Planting a cover crop between the main cropping seasons is an agricultural management measure with multiple potential benefits for sustainable food production. In the maize production system of the Netherlands, an effective establishment of a winter cover crop is important for reducing nitrogen leaching to groundwater. Cover crop establishment after maize cultivation is obliged by law for sandy soils and consequently implemented on nearly all maize fields, but the winter-time vegetative ground cover varies significantly between fields. The objectives of this study are to assess the variability in winter vegetative cover and evaluate to what extent this variability can be explained by the timing of cover crop establishment and weather conditions in two growing seasons (2017–2018). We used Sentinel-2 satellite imagery to construct NDVI time series for fields known to be cultivated with maize within the province of Overijssel. We fitted piecewise logistic functions to the time series in order to estimate cover crop sowing date and retrieve the fitted NDVI value for 1 December (NDVI_Dec). We used NDVI_Dec to represent the quality of cover crop establishment at the start of the winter season. The Sentinel-2 estimated sowing dates compared reasonably with ground reference data for eight fields (RMSE = 6.6 days). The two analysed years differed considerably, with 2018 being much drier and warmer during summer. This drought resulted in an earlier estimated cover crop sowing date (on average 19 days) and an NDVI_Dec value that was 0.2 higher than in 2017. Combining both years and all fields, we found that Sentinel-2 retrieved sowing dates could explain 55% of the NDVI_Dec variability. This corresponded to a positive relationship (R² = 0.50) between NDVI_Dec and the cumulative growing degree days (GDD) between sowing date and 1 December until reaching 400 GDD. Based on cumulative GDD derived from two weather stations within Overijssel, we found that on average for the past three decades a sowing date of 19 September (± 7 days) allowed to attain these 400 GDD; this provides support for the current legislation that states that from 2019 onwards a cover crop should be sown before 1 October. To meet this deadline, while simultaneously ascertaining a harvest-ready main crop, in practice implies that undersowing of the cover crop during spring will gain importance. Our results show that Sentinel-2 NDVI time series can assess the effectiveness and timing of cover crop growth for small agricultural fields, and as such has potential to inform regulatory frameworks as well as farmers with actionable information that may help to reduce nitrogen leaching.

在主要农作季节之间种植覆盖作物是一项农业管理措施，对可持续粮食生产具有多种潜在利益。在荷兰的玉米生产系统中，有效建立冬季覆盖作物对于减少氮向地下水的淋溶非常重要。根据法律，在玉米栽培后，必须对沙质土壤进行覆盖作物的种植，因此几乎在所有玉米田都采用这种方法，但是不同田间的冬季植物性地被植物差异很大。这项研究的目的是评估冬季植物被覆的变异性，并评估该变异性在多大程度上可以通过两个生长季节（2017-2018）的被覆作物建立时间和天气状况来解释。我们使用Sentinel-2卫星图像为Overijssel省内已知要用玉米耕种的田地构造NDVI时间序列。我们将分段逻辑函数拟合到时间序列，以便估算覆盖作物的播种日期并检索12月1日拟合的NDVI值（NDVI_十二月）。我们使用NDVI _Dec来代表冬季开始时的覆被作物种植质量。Sentinel-2估算的播种日期与八个田地的地面参考数据进行了合理比较（RMSE = 6.6天）。所分析的两个年份相差很大，2018年夏季更加干燥和温暖。干旱导致较早的预估农作物播种日期（平均19天），NDVI _Dec值比2017年高0.2。结合年份和所有领域，我们发现Sentinel-2检索到的播种日期可以解释55％ NDVI _Dec变异性 这对应于 NDVI _Dec之间的正关系（R ² = 0.50）从播种日期到12月1日直至达到400 GDD为止的累积生长天数（GDD）。根据上艾瑟尔内两个气象站的累积GDD，我们发现在过去的三十年中，平均播种日期为9月19日（±7天），可以达到这400 GDD。这为当前的法律提供了支持，该法律规定从2019年开始，应在10月1日之前播种表层作物。为了达到这个截止日期，同时要确定已准备好收获的主要农作物，实际上意味着在春季播种覆盖作物将变得很重要。我们的结果表明，Sentinel-2 NDVI时间序列可以评估小型农田的覆盖作物生长的有效性和时机，