Weed Productivity and Composition in Sole Crops and Intercrops of Barley and Field Pea

Abstract

(1) The hypothesis that intercrops have fewer, smaller weeds than their component sole crops was tested by sowing pure and mixed crops of barley and field pea at commercial densities and at double these densities in two fields near Ithaca, New York, U.S.A. The effect of these cropping systems on the species composition of associated weeds was also investigated. (2) For both sole crops and intercrops weed productivity decreased and crop productivity increased with higher crop density. (3) Crop treatments had no effect on weed numbers relative to unplanted control treatments. (4) For both highand low-density treatment series, weed productivity decreased in the order unplanted controls > pea sole crops > intercrops > barley sole crops. Weed productivity in the high-density intercrop, formed by sowing together the two low-density sole crops, was not significantly different from weed productivity in the low-density barley sole crop. (The estimated difference+S.E. was 0 77+21 8 g m-2.) (5) Weed suppression by barley may have resulted from competition for soil moisture since weeds which grew in barley sole crops or barley/pea intercrops had lower predawn water potentials than weeds in unplanted controls or in pea sole crops. (6) Within both density series, above-ground crop productivity decreased in the order barley > intercrop > pea. Yield of the high-density intercrop was nearly equal to the sum of yields of the two low-density sole crops. (7) Relative abundance of the major weed species differed among crop treatments. In general, the most dominant weed species was more suppressed than other species as crop productivity increased. This response is explained in terms of a dominance hierarchy.