Strategies for cut-and-carry green manure production

To fulfill an increasing demand for certified organic fertilizers, there is a need to develop new fertilizer sources for high-value organic cash crops. One possibility is to grow green manure crops that are harvested and transported to fields where the fertilizer is required. To test this strategy, a range of legume crops were harvested at three to four developmental stages and each green manure crop was re-harvested two to four times during the season, according to the specific developmental stages. The legume species investigated were alfalfa, white clover, red clover, and a grass clover mixture (Italian ryegrass and red clover). In addition to these perennial crops, some annual crops were also tested: broad bean, lupin and pea. The results showed that during a full growing season, 400-500 kg ha(-1) of nitrogen (N) were produced by the perennial green manure crops. This production was obtained by four cuts at the early stage, three cuts at the middle stage or two cuts at the late developmental stage. The annual crops produced around 200 kg N ha(-1) from two harvests of the above-ground plant parts per season. Harvesting the seeds, which is one harvest only, resulted in equal amounts of nitrogen ha(-1). Harvest at later developmental stages resulted in the decreased concentration of nitrogen in the plant material and consequently increased carbon to nitrogen (C:N) ratios. A strategy with frequent cuts therefore resulted in cutand-carry green manures with low C:N ratios suitable for in-season fertilization of high-value cash crops.