Invasive Plant Control by Livestock: From Targeted Eradication to Ecosystem Restoration

Targeted grazing (TG), the use of livestock to accomplish specific management objectives, is an effective technique for controlling invasive plants (Launchbaugh et al. 2006 and references therein). Typically, the area targeted for treatment is fenced, and modifications, such as the cutting of dense vines, are made to ensure the effectiveness of the treatment. The animals are deployed for periods of time adequate to severely “damage” the targeted species. Additional treatments, such as herbicide application, may be used as needed after the livestock has been removed.

The effectiveness of TG is illustrated by its use in a cattle pasture invaded by multiflora rose (Rosa multiflora) at Glynwood Center, in Cold Spring, New York. Because cattle lack the dentition to browse the thorny rose stems, this invasive has spread rapidly, resulting in the loss of thousands of acres of pasture intended for dairy and beef cattle production. In May 2009, we deployed 13 Boer goats (Capra hircus) into an infested (20–30 plants, 1–2 m high, per 10 m²) 0.2-ha portion of pasture at Glynwood Center. The goats remained on pasture until October and readily browsed the multiflora rose. Fecal pellets processed for histological analysis were examined under an inverted microscope, revealing that 42% of the diet of the goats consisted of multiflora rose (Caggiano et al. 2010). Over the course of the season, the vitality (percent of plant with foliated, pliable stems) of multiflora rose in the grazed area declined, and after 2 seasons, many plants were dead (Figure 1). In ungrazed portions of the pasture, multiflora rose appeared healthy. In addition to the invasive species, however, the goats also depleted existing graminoids and forbs and, by September, hay supplementation was needed to support their nutrition. In 2010, the treatment flock was reduced to 6 goats, but hay supplements were necessary by mid-July. We removed the animals in mid-September. The lesson learned from the project is that successful suppression of the invasive may come at a cost to the larger plant community.

Figure 1.

Multiflora rose vitality (percent of plant with foliated, pliable stems). Mean (± 1 standard error) of 20–25 randomly selected plants in grazed (Boer goats; dashed line) and ungrazed (solid line) portions of a pasture at Glynwood Center, Cold Spring, New York, May–October 2009 and April–October 2010.

Grazing, like other manipulations of landscapes, disturbs the plant community. However, the disturbance caused by grazing need not disrupt the ecosystem. If the rate of plant biomass removal is lower than the rate of biomass production, overgrazing should not occur. The amount of time animals spend on the landscape is most critical, more so even than the number of animals on that landscape. Wild ungulates tend to pack densely on the landscape but move constantly.

We are using an intensive rotational grazing (IRG) protocol in our studies of the restoration of biologically invaded landscapes with livestock. IRG mimics the distribution of wild ungulates. With IRG, the landscape is divided into paddocks. Animals are stocked at 2–4 times conventional densities (5–9 tons biomass hectare⁻¹) and rotated through the paddock system at high frequencies (2–3 d paddock⁻¹). The grazers open the canopy, and we have observed rapid increases in species richness in grazed paddocks during 30–60 d periods, when grazers are present in any paddock only about 20% of the time (Table 1). We use Romney sheep (Ovis aries), a common breed, inexpensively obtained as culls. Before and after the grazing phase of a study, we perform a suite of measurements on individual plants of the target invasive species and on the plant community. Measurements germane to the present discussion are canopy height, measured with a meter stick or tape, and species richness measured in 10–25 randomly selected 0.25 m² quadrats in each paddock.

Table 1.

Attributes and results of intensive rotational grazing studies in 2 invaded landscapes in the upper Hudson River Valley of eastern New...