In the Red River Delta, farmers have experience in growing an upland crop between two rice crops. Spring rice is harvested in June and early 'Mua' rice crop is harvested at the beginning of November. The possible upland crops to follow in this cropping pattern are potato, vegetables, beans etc., or at the end of October it could be corn, sweet potato or soybean. They are sown immediately after harvesting rice to avoid low temperature in winter. Short duration and non-photosensitive rice varieties are chosen for the early Mua rice crop. When necessary, crop seedlings for each hill are prepared by sowing seed in separate boxed seedbeds made of leaves or plastic. Sometimes farmers put seedlings of the upland crop in rice fields about 10 days before harvesting the rice crop. Such cropping systems (rice-rice-upland crop) are practiced on hundreds of thousands of hectares in the North, including the Red River Delta. Farmers can get more cash from the upland crops, especially by growing temperate vegetables such as cabbage, cauliflower and potato which can yield 15-20 tonnes/hectare. There is a residual effect from the upland crop for the subsequent spring rice crop which includes minimizing land preparation and weed control costs, and spending less on fertilizer.
In the Mekong delta, there is no problem of low temperatures. Farmers have grown many upland crops in rotation with rice, except crops of temperate origin such as cauliflower. Farmers grow winter-spring rice from November to February, then follow up with an upland crop before summer-autumn rice. It is interesting to note that in Longxuyen quadrangle, peanut has been grown on heavy soils (60 percent clay) on thousands of hectares, which can yield 3-4 tonnes/hectare. The soils in this region are perhaps high in organic matter (5-6 percent) and the difference of diurnal day/night temperature is high (6-10°C). In terms of residues for the next rice crop, groundnut and vegetable soybean (65 days) are considered the best. Generally, no tillage is applied for upland crops. After burning rice straw, farmers make planting holes and place the seed. One of the problems of crop intensification for crop diversification on rice land is damage from floods in the rainy season. Floods occur from the middle of August and recede in November. Farmers are attempting to experiment with new technologies to overcome such constraints, for example, by using rice varieties with a duration below 90 days, or to apply a method of water seeding for the winter-spring rice crop in order to shorten the duration of the crop in the field.
In the 1960's and 1970's, Azolla was cultivated on a large scale in about 0.5 million hectares in the north, especially in the Red River Delta. Azolla could replace 30-50 percent of N fertilizer for winter-spring or spring rice crop, but this practice has since been abandoned. The reason is that Azolla cultivation requires a lot of labour with very precise conditions in the winter season and farmers had to spray insecticides and apply phosphorous fertilizer every 5 days. They also had to manage water and remove dew to dry Azolla every day when temperature was low (below 15° C), if not, Azolla would die. Leguminous crops such as groundnut, soybean, mungbean for either cash crop or green manure are feasible cultivation alternatives under these conditions. These leguminous crops can be grown in rotation with rice or intercropped with corn as well. Data from production experiments show that vegetable soybean (65 days for green soybean, 90 days for soybean seed) and peanut are the best for the above purpose, grown either as a cash crop or for green manure. Yield of the following rice crop can be increased by 20-30 percent, or it can decrease N requirement by 30-40 kg/ha. When grown as an intercrop with corn, spacing of 80 cm × 25 cm should be replaced by spacing of 120 cm × (25 × 40 double lines), as by these configurations the corn population will have the same plant density of 50,000 plants/hectare, which gives the same yield. Three lines with 40 × 20 cm of leguminous crop can be grown between two lines of corn with a spacing of 120 cm.
In the Red River Delta, farmers have experience in growing an upland crop between two rice crops. Spring rice is harvested in June and early 'Mua' rice crop is harvested at the beginning of November. The possible upland crops to follow in this cropping pattern are potato, vegetables, beans etc., or at the end of October it could be corn, sweet potato or soybean. They are sown immediately after harvesting rice to avoid low temperature in winter. Short duration and non-photosensitive rice varieties are chosen for the early Mua rice crop. When necessary, crop seedlings for each hill are prepared by sowing seed in separate boxed seedbeds made of leaves or plastic. Sometimes farmers put seedlings of the upland crop in rice fields about 10 days before harvesting the rice crop. Such cropping systems (rice-rice-upland crop) are practiced on hundreds of thousands of hectares in the North, including the Red River Delta. Farmers can get more cash from the upland crops, especially by growing temperate vegetables such as cabbage, cauliflower and potato which can yield 15-20 tonnes/hectare. There is a residual effect from the upland crop for the subsequent spring rice crop which includes minimizing land preparation and weed control costs, and spending less on fertilizer.In the Mekong delta, there is no problem of low temperatures. Farmers have grown many upland crops in rotation with rice, except crops of temperate origin such as cauliflower. Farmers grow winter-spring rice from November to February, then follow up with an upland crop before summer-autumn rice. It is interesting to note that in Longxuyen quadrangle, peanut has been grown on heavy soils (60 percent clay) on thousands of hectares, which can yield 3-4 tonnes/hectare. The soils in this region are perhaps high in organic matter (5-6 percent) and the difference of diurnal day/night temperature is high (6-10°C). In terms of residues for the next rice crop, groundnut and vegetable soybean (65 days) are considered the best. Generally, no tillage is applied for upland crops. After burning rice straw, farmers make planting holes and place the seed. One of the problems of crop intensification for crop diversification on rice land is damage from floods in the rainy season. Floods occur from the middle of August and recede in November. Farmers are attempting to experiment with new technologies to overcome such constraints, for example, by using rice varieties with a duration below 90 days, or to apply a method of water seeding for the winter-spring rice crop in order to shorten the duration of the crop in the field.In the 1960's and 1970's, Azolla was cultivated on a large scale in about 0.5 million hectares in the north, especially in the Red River Delta. Azolla could replace 30-50 percent of N fertilizer for winter-spring or spring rice crop, but this practice has since been abandoned. The reason is that Azolla cultivation requires a lot of labour with very precise conditions in the winter season and farmers had to spray insecticides and apply phosphorous fertilizer every 5 days. They also had to manage water and remove dew to dry Azolla every day when temperature was low (below 15° C), if not, Azolla would die. Leguminous crops such as groundnut, soybean, mungbean for either cash crop or green manure are feasible cultivation alternatives under these conditions. These leguminous crops can be grown in rotation with rice or intercropped with corn as well. Data from production experiments show that vegetable soybean (65 days for green soybean, 90 days for soybean seed) and peanut are the best for the above purpose, grown either as a cash crop or for green manure. Yield of the following rice crop can be increased by 20-30 percent, or it can decrease N requirement by 30-40 kg/ha. When grown as an intercrop with corn, spacing of 80 cm × 25 cm should be replaced by spacing of 120 cm × (25 × 40 double lines), as by these configurations the corn population will have the same plant density of 50,000 plants/hectare, which gives the same yield. Three lines with 40 × 20 cm of leguminous crop can be grown between two lines of corn with a spacing of 120 cm.
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