Weed Suppression in Summer Annual Grass-Legume Intercrops.

Compared to monocultures, growing multiple crops together as a mixed intercrop can increase weed suppression by more fully utilizing light and soil moisture that would otherwise be available to support weed growth. We conducted an experiment across the northeastern United States in 2013 and 2014 to examine the effects of species richness and functional diversity on weed suppression. We compared four annual crop species that differed in stature and nitrogen acquisition traits: (i) pearl millet [Pennisetum glaucum (L.) R. Br.], (ii) sorghum sudangrass [Sorghum bicolor (L.) Moench × S. sudanense (Piper) Stapf], (iii) cowpea [Vigna unguiculata (L.) Walp], and (iv) sunn hemp (Crotalaria juncea L.). Crops were seeded in monoculture and in three- and four-species mixtures using a replacement design where monoculture seeding rates were divided by the number of species in the intercrop. Crop and weed biomass were sampled at ~45 and 90 d after planting across the six site-years. In general, crop biomass production of the legume monocultures was lower than the grass monocultures and intercrops at both sampling dates. Weed biomass ranged from 60 and 280 kg ha^-1 (in the 2013 Aurora, NY site-year) to 1,050 and 2,110 kg ha^-1 (in the 2014 Willsboro, NY site-year) in the first and second sampling, respectively. When averaged across treatment categories (i.e. monocultures and the three and four species mixtures), weed biomass was 26% and 46% lower at the second sampling date for three and four species mixtures respectively, compared to average weed biomass in the monocultures. However, when examined at the functional group level, grass monocultures had relatively low weed biomass levels and thus there was no weed suppression advantage nor disadvantage to adding additional crops to grass monocultures. Results illustrate the importance of considering functional group differences when examining the relationship between crop species richness and weed suppression.