| Popis: |
Upland rice, i.e. rainfed rice grown on flat to strongly sloping fields that are welldrained and not bunded, is the major or an important component of the livelihood systems of an estimated 100 million poor farmers in many parts of the humid tropics. It is mostly grown as a subsistence crop in swidden farming systems, which produce highly variable but generally low yields, and exert strong pressure on the environment. Scientific knowledge about these systems is relatively scarce. The aim of this comprehensive diagnostic study, carried out for the greater part in farmers' fields, and partly also under controlled experimental conditions, was to identify, rank, and explain the yield-limiting factors of upland rice, in order to formulate guidelines for further fundamental and applied research. The field study was carried out in Mae Haeng, a Lahu minority village in upper northern Thailand, over a period of 4 years (1993-1996). Farmers grew only local traditional upland rice cultivars, which all belonged to the tropical japonica germplasm. These cultivars displayed a relatively high yield potential, varying from 3.1 t ha-1 for early-maturing cultivars (110 days) to 4.2 t ha-1 for late-maturing cultivars (160 days), but average yields were low for both types of cultivars (1.1 and 1.3 t ha-1, respectively). Yield losses in individual fields were associated with multiple constraints, occurring at various stages throughout the crop cycle, and imposing cumulative and varying degrees of stress on the rice crop. Root aphids (Tetraneura nigriabdominalis) were identified as the most important yield-limiting factor. Other important yield-limiting factors were drought stress, soil erosion, and weeds. The negative impact of these limiting factors is aggravated by shallow rooting of the upland rice crop, which is thought to be a consequence of chemical fertility gradients in the soil profile. Effects of topsoil nutrient status and nematode infestation on rice yields could not clearly be demonstrated. Various other pests and diseases were observed but were of minor importance. Upland rice cultivation practices varied little among fields, and their effects on soil, weeds, and upland rice growth and yield performance were limited or difficult to assess under on-farm conditions. Additional knowledge on the effects of selected cultivation practices, namely (1) burning of fields, (2) field preparation by means of hoe-tillage, and (3) the local use of a herbicidal salt (NaCI) solution for weed control was obtained through complementary experimental research. |
