Legume Suppress Weeds in Pigeonpea

Pigeonpea (Cajanus cajan) is often intercropped with legumes or cereals in rainfed areas. Recent interest in integrated weed management has brought attention to the possibility of suppressing weeds with intercrops. Short duration legumes which have fast early growth and close canopies are good competitors with weeds. Studies in Kanpur, India at the Directorate of Pulses Research (ICAR) indicate that intercropping with these legumes can effectively suppress weed growth in pigeonpea. Short duration of a legume intercrop means that it only competes with the pigeonpea for a short period of time; this minimizes pigeonpea yield loss due to intercropping. In addition, the intercrop itself can contribute to total productivity of the plot. The results suggest that intercropping with legumes in pigeonpea could greatly reduce the need for other financially and environmentally costly weed control measures.

The studies examined the weed suppressing ability and total productivity of four short duration legumes and one cereal intercropped with pigeonpea. Intercrop species were urdbean (Vigna mungo), mungbean (Vigna radiata), cowpea (Vigna unguiculata), soybean (Glycine max), and sorghum (Sorghum bicolor). Two planting systems were tested for each crop combination: uniform rows of alternating pigeonpea and an intercrop, rows 30 cm apart; and paired rows of pigeonpea alternating with one row of an intercrop, all rows 40 cm apart. Trials were conducted in 1984, during an unusually dry season (662 mm rainfall) and in 1985, with 1127 mm rainfall during the crop season.

The intercrops suppressed weed growth by 20-45% over an unweeded pigeonpea monocrop. Overall weed intensity was greater in 1984, the dry year, than in 1985. The intercrops suppressed weeds more effectively in uniform row than in paired row plantings, although in 1985 there was less difference in weed suppression between the planting systems. Rows were more closely spaced in uniform row plantings than in paired row plantings; this probably increased the intercrop's ability to compete with weeds. Among the intercrops, cowpea was the best competitor with weeds. This was attributed to cowpea's fast growth rate and good canopy structure. Weed suppressing ability of the other intercrops varied between the years. Mungbean and urdbean were very effective in 1985, but did not differ from soybean and sorghum in 1984, the dry year.

Pigeonpea seed yield in a weed free monocrop was 2811 kg/ha in 1984 and 2367 kg/ha in 1985. Unchecked weeds reduced these yields by 25.3% and 27.3% respectively. Intercropping with legumes gave pigeonpea yields similar to the unweeded monocrop, but intercropping with sorghum further reduced pigeonpea yields by 46.6% and 34.3% from the weed free monocrop yield in 1984 and 1985 respectively. The sorghum competed with pigeonpea for a longer period of time than did the legumes; thus although sorghum suppressed weeds, it also suppressed pigeonpea growth. Planting system had no consistent effect on pigeonpea yield. Sorghum yield was 625 kg/ha in 1984 and 855 kg/ha in 1985, higher than that of the best-yielding legume intercrop, cowpea (44 kg/ha in 1984, 470 kg/ha in 1985).

The pigeonpea seed equivalent yield reflects the total productivity of each intercropping system. During both years, all intercrops of legumes with pigeonpea were more productive than the unweeded pigeonpea monocrop. The pigeonpea + mungbean system was most productive, followed by pigeonpea + urdbean and pigeonpea + cowpea. In addition, the pigeonpea + mungbean system was as productive as the weed free pigeonpea monocrop. This suggests that intercropping with mungbean may replace other weed control measures in pigeonpea, with no real loss in productivity.

The weed suppression, pigeonpea yield, and total productivity of each crop combination varied with both the planting system and the year (or rainfall). For example, during both years total productivity was higher in uniform row planting than in paired rows for pigeonpea + urdbean and pigeonpea + mungbean, but differences between planting systems for the other crop combinations varied between years. Thus while short duration legume intercrops have a potentially important role in integrated weed management systems, both formal studies and on-farm trials are needed to help farmers determine the crop combinations and planting systems which most effectively suppress weeds in their regions.

M. Ali. 1988. Tropical Pest Management 34(4): 384-387.