Winter-forage crop grazing in the Gore-Mataura area of Southland: using time-series mapping to estimate location and frequency of cropping

Winter grazing of forage crops is a key land-use in southern New Zealand, providing important feed for livestock but has been identified as risky if not managed well, potentially resulting in soil degradation and nutrient losses. We hypothesised that analysing an existing time series of winter-forage maps, derived from satellite imagery could be used to identify how often paddocks are re-used for winter forage. A pilot study was undertaken to explore the practicality and utility of this new method by examining maps derived from satellite images of the Gore-Mataura area, Southland taken in 2013, 2014, 2017, and 2018. Within the study site (67,618 ha), 8925 ha was classed as winter forage in one or more of the source maps. Eighty-five percent of this area was used in only one of the four years, and just 1% in three or four years. High-certainty class pairs for 2013/14 and 2017/18 show two consecutive years of winter forage in the same paddock, 31% or 21% of the time, respectively. These winter-forage crops were generally grown on Brown soils (63%), followed by Pallic and Gley soils. Although, this study was limited by differences in the mapping methodologies of the source maps, it nonetheless  demonstrated that potentially valuable data can be derived. It showed a low level of repeat use of paddocks for winter forage grazing over all the years studied, and that Brown soils are more commonly used for winter forage than previous studies suggested.