Crop biomass not species richness drives weed suppression in warm-season annual grass-legume intercrops in the Northeast

Authors: BYBEE-FINLEY, ANN - Cornell University; Mirsky, Steven; RYAN, MATT - Cornell University

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2017
Publication Date: 9/1/2017
Citation: Bybee-Finley, A., Mirsky, S.B., Ryan, M. 2017. Crop biomass not species richness drives weed suppression in warm-season annual grass-legume intercrops in the Northeast. Weed Science. 65:669-680.

Interpretive Summary: The use of two or more cover crop species in mixture (intercrops) when cash crops are not present has gained popularity among farmers. Different cover crop species use water and nutrients from different soil depths, have varying growth architectures (i.e. vining vs. upright), and individually provide different benefits to a cropping system. The complementary features of intercropped species may make the intercrop more competitive against weeds than a single monocrop species by more effectively monopolizing resources such as nutrients and light. This potentially enhanced competitiveness against weeds is valued by organic farmers (who have limited weed-control options) and by all farmers concerned about the rise of herbicide resistant weeds. We conducted a two-year experiment across the Northeastern US to examine the effects of functional diversity and species richness on weed suppression. We compared four annual crop species that differed in stature and nitrogen acquisition traits: the upright grasses pearl millet and sorghum-sudangrass, and the N-fixing legumes cowpea (vining) and sunn hemp (upright). Crops were seeded in monoculture and in 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 days after planting. We found some evidence that increasing crop species richness can increase weed suppression: weed biomass in mixtures was 26% and 46% lower than in monocultures at the second sampling date. However, this finding was driven by the fact that legume monocultures had relatively high weed biomass levels, resulting in a negative relationship between weed biomass and species richness. It has long been recognized that legume cover crops are not the best choice for weed suppression, hence the interest in growing legumes with grasses. Legumes provide other agroecosystem benefits including biological nitrogen fixation, increased forage quality, and improved soil health. Our results support current recommendations that farmers who seek the benefits of legume cover crops but also need weed suppression grow a legume in combination with a grass. This study will be of use to Extension and ag-industry personnel who advise farmers on the use of cover crops.

Technical Abstract: Intercropping with functionally diverse crops can reduce the availability of resources that could otherwise be used by weeds. An experiment was conducted six times across the northeastern United States in 2013 and 2014 to examine the effects of functional diversity and species richness 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. In general, crop biomass production of the legume monocultures was lower than the grass monocultures and intercrops at both sampling dates. Intercrops tended to perform similarly to grass monocultures at the second sampling date. Across the six site-years, 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. Intercrop treatments generally produced greater crop biomass and lower weed biomass than the monoculture treatments. When averaged across treatment categories (i.e. monocultures, three species mixtures, and the 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, which suggests that increasing crop species richness can increase weed suppression. However, when examined at the functional group level, grass monocultures had relatively low weed biomass levels and thus there was no clear relationship between crop species richness and weed biomass. On the other hand, legume monocultures had relatively high weed biomass levels, which resulted in a negative relationship between weed biomass and crop species richness. Despite the weak weed suppression from the two legume crops tested in this research, legumes provide other agroecosystem benefits including biological nitrogen fixation, increased forage quality, and improved soil health.