The Sixth Action Program (AP) imposes stricter requirements on catch crops after maize on sand and loess soils (measure 5.4.3) and introduces an obligation to grow catch crops after potatoes (measure 5.4.4). The Sixth AP also aspires to stimulate catch crops after other crops (measure 5.6.3). Field trials can fill in the lack of knowledge about the effect of catch crops on nitrate leaching, define more clearly the conditions under which catch crops are effective, and increase support for the cultivation of catch crops in the sector

Research shows that catch crops can limit nitrogen (N) loss (Schröder et al., 1996; -, 2013) but that the date on which the growth of the catch crop starts in late summer or autumn is strongly decisive.

The Sixth Action Program envisages an important role for catch crops. The requirements for a timely start of growth are limited, for the time being, in order to limit resistance in the sector. In the case of sowing a catch crop after harvest of maize and potatoes, the catch crop must be sown by 1 October and 31 October respectively. In the case of underseeding in maize, no requirements are set for the harvest date of the maize.

The implications of those requirements for limiting N and P leaching and runoff are not fully known. It is therefore proposed to carry out field trials for both potatoes and maize for several years. The treatments to be investigated consist of variants of the sowing dates (stubble sowing) or the harvest date of the main crop (stubble and underseeding), variants of the establishment of the main crop (variety type, N fertilization, plant number) aimed at bringing forward the harvest date, and variants of the sowing density of the catch crop. In the selection of the research objects and the measurement campaign, attention is paid to both the nitrate leaching to groundwater and the run-off of N and P to surface water.