Towards a more productive and sustainable cropping system in the democratic People's Republic of Korea. I. Rice production

To improve food security in the Democratic People's Republic of Korea, an increase in grain production is essential. However, the access to and affordability of fertilizer is limited. At two cooperative farms (Hyongsan and UnSong) in the central western grain producing region, field trials with rice were established in 2002 and continued to 2005. The aim was to quantify the yield response of lowland rice to a legume green manure (hairy vetch; Vicia villosa) sown in autumn-winter of each year. In other cropping treatments one, two or all vetch crops in the three-year sequence were replaced by wheat, or by fallow (traditional). Thus, over the course of the experiment the number of vetch crops relative to rice crops (vetch : rice) varied 1.0, 0.67, 0.33, and 0.0 between the rotation treatments. Each rotation treatment was amended with nil or plus urea (60 kg N/ha) applied only to the rice crop. Basal nutrients of P, K, and Mo were applied to all treatments and the continuousrice-fallow and rice-wheat rotations were duplicated without the basal nutrients. The yields of rice sites were limited by N fertility. Rotations that included vetch supported significantly higher mean rice yields than the continuous rice-fallow or rice-wheat rotations. In the absence of urea, the mean yield of rice in the treatment with vetch : rice = 1.0 was 0.55 t/ha (12%) greater than the yield in the rice-fallow treatment, and was 68% of the mean rice yield achieved with the annual applications of urea in the rice-fallow rotation. The yield of rice was positively related to the amount of vetch preceding each rice crop, and the mean annual gain in rice yield was estimated at 67 kg/fresh weight tons of vetch/ha. However, the biomass of vetch was severely reduced in some plots by waterlogging, by grazing or foraging, and by cold sensitivity such that improved vetch management may feasibly support greater increases in rice yield. A combination of both urea and vetch produced the highest mean rice yields, but it also increased the risk of blight in rice. It was concluded that widespread adoption of winter legume green manuring in the DPRK would significantly improve food security and reduce reliance on urea aid; however, further research is required to optimize this strategy for rice yield.