By Stella Belliss, Alexander Amies and Heather North
Winter forage can be any crop fed to stock, but the main ones are brassicas (especially kale), fodder beet, and cereals. These crops are commonly strip-grazed, with each daily strip of feed being eaten down to near bare ground and usually not re-growing afterwards. Alternatively, stock can be given access to the whole paddock, which they gradually graze down to near bare soil over a period of weeks or longer. New guidelines for growing and managing winter forage crops mean we need to monitor where these crops are so we can mitigate any potential risks – nitrates and sediment passing into waterways, for example.
Manaaki Whenua – Landcare Research have been using medium resolution satellite imagery to map vegetation and bare soil for many years and now have a semi-automated methodology for mapping these winter forage crops. This uses a March-September times series of imagery from the European Space Agency’s (ESA) Sentinel-2A and -2B satellites, processed on the national high speed computing system (NeSI) from raw imagery to analysis-ready data – geometrically, atmospherically, and topographically corrected, cloud and shadow removed, plus topographic flattening to minimize slope-induced brightness variations.
The winter forage mapping method is a combination of spectral analysis and classification rules. The main steps are:
Masking out all the land areas that are not used for agriculture
Using ground truth data to define spectral signatures for forages, non-forage land cover classes, and bare ground.
Creating GIS paddock polygons from an automated paddock boundary methodology, enabling analysis at the whole-paddock – rather than pixel-level
Combining the evidence from the time series sequence of land cover classifications to identify winter forage as a land use
Assigning each winter forage paddock to one of three certainty levels, depending on the strength of the evidence available.
For the highest certainty of winter forage identification, we would have an unequivocal spectral signature for the paddock land cover and a full sequence of imagery so we can see it right through from full canopy cover early in the winter grazing season through to a bare or nearly bare paddock by the end. However, persistent cloud cover over some regions means we may not be able to see the agricultural land there for weeks at a time, even though the Sentinel-2 satellite coverage is every 5 days.
For each paddock polygon, we also record the average slope within that paddock. These slopes are from an in-house DEM with 8m postings, derived from LINZ contours. If required, we can also record when a paddock/a proportion of a paddock became bare and how long it stayed that way.
In any given year, around 2% of New Zealand’s agricultural/horticultural land is used for intensive grazing with the coldest regions; Southland, Canterbury and Otago having the largest areas of winter forages and the northernmost regions the least.