Free-ranging cattle herd spatial associations and social interactions in a semi-arid rangeland in Texas, USA

Background/Question/Methods Livestock grazing in arid rangelands often exhibits uneven grazing distribution patterns as animals select areas of highly palatable forage. This may result in soil erosion and degradation of vegetation. Strong social associations between individuals could have direct influence on the evenness of pasture use because strong cohesion might limit exploration of unused pastures. The goal of the study is to increase our understanding of spatial patterns and social dynamics in a free-ranging cattle herd. Our main objective is to determine the seasonal and diurnal pattern of spatial associations among individuals and its relationship to herd hierarchy. Positional information of eleven free-ranging Bonsmara cows was obtained at five minute intervals using GPS collars. Four trials each of three weeks were conducted, two in the 2008 winter season and two during the 2009 summer season in a 457 ha pasture in South Texas. To quantify spatial proximity we used ASSOC 1, a computerized spatial grouping algorithm program.

Results/Conclusions
Preliminary results indicate that the herd behaved as a composition of sub-herds. The average number of sub-herds (range [1.8, 2.2]) and cows per sub-herd (range [5.1, 6.7]) remained steady with no significant differences between the time periods (χ² = 4.080, df = 4, P = 0.395), but tended to be higher during the morning and lowest during mid-day. Sub-herd membership affiliation varied as a function of different periods of the day suggesting a dynamic movement of individuals between sub-herds. Some individuals showed high degree of spatial attraction while others showed spatial repulsion, but the relationship was not consistent throughout the day. Further analysis of the complete dataset and the influence of the hierarchical behavior of the herd is in progress to shed more light on the relationship between spatial associations and social interactions among members of the group. Our findings on sub-herd formation and membership stability could prove useful on management strategies that alter herd composition and size to affect the movement behavior and distribution of a herd aimed for a more even pasture use.