GIS hot-spot analysis of pasture utilization of two separate herds of goats over time

Abstract Text: An understanding of pasture landscapes that promotes or hinders efficient utilization is essential for proper management. The objective of this study was to characterize pasture utilization of two separate herds of goats utilizing the same pasture in different years. The study area was a 14.1-ha pasture of predominantly fescue, bermudagrass, panicums, bahia grass, and broomsedge bluestem but was reverting to a wooded area containing predominately pecan, elm, and honey locust sapling-size trees. In year one (Y1), the study area was stocked with 36 Spanish goats, of which 10 were fitted with GPS collars and in year two (Y2), the study area was stocked with 58 Spanish goats, of which 19 goats were fitted with GPS collars. Different goats were used in Y1 and Y2. For the first 2 wk of pasture introduction, goats wore the collars, which recorded a fix every 5 min. An average nearest neighbor analysis yielded a z score of -150.2 (P < 0.01) for Y1 and -150.1 (P < 0.01) for Y2, indicating highly clustered events for both years. A GIS point-in-polygon (PiP) analysis was conducted for each year using the same grid (1,792 10 × 10 m squares) for each year and with unique grid identifiers. Moran’s I, a measure of spatial autocorrelation, indicated a peak at 30 m and that value was used in the hot-spot (Getis-Ord Gi* statistic) analysis conducted on the resulting PiP. Based upon the resulting z-scores from the hot-spot analysis, each square was classified as very low (VL), low (L), moderate (M), high (H), and very high (VH) usage. Y1 had greater (χ² = 13.89, P < 0.01) VL and lower VH squares (82% and 1%, respectively) compared with Y2 (80% and 3%, respectively). Hot-spot analysis revealed two areas of H and VH usage for both years. One of the areas was a small grove of trees that had almost a 100% overlay for both years. The degree of similarity in pasture usage was high as indicated by a Spearman’s rank correlation coefficient (0.76; P < 0.01) of the square z-scores for Y1 and Y2. Even though the two herds of goats never interacted and were separated by time, their pasture utilization was strikingly similar. Further work is needed to investigate the physical features of the pasture to understand the causes behind this similarity.

Keywords: GIS, GPS, goats