Cattle grazing distribution in shortgrass steppe: Influences of topography and saline soils

Authors: Gersie, Samuel; Augustine, David; Derner, Justin

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2019
Publication Date: 6/15/2019
Citation: Gersie, S., Augustine, D.J., Derner, J.D. 2019. Cattle grazing distribution in shortgrass steppe: Influences of topography and saline soils. Rangeland Ecology and Management. 72:602-614. https://doi.org/10.1016/j.rama.2019.01.009.
DOI: https://doi.org/10.1016/j.rama.2019.01.009

Interpretive Summary: Cattle grazing pressure is often distributed unevenly across rangeland landscapes. To improve management of these lands, we need to understand factors that influence this uneven distribution. We used data from widely available digital elevation models (DEMs), which are a digital form of topographic maps, to classify a landscape in the shortgrass steppe using two different methods. The first was a topographic wetness index (TWI), which measures the way that water flows across a landscape based on the shape of the land surface, and the second was a suite of topographic position classes (TPC) calculated from the DEMs. We used global positioning system (GPS) collars to track the grazing locations of cattle within several pastures, and used statistical modelling approaches to determine if grazing distribution was related to TWI or TPC. Additionally, we examined whether presence of vegetation growing on saline soils in lowlands altered our predictions of cattle grazing compared to pastures without saline soils. The models indicate that TPC more effectively predicts grazing distribution than TWI, and that the patterns are strongest in the second half of the growing season. The degree to which cattle grazed in vegetation on saline soil was not consistent across multiple pastures. These models, in combination with local knowledge, can be used by managers to predict and manage livestock distribution even in landscapes with relatively subtle topographic variability.

Technical Abstract: The distribution of livestock across heterogeneous landscapes is often uneven, which has important implications for vegetation dynamics and how rangeland managers achieve desired outcomes from these landscapes. Here, we use data from widely available digital elevation models (DEMs) to classify a landscape in the shortgrass steppe using two different approaches: topographic wetness index (TWI) and topographic position classes (TPC) derived from topographic position indices. We used global positioning system (GPS) collars to track the grazing locations of cattle within replicate pastures, and fit generalized linear mixed models to their locations to quantify the influence of topography on grazing distribution. Additionally, we examine the influence of the presence of saline vegetation communities on cattle utilization of lowlands. The resulting models indicate that TPC more effectively predicts grazing distribution than TWI, and that the patterns are strongest in the second half of the growing season. Model parsimony was improved with the inclusion of saline vegetation communities, although the magnitude of utilization of these communities by cattle was not consistent across multiple pastures. These models, in combination with local knowledge, can be used by managers to predict and manage livestock distribution even in landscapes with relatively subtle topographic variability.