitrogen dynamics, cycling, and temporal availabilities in green manure and pulse crop systems in the northern Great Plains.

This study has provided strong evidence supporting the inclusion of summer fallow acreage during times of drought in the northern Great Plains. An important change in grower perception of replacing traditional cereal rotations with diversified, intensified cropping systems is in the value of summer fallow. Spring wheat yields have been significantly depressed in nearly all recrop situations in this study. Spring wheat in diversified rotations yielded almost 30% less than spring wheat in traditional cereal rotations, however, grain protein was about 1.5 units higher in diversified rotations. Furthermore, Fusarium crown rot was higher in wheat grown in intensified systems than in traditional systems.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.