Nitrogen fertilization and utilization of sorghum as an extra forage crop on dairy farms

Sorghum may have potential as an extra forage crop on dairy farms. It can be an addition to the crop rotation with maize and grass-clover leys, in order to prevent disadvantages of continuous maize cropping. However, concerning the cultivation of sorghum, questions arise around fertilization and nitrate leaching caused by residual mineral nitrogen (N) after harvest. Therefore, in 2018 and 2019, we performed experiments to quantify N utilization of sorghum and to estimate optimal N fertilization, including the risk of nitrate leaching. Two types of sorghum (a type high in starch (ST) with a potential for high starch content and type high in fibre (FT) with high cell wall content) and maize (reference) were compared, each with three levels of N fertilization (0, 70, 140 kg N ha^-1). In both years, dry matter (DM) and N yield were highest in maize (P<0.001; up to 6 t ha^-1 more DM yield for maize than for ST or FT). The two sorghum types differed in N response: FT showed an N response, whereas ST did not. The type high in fibre had the lowest residual N in the soil after harvest (P<0.001; up to 40 kg ha^-1 less residual N on FT plots), indicating a possible reduction of nitrate leaching. Optimal level of fertilization was expected to be lower for sorghum than for maize. A relatively high yield of ST under N limitation indicates that sorghum cultivation is possible on relatively poor soils. In conclusion, because of the low required N inputs and the potentially reduced risk on nitrate leaching, sorghum is a promising crop to extend crops rotations on dairy farms.