EFFECTS OF CATTLE STOCK DENSITY ON TRAMPLING LOSS OF SIMULATED GROUND NESTS

Abstract

Short-duration grazing (SDG) involves subdividing larger pastures into smaller paddocks (generally 8-20) and rotating a herd of livestock throughout. The recent popularity and controversy surrounding SDG on rangelands has spawned a variety of grazing impact-related studies relative to livestock performance (Heitschmidt et al. 1982b), watershed ecology (Thurow et al. 1986), herbage dynamics (Heitschmidt et al. 1982a, Brummer et al. 1988), and wildlife (Koerth et al. 1983, Bareiss et al. 1986, Schulz and Guthery 1987, 1988). The potential for disruption of ground-nesting birds from livestock trampling is especially important to wildlife managers. Higher stocking rates and increased paddock numbers advocated by proponents of SDG result in higher stock densities, thus increasing the potential for nest disruption by grazing livestock. In the context of this discussion, we express stocking density as head/ha (hd/ha), rather than animal units/ha (AU/ha). This deviates from the normal expression of stocking density as AU/ha which implies forage demand based on the weight of the animal. However, from a trampling standpoint, the emphasis is on the number of animals, i.e., hooves, and not on livestock weights or forage demand per se. Koerth et al. (1983) evaluated ground-nest loss from trampling under SDG and continuously grazed (CG) pastures and reported nest losses averaging 9% for SDG and 15% for CG. They concluded that trampling losses were not a major management concern at the stocking density studied (1.2 hd/ha). Similarly, Bareiss et al. (1986) compared SDG versus CG relative to trampling nest loss and reported insignificant nest loss from trampling at the stocking densities studied (0.1 to 3.6 AU/ha) (hd/ha not reported). However, they hypothesized that as stock density exceeded 2.5 AU/ha, nest loss from trampling could become a significant management concern. Stock density is related to herbage availability, i.e., areas with higher herbage production potential can accommodate more grazing animals. The previous trampling studies (Koerth et al. 1983, Bareiss et al. 1986) were conducted in areas with annual forage production of less than 3,000 kg/ha annually. More productive rangelands (tallgrass prairie) may be subjected to higher stock densities, possibly increasing the likelihood for ground-nest disturbance from livestock grazing. Stock densities in excess of 2.5 hd/ha are likely on such sites when an intensive grazing system is employed. As an example, a common stocking density for summer yearling production under continuous grazing in tallgrass prairie would be 0.6 hd/ha. Under an 8-paddock short-duration grazing program, density would increase to 4.9 hd/ha. A 16-paddock system would increase density to 9.9 hd/ha. Because tallgrass prairie represents an important habitat for ground-nesting bobwhites, ring-necked pheasants (Phasianus coichicus), greater prairie-chickens (Tympanuchus cupido pinnatus), wild turkey (Meleagris gallopavo), and a variety of nongame birds, potential impacts from SDG have economical and ecological significance. Further, other intensively stocked grazing methods, like intensiveearly stocking (Smith and Owensby 1978), are becoming more popular with ranchers in the tallgrass prairie region. In order to address the potential for ground-nest disruption from in-