Degree Day Modeling and Economic Considerations of Insects and Weeds in Sheep Grazed Alfafla, Grain, and Range Production Systems

Project Overview

Project Type: Research and Education
Funds awarded in 2011: $206,700.00
Projected End Date: 12/31/2014
Region: Western
State: Montana
Principal Investigator:
Dr. Hayes Goosey
Montana State University

Annual Reports


  • Agronomic: wheat, grass (misc. perennial), hay
  • Animals: sheep


  • Animal Production: feed/forage, grazing management, grazing - rotational, stocking rate, winter forage
  • Crop Production: conservation tillage
  • Education and Training: demonstration, extension, farmer to farmer, mentoring, networking, on-farm/ranch research, youth education
  • Farm Business Management: new enterprise development, budgets/cost and returns, value added
  • Natural Resources/Environment: habitat enhancement, wildlife
  • Pest Management: biological control, cultural control, economic threshold, integrated pest management
  • Production Systems: agroecosystems
  • Sustainable Communities: community planning, public participation, urban agriculture, urban/rural integration, employment opportunities

    Proposal abstract:

    In 2009, Montana harvested 688,259 ha of alfalfa (Medicago sativa, L.) hay worth $3.42 million and 28,745 ha of alfalfa seed worth $6.3 million (NASS 2009). Alfalfa represents the foremost forage crop in many semiarid and temperate states with viable seed production being the core of this forage production system. Two biological stressors (insects and weeds), combined with poor field management, are primarily responsible for reduced yields. Western U.S. sheep producers rely on regrowth of alfalfa as a source of winter pasture. The highest variable cost associated with profitable production is winter feed. In recent years the U.S. sheep industry has been subjected to competition on the world market through imports of relatively cheap lamb. Inexpensive feed costs become critical for American lamb producers to remain competitive. Integrating sheep into farming systems has the potential to reduce pesticide usage and to create new low-capitol, entrepreneurial opportunities. Current insect management is insecticide-based. When populations reach the economic threshold (ET), producers are justified to chemically treat. Insecticides are an effective but costly practice that is remedial in nature. Sheep grazing, however, is preventative and if implemented correctly, stops pests from building to economic levels. This represents a savings to producers and has the potential to reduce insecticide use by 6.6 million g ai while alleviating concerns with non-target insects. Published research (Goosey et al. 2004) reports spring sheep grazing keeps alfalfa weevil (AW) (Hypera postica, Gyenhall) larvae below the ET without reducing crop yield or value. We received USDA Western SARE competitive funds to complete a two-year project of developing a degree day (DD) based model of the grazing system. Research was conducted during 2008 and 2009 in central MT. The recommendation to producers from this research is to stock alfalfa fields in the spring prior to 34 DD with rates between 251 and 582 sheep d/ha. The model predicts that sheep should be allowed to graze to a minimum of 106 and maximum of 150 DDs before removal. The model is published (Goosey 2009) and is in the validation stage prior to journal publication. Cropping systems are highly influenced by plant and arthropod species. Arthropod dynamics are highly influenced by plant community structure and land management practices. Therefore we proposed a two-year project aimed at increasing the sustainability of both livestock and crop production by 1) expanding the previously developed DD grazing model to two additional alfalfa production pests, and 2) assessing the influence sheep grazing has on plant and arthropod community structures in crops, pastures and rangelands. Losses from alfalfa seed chalcid (ASC) (Bruchophagus roddi Gussakovskii) average 2-15% (Lauderdale 1991). Chemical ASC control is ineffective due to the prolonged adult emergence period and the persistence of attack (Peterson and Baird 1993). Timing of crop maturity (Ahring et al. 1984) is not feasible because of climatic differences between the northern and southern Great Plains (Thoenes and Moffett 1990). Infestations levels of ASC are a predictable response to climatic conditions. Erdélyi et al. (1994) calculated correlations between ASC damage and weather parameters in Europe. In the U.S., Soroka and Spurr (1998) used hydrothermal quotients of the current and previous production years to predict, using a logistic model, which years would be more severe for ASC damage. Larvae overwinter in seeds in the field and are considered a univoltine pest (Soroda and Spurr 1998). Disruption of the overwintering survivorship of ASC larvae through strategic fall sheep grazing has potential to reduce breeding adults and subsequent seed infestation. Pea aphids (Acyrthosiphon pisum Harris) overwinter as eggs glued to alfalfa aftermath that remains in the field. Eggs hatch in the early spring and nymphs feed on alfalfa. Asexual reproduction continues through the summer. Males are produced, and sexual reproduction results in overwintering eggs. Dry cool spring conditions (12.8 to 15.6 °C) favor development of dense populations (Blodgett 2006). Fall sheep grazing removes crop residue and with it pea aphid eggs. Spring grazing sheep consume alfalfa and, with it, feeding pea aphids which disrupts the crop/pest synchronization (Goosey 2009). In Australia, Bishop et al. (1980) observed that spring grazing reduced spotted alfalfa aphid (SAA) damage, but reported substantial infestations were frequently found on crop regrowth. The pea aphid differs from the SAA in that it prefers cool rather than hot conditions. Grazing removes early crop growth which increases microclimate temperature (Goosey unpublished data), resulting in increased pea aphid mortality. McClure et al. (1994) compared rotational grazing of alfalfa to an all-concentrate diet fed in drylot. Performance of lambs grazing alfalfa approached that recorded for the drylot diet, indicating that grazing systems which allow sheep to harvest their own forage offer economic benefits. In past years, harvest residue was grazed by livestock. However, many of the currently registered pesticides eliminate grazing residues because treated plant material is prohibited from entering the human food chain. By developing DD-based grazing models which manage insect populations preventatively before and during the growing season, insecticides become unnecessary and harvest biomass can once again enter the food chain. Beneficial arthropods, of agronomic systems, include species that contribute to maintaining or increasing crop yield or enhance the ecological stability of the cropping system. Arthropods that act as pollinators or predators, parasitoids or herbivores of insect and weed pests are recognized as beneficial in crop ecosystems. Predators and parasitoids benefit from a variety of plant resources (nectar, pollen or seeds) and interact with plant diversity at varying scales (Wilkinson and Landis 2005). There is a well-known functional relationship between plant resources and insect biology, and entomologists have documented the positive role of weeds in enhancing beneficial insect survivorship in crop ecosystems (van Emden 1965). Despite improved labor efficiency and yield, economic, environmental, and societal concerns indicate that an increased reliance on herbicides threatens the sustainability of the farming enterprise (Liebman 2001). Currently, mechanical tillage is the only practiced alternative to chemical fallow for weed control. However, tillage can bury crop residues, decreasing residue cover, thus increasing the potential for soil erosion. Strategic grazing may offer an alternative to traditional weed management systems, resulting in a significant reduction in herbicides or tillage (Hatfield et al. 2007). Despite its potential advantages, the impact that grazing has on crop and weed communities is largely unknown. In a preliminary study (Hanson et al. 2010), we determined an increase in weed species richness and diversity in sheep grazed compared to mechanical and herbicide systems. Furthermore, we observed a shift in weed species composition with communities dominated by perennial weeds, such as dandelion, annual dicotyledonous species, such as prostate pigweed (Amaranthus blitoides) and shepherd's purse (Capsella bursa-pastoris), and volunteer wheat. A growing body of evident suggests that systems (including arthropod predator/prey) with increased weed diversity are more stable in terms of self regulating ecosystem processes. Our preliminary data suggests that sheep grazing may increase beneficial weed diversity without impacting crop yield or nutrient characteristics. Management of pests and conservation of beneficial insects associated with grazing has been investigated primarily in rangelands (O’Neill et al. 2003, O’Neill et al. 2008, Onsager, 2000, Sjodin 2007). Therefore our project will further investigate the influence of sheep grazing on plant and insect community structure and the contribution this has to improving sustainable agriculture practices.

    Project objectives from proposal:

    1) Compare various intensities (0-200 DD) and model results of sheep grazing (fall and spring) and a no-input control on:

    a) pea aphid and ASC populations in alfalfa (Goosey, O’Neill, Johnson, Helle, Lehfeldt, Thomason, Baucus).

    The concept of this management tactic is based on sheep consuming and/or trampling pea aphid eggs or ASC infested seed in the fall and/or spring. For targeted grazing to manage these pest populations, appropriate stocking rates must be determined. Simply turning a few sheep onto vast acreage will not be sufficient. Fall stocking rates can be expressed on a purely animal per unit area basis, and the performance target will be calculating what rates are necessary to sufficiently remove pea aphid eggs and ASC infested seed.

    Spring grazing, prior to and during the alfalfa green up period, will need to be expressed on a DD basis, similar to Goosey (2009). The spring grazing performance target will be calculation of appropriate stocking rates and grazing durations based on DD rather than calendar dates. To utilize targeted grazing as an IPM tactic, implementation needs to be expressed in terms of temperature.

    b) change in alfalfa field plant community and alfalfa aftermath (Goosey, Menalled).

    Decreased plant biodiversity in agro-ecosystems has negative consequences to insect biodiversity (Gaines and Gratton (2010). The loss of insect diversity is especially pertinent in agroecosystems, as insects provide a variety of ecosystem services vital to farming (Isaacs et al. 2009). Because of the value of plant biodiversity in agroecosystems, agricultural practices that favor conservation of farmland biodiversity should be encouraged (Butler et al. 2007). Our preliminary data indicates an increase in weed species diversity in a sheep grazed wheat/pea/summer fallow system (Hanson et al. 2010). The performance target is to monitor treatment effect on plant biodiversity, document any changes in yield and/or nutritive quality and correlate this to any recorded changes in insect diversity.

    c) pollinator, predator, and parasitoid populations in alfalfa (Goosey, O’Neill, Johnson).

    One driving force behind sustainable agriculture practices is utilizing production strategies which enhance beneficial arthropod populations and success rates of pollination, predation and parasitism. Losey and Vaughan (2006) estimated the annual value of pest control and crop pollination attributed to wild insects in the U.S. at $4.5 and $3.1 billion, respectively. However, little is known about how targeted grazing systems impact beneficial species. Our preliminary data indicates greater parasitic hymenoptera in sheep grazed spring and winter wheat/summer fallow farming systems when compared to mechanical and chemical managerial practices. Our team will compile an inventory of beneficial species and assess the treatment influence on populations. The performance target will be a preliminary understanding of how targeted grazing influences beneficials of alfalfa production with the expectation that this knowledge will lead to further research hypotheses and measurable objectives.

    d) Carabidae spp. diversity between sheep grazed cropland, improved pasture and rangeland (Goosey, O’Neill, Johnson, Menalled).

    Carabid species assemblages differ between habitats and the pest management potential of an assemblage is dependent on species identity (Gains and Gratton 2010). The functional diversity of an assemblage is a measure of the functional roles represented within a community, as compared to diversity per se, which is the number of species present. Previous research has recorded that more functionally diverse assemblages are more capable of providing ecosystem services (Straub and Snyder 2006). Our performance target is to identify differences in species assemblage between natural (i.e., rangeland) and cropped (i.e., alfalfa, wheat/pea/fallow and continuous wheat rotations) habitats to further document how animal grazing acts to alter diversity and arthropod ecosystem services.

    2) Develop an economic decision support tool to evaluate long term cost-benefits of sheep grazing in alfalfa production (Goosey, Griffith, Helle).

    We propose to utilize an existing decision support tool to analyze the economics of this grazing system. The intent of this program is to be a tool available to producers to assist when developing a grazing program. This program was developed with Western SARE funds (SW07-013) and is currently available online at:
    Our performance target will be developing a grazing protocol that is effective in terms of dollar cost and pest management potential.

    3) Develop and conduct large, multi-farm demonstrations. Communicate results to producers, students, scientists and the public on the advantages of incorporating prescriptive sheep grazing into alfalfa and cereal production 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.