This study found five important effects of goat browsing on buckthorn after either a single browsing period or after repeated browsings. First, goats exhibited size-selective feeding when stripping bark from buckthorn stems. Second, goats girdled approximately 60% of the stems strip barked, but a large majority of stems were not stripped during an initial browsing. Third, repeated browsing reduced densities of buckthorn > 5 mm stem diameter by 90% by killing the abundant, smaller buckthorn within the population. Fourth, goats browsed on seedling and 2nd -year buckthorn, but did not reduce their densities after five browsings. Finally, goats did not kill most large (> 60 cm diameter) buckthorn, as they were unwilling or unable to strip bark from larger stems.
Size-selective bark stripping by goats on buckthorn was evident after both an initial period of browsing and after five periods of browsing. Goats stripped bark and girdled buckthorn stems primarily within the 20–50 mm diameter range, which comprised the majority of buckthorn present on the savanna sites. Smaller (< 20 mm diameter) plants were seldom bark stripped even though they represented > 40% of buckthorn present; observations suggested that small stems were not stiff enough for easy gnawing, and goats could straddle and bend these plants to access foliage and terminal buds. Larger stems were less frequently bark stripped, likely the result of decreased bark palatability in older or larger trees/shrubs (Bergström 1992; Gill 1992a). Similar, size-selective bark stripping on young tree stems has been observed in red deer (Cervus elaphus) in Sweden (Månsson and Jarnemo 2013).
Over 60% of buckthorn stems stripped by goats during the initial browsing session were completely girdled, producing at least top-kill (if not complete kill) of those plants. This magnitude of girdling via bark stripping by browsing wildlife can result in rapid death of browsed woody trees and shrubs (e.g., Michael 1987; Akashi and Nakashizuka 1999). With only 14–17% of stems bark stripped after initial goat browsing, possibly only 11% of total buckthorn stems in the Garvin Heights savannas faced death or top-kill from bark stripping. However, since most bark stripping was focused on stems 20–60 mm in diameter, goats potentially girdled 25–30% of buckthorn stems within that size group. Killing or suppressing this proportion of buckthorn after a single browsing period is significant, as buckthorn in this size group often represent the majority of fruiting stems in buckthorn populations (Delanoy and Archibold 2007). If this result is typical, and goats continue this level of bark-stripping effort during additional repeated browsings, significant reductions in reproductive output of buckthorn can be achieved.
After five browsing episodes, goats reduced buckthorn stem densities by 90% compared to densities assessed after the initial browsing. This likely was achieved through a combination of girdling and killing of stems 20–60 mm in diameter and bending and defoliating plants with stem diameters < 20 mm. Buckthorn stems < 20 mm in diameter declined from 43% of the population immediately after the initial browsing to only 8% after five browsings, suggesting that goats were very successful at defoliating and killing the smaller stems, similar to the effects of other browsers on smaller woody plants (Gill 1992b; Akashi and Nakashizuka 1999; Horsley et al. 2003; Royo et al. 2010). In addition, continued bark stripping during repeated browsing sessions left only one-third of buckthorn stems unstripped after five browsings. Such cumulative bark stripping is typical when young woody stems are subjected to intense feeding pressure from ungulates, often when alternative foods are limited or unavailable, or when additional fiber is needed in the diet (Gill 1992a). Observations during the current study suggest that goats initially consumed a combination of buckthorn foliage, branch tips, and bark when first released into a new habitat, switching more to bark stripping after the other food resources were reduced. By defoliating buckthorn < 20 mm in diameter and stripping bark from larger stems, goats were successful in killing the majority of buckthorn that were present initially within the savanna habitat.
Densities of very small buckthorn (seedling and 2nd -year plants, < 0.5 mm stem diameter) were not reduced by goat browsing. Even though goats browsed on > 90% of 2nd -year plants and > 25% of seedlings, densities were not suppressed over the 3-year period, with seedlings actually becoming more abundant. This suggests the presence of an abundant seedbank, supplying more new plants each year than periodic browsings could control. Goats typically do not browse near the ground, where they may ingest eggs of harmful parasites (Hart 2001), and goat contractors usually remove goats from habitats before ground-level browse becomes the only food option (K. Johnson, Diversity Landworks LLC, personal communication). Conditions for germination of the buckthorn seedbank apparently improved as densities of intermediate-sized (5–60 mm stem diameter) buckthorn were reduced by goats during repeated browsing (Hart 2012). A similar increase in seedling buckthorn density was observed in an adjacent savanna parcel (Parcel 2), where buckthorn had been removed by cutting/treating followed by attempted control of new growth via prescribed burns and propane weed torches. It appears that goats are not effective against very young buckthorn, only browsing on it heavily once it grows beyond the seedling stage. If a buckthorn seedbank is present, or if large buckthorn remain and continue to produce seeds, periodic goat browsing will be necessary to keep the plants in check until the seedbank is depleted and/or the mature buckthorn are removed.
Repeated managed browsings by goats over a period of three years had little to no effect on larger (> 60 mm stem diameter) buckthorn within the Garvin Heights savanna. Goats were unable to access foliage or terminal buds (usually 3 m or more aboveground) on these plants, and were unwilling or unable to strip bark from these larger stems. Even though large buckthorn represented only slightly more than 2% of the buckthorn population in the Garvin Heights savannas, this size group included the major berry- and seed-producers within the population. Inability of goats to feed on large buckthorn was not unexpected, as goat contractors generally recommend felling or otherwise pretreating large buckthorn prior to goat browsing (Hart 2012; Nolden 2020; K. Johnson, Diversity Landworks LLC, La Crescent, MN, personal communication). Because goats likely are ineffective on large buckthorn, using goats along with other practices to control woody invasives would improve the chance of successful management (Nolden 2020).
Managed browsing with goats proved to be an effective tool for suppressing established populations of common buckthorn within bluffland savannas in southeastern Minnesota. Suppression was accomplished through 1) direct consumption of foliage and terminal shoots on small buckthorn (< 20 mm stem diameter) that goats could reach or bend over, and 2) bark-stripping of intermediate-sized (20–60 mm) stems that often resulted in complete girdling and eventual top-kill. Girdled and top-killed plants often produced basal sprouts, which goats readily browsed, leading ultimately to complete plant death.
Although goats were successful at eliminating most buckthorn with stem diameters between 5 and 60 mm, they were ineffective against both seedling buckthorn and plants with stem diameters > 60 mm. Prior to using goats to manage buckthorn, all large plants should be felled or chemically treated to eliminate on-going seed production. If a substantial buckthorn seedbank is present, goat browsing will open up the habitat and likely accelerate seed germination, favorably leading to more rapid depletion of the seedbank (Hart 2012). However, during that depletion process that could last from 2 to 6 years (Zouhar 2011), periodic goat browsing will be needed to cull plants as they reach browsable sizes. Managing buckthorn and other woody invasives using goats will be a multi- to many-year undertaking, and landowners should be fully aware that additional control methods will be necessary to successfully suppress and control undesirable vegetation like buckthorn.