Palatability of common cover crops to voles (Microtus)
Introduction
Use of cover crops in conventional row-crop agriculture has increased greatly over the last decade (White, 2014). In corn-soybean rotations of the Midwestern U.S., producers plant cover crops after harvesting the commodity crop in fall and terminate in spring before the next commodity crop germinates. Cover crops are used to improve soil health by retaining topsoil, providing essential nutrients, and maintaining soil moisture (Fageria, 2007).
In addition to improving soil health, cover crops provide forage and vegetative structure in fields that, under conventional tillage practices, contain only bare soil or crop stubble from late fall to early spring. Improved overhead cover likely increases diversity of the small mammal community in row-crop fields (Berl et al., 2018; Getz et al., 2007; Jug et al., 2008) and could enable population growth of herbivorous small mammals that can incorporate the cover crops into their diets. Although increased biodiversity can benefit farm management (Altieri, 1999), some species, such as voles (Microtus), may depredate the commodity crop when their populations grow too large (Wiman et al., 2009; Witmer et al., 2007). When appropriate cover and food are available, voles reproduce year-round, and populations can quickly grow to large numbers (Cole and Batzli, 1978; Getz et al., 2007; Goswami et al., 2011).
Vole depredation of crops in row-crop fields has been reported previously (Witmer et al., 2007), and complaints of vole damage to soybeans (Glycine max) in cover-cropped fields are numerous (Fisher et al., 2014; Prieur, 2019; Joe Rorick, [Conservation Cropping Systems Initiative, West Lafayette, IN] personal communication, [August 2017]). However, we are unaware of research designed to evaluate which cover crops used in corn-soybean rotations may, by virtue of their relative palatability, encourage vole use of fields and increase risk of damage to soybeans. Our objective was to rate commonly used overwinter cover crops, including grasses, clovers, brassicas, and others, for selection by meadow (M. pennsylvanicus) and prairie (M. ochrogaster) voles. The geographic ranges of these two vole species encompass the bulk of the Midwestern U.S., where row-crop agriculture dominated by soybean and corn (Zea mays) is prevalent. Identifying cover crops that are avoided by voles provides producers with options for planting cover crops of lower value to voles, which may discourage immigration and recruitment in fields and thus reduce potential for feeding damage by voles to young soybean plants. Alternatively, producers can anticipate damage in fields planted with highly preferred cover crops and act preemptively to minimize damage in these fields by using farming strategies such as spot tillage or manipulation of nearby grassland habitat (Prieur, 2019).
We expected clovers (Trifolium) and alfalfa (Medicago sativa) to be preferred, as they ranked high in previous vole diet studies comparing plants from permanent vole habitats (DeJaco and Batzli, 2013; Lindroth and Batzli, 1984). We anticipated that vetches, specifically hairy vetch (Vicia villosa) and cicer milk vetch (Astragalus cicer), would be avoided, as Vicia was suggested by Sullivan (2006) to be the most likely group of cover crops avoided by voles, and A. cicer was reported to deter voles from entering fields (Lisa Holscher, [Conservation Cropping Systems Initiative, West Lafayette, Indiana], personal communication, [August 2017]).
In addition to establishing relative preference by voles, we evaluated factors hypothesized to influence selection and avoidance of plant species. These objectives were operationally motivated; plant species that consistently are avoided by voles are less likely to yield variable results when used by producers compared to plant species for which avoidance varies with factors such as vole sex or availability of alternative foods. Swihart (1990) found that woodchucks (Marmota monax), a generalist herbivore, more consistently selected and avoided highly and minimally preferred species of orchard ground cover, respectively. In contrast, moderately preferred species were selected with more variation among feeding trials. Hence, we predicted a similar unimodal relationship between relative preference and variation in selection of cover crops for our vole species.
Meadow and prairie voles are generalist herbivores (Reich, 1981; Stalling, 1990) that can adjust diets to account for changes in plant availability (Haken and Batzli, 1996). Thus, we hypothesized that voles would become increasingly willing to consume a plant as available plant diversity and quality of forage declined (Haken and Batzli, 1996), resulting in greater relative preference. To control for possible effects of plant diversity, we also tested whether voles consumed less when presented with an equally diverse offering of plants rated as avoided versus preferred based on prior trials. We expected voles to consume less of unpreferred forage, as some minimally selected plants may be consistently avoided (Swihart, 1990).
Section snippets
Study site
We captured voles and performed captive feeding trials at the Purdue University Wildlife Area (PWA) located 11 km west of West Lafayette, Indiana. PWA encompasses 1.17 km2 of restored tallgrass prairie, savanna, and wetland habitat and is surrounded by row-crop agriculture. We captured five female and five male meadow voles and two male prairie voles within restored prairie at the site and at a nearby Purdue property. Low population levels during the study prevented us from capturing sufficient
Relative preference
No plant species was wholly avoided, as both meadow and prairie voles sampled all plant species at least once. However, canola (Brassica napus) ranked lower than most plant types for both vole species and was chosen less than expected by meadow voles in trials 1, 2, and 4 (Table 1). Meadow voles also ate turnip (B. rapa) less than expected in trials 1 and 3.
Of the 17 plant species presented to both vole species, red clover (T. pratense), alfalfa, and hairy vetch were consistently selected more
Discussion
We found pronounced differences in selection of commonly used cover crop species. Elimination of the most preferred plant species in successive trials allows us to present an estimated ranking of relative palatability for the species tested to aid producers in selection of cover crops to use in fields with historic vole issues (Fig. 3). However, we suggest consideration of all results presented before applying the rankings, as the order is subjective and based on the cumulative results of all
Conclusion
Common cover crop species range in attractiveness to meadow and prairie voles, and palatability depends on availability of other forage. Red clover, alfalfa, and hairy vetch were commonly preferred, and canola was avoided. Highly and minimally preferred species were selected or avoided, respectively, more consistently than moderately preferred species. Farmers can plant minimally preferred cover crops to deter voles, however, our findings suggest that the effectiveness of this strategy may
Funding
This work was supported by the U.S. Department of Agriculture Natural Resources Conservation Service [grant number 68-3A75-18-127], the U.S. Department of Agriculture National Institute of Food and Agriculture [Hatch project 1014271], the Iowa Academy of Science [grant number 40001934], and the Purdue University Department of Forestry and Natural Resources.
CRediT authorship contribution statement
Abby-Gayle A. Prieur: Methodology, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization, Supervision, Project administration, Funding acquisition. Robert K. Swihart: Conceptualization, Methodology, Formal analysis, Resources, Writing - original draft, Visualization, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors, Abby-Gayle Prieur and Rob Swihart, declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Technical support was provided by D. J. Hixon, C. Harms, and L. Estrada. Seeds were donated by CISCO Seeds, Ceres Solutions, and Kester's Wild Game Food Nurseries. L. Holscher and J. D. Rorick offered valued advice on project development. We thank E. A. Flaherty S. Zezula, and two anonymous reviewers for improving the manuscript.
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