Field attributes and farming practices associated with vole (Microtus) damage in cover-cropped fields

Highlights

• Lack of snow cover was most important to predicting probability of vole damage.

• Fields with high proportions of loamy soils were at greater risk for vole damage.

• Fields with 5–7 % grassland landcover within 50 m were at greater risk for damage.

• Farmers may employ conservation tillage if damage is expected to be high.

Abstract

Increased use of cover crops to promote soil health in high-intensity row-crop agroecosystems has resulted in more frequent reports of damage to soybean (Glycine max) commodity crops by voles (Microtus). Meadow (M. pennsylvanicus) and prairie voles (M. ochrogaster) may use overhead cover and forage provided by cover crops and browse soybean seedlings after emergence in spring. Because cover cropping is a recently adopted conservation practice, few methods have been evaluated for their relative effectiveness in preventing vole damage in cover-cropped fields. We used boosted regression tree models to assess how farming practices, physical and landscape attributes of cover-cropped soybean fields, and seasonal weather conditions were associated with severity of damage by voles across Indiana, USA. We found non-linear, asymptotic relationships between risk of vole damage and number of days of snow cover the preceding winter, percent of well-drained soils, and number of years of cover-crop use in a field. Damage risk was greatest at intermediate levels of grassland vole habitat within 50 m, and risk was lower when conservation tillage was applied. Our results can be used by producers to identify fields and years where vole damage is most likely to occur and identifies farming practices most likely to mitigate risk in those fields.

Introduction

Use of cover crops in corn (Zea mays)–soybean (Glycine max) rotational agriculture has increased in the midwestern United States over the last decade (White, 2014). Cover crops are non-commodity plant species sown in the fall after harvest that die during the first frost or are terminated by mechanical or chemical means in the spring. Producers plant cover crops to aid in soil conservation, moisture retention, and weed suppression (White, 2014). Additionally, cover crops provide wildlife habitat that does not exist in traditionally farmed row-crop fields (Berl et al., 2018; Jug et al., 2008; Wilcoxen et al., 2018). Though increasing wildlife habitat in highly fragmented agricultural ecosystems is generally viewed as a positive aspect of conservation agriculture (Altieri, 1999), some animals that use habitat provided by cover crops may cause conflict when they consume the commodity crop (Wiman et al., 2009; Witmer et al., 2007). In Indiana and elsewhere in the midwestern United States, reports of damage by voles (Microtus) have occurred for several years (Fisher et al., 2014; J. Rorick, Agronomist, Conservation Cropping Systems Initiative, pers comm.).

Meadow (M. pennsylvanicus) and prairie vole (M. ochrogaster) ranges span the state of Indiana (Reich, 1981; Stalling, 1990) and are associated with grassland habitat. Both species are found in areas of dense overhead cover, though prairie voles also persist in habitats that provide less protection from predators (Getz et al., 2001). In years when cover crops establish well in fields, they likely provide sufficient cover for both vole species. Cover crops also provide forage for voles (Prieur and Swihart, 2020) that may allow them to persist in fields overwinter. However, when cover crops are terminated in the spring and are replaced by newly planted soybeans, voles may forage upon the commodity crop, which is the only abundant source of green vegetation after cover-crop termination. Deer mice are also found in crop fields year-round, however, they typically persist on weed seeds and waste grain and cause little damage to planted crops (Berl, 2017; Berl et al., 2017).

Because adoption of cover cropping in intensive row-crop agriculture is recent, no systematic assessment of factors related to damage has been conducted. We addressed this deficiency by quantifying the association between farming strategies, attributes of fields and surrounding landscapes, abiotic factors, and risk of soybean damage incurred by voles.

With respect to farming strategies, we considered tillage, planting, and habitat management methods. Soil disturbance caused by tillage reduced common vole (M. arvalis) abundance in European wheat fields compared to no-till fields (Heroldová et al., 2018). However, reduced or no-till practices are implemented as forms of soil conservation used in conjunction with cover cropping to improve soil health in agricultural fields. Thus, farmers might be reluctant to use full-width tillage to reduce vole populations if doing so sacrifices soil conservation efforts. Reduced tillage strategies, where a lesser portion of the soil area is disrupted, have been shown to reduce vole presence in agricultural fields (Roos et al., 2019), but it is not known if such disturbance would be similarly effective in cover-cropped soybean fields. Methods for planting cover crops and soybeans also have been suggested to reduce soybean depredation by voles (Fisher et al., 2014). Seed drills and planters intersect the soil surface, potentially disrupting burrows and protecting seeds from predation by placing them beneath the soil surface or covering them over with soil. Other methods, such as broadcast spreading, leave seeds on the ground’s surface and do not disturb soil, thereby leaving soybeans open to predation and burrows and nests intact. Managing quality of vole habitat in focal fields may also reduce risk of crop depredation. One strategy is to deplete available habitat before soybeans are planted by increasing the time between cover crop termination and soybean planting (Fisher et al., 2014). Voles may consume soybean plants as a consequence of the commodity crop being the only growing source of forage in a field after cover crops are terminated. Eliminating vole habitat provided by cover crops well before soybeans are planted may increase exposure to predation and encourage emigration to adjacent permanent habitats that provide more ideal cover and food (Lin and Batzli, 2001; Smith and Batzli, 2006). Additionally, maintaining minimal vegetative cover in fields by mowing or grazing the cover crop may reduce vole populations by decreasing cover and opening the vegetative canopy to predators (Lin and Batzli, 2001; Peles and Barrett, 1996; Slade and Crain, 2006).

In addition to farming strategies that may be altered to reduce damage risk, we compared the relative importance of biotic and abiotic features of fields and surrounding landscapes, such as soil type, adjacent permanent habitat available to voles, and weather. Though farmers cannot manipulate these conditions to mitigate risk, identifying influential factors will nonetheless allow farmers to identify fields or environmental conditions where damage risk is greatest and take preventative action to protect crops. Soil texture and moisture retention affect voles’ ability to burrow (Blank et al., 2011; Rhodes and Richmond, 1985), and well-drained soils supported higher common vole populations in Europe (Santos et al., 2011). Suitable meadow and prairie vole habitat adjacent to cover-cropped fields, often provided by lands enrolled in the federal Conservation Reserve Program, grass waterways and road-side verges, can harbor vole populations in fragmented agroecosystems (de Redon et al., 2010; Rodriguez-Pastor et al., 2016; Santos et al., 2011). These permanent habitats enable voles to move into field interiors once cover and food are available (Lin and Batzli, 2001; Smith and Batzli, 2006). Alternatively, adjacent forest landcover may be associated with reduced vole abundance. Many birds of prey, such as hawks, falcons, and owls are predators of small mammals and can help manage rodent populations in agricultural settings but often require perch sites from which to hunt (Kay et al., 1994; Machar et al., 2017; Motro, 2011). Lastly, some weather conditions can affect vole survival and reproduction and may help farmers anticipate high risk for crop damage in years that have weather conditions favorable for voles. Sufficient snow cover provides protection from predators (Lindstrom and Hornfeldt, 1994) and insulation from extreme weather conditions during winter months (Esther et al., 2014; Tkadlec et al., 2006). However, ice may form below the snowpack and hinder movement and foraging activity (Korslund and Steen, 2006). Rainfall may also influence vole population fluctuations (Deitloff et al., 2010; Esther et al., 2014; Heisler et al., 2013), though effects may differ for prairie voles, which prefer drier sites, and meadow voles, which inhabit wetter sites (Findley, 1954; Getz, 1970).