Assessing Competition in Warm Season Grass-Legume Intercrops Using Partial Land Equivalent Ratios Across Nine Site-Years.

A warm-season annual intercropping experiment was conducted across the northeastern United States at four sites in 2013 and five sites in 2014. We compared four crop species that differed in stature and N 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 biomass was sampled at ~45 and ~90 d after planting. When averaged across the nine site-years, biomass at the first and second sampling dates, respectively, of the monoculture treatments ranged from 1,040 and 2,500 kg ha⁻¹ (cowpea) to 3,000 and 9,300 kg ha⁻¹ (pearl millet). In general, biomass production of the legume monocultures was lower than the grass monocultures and intercrops at both sampling dates. All intercrops had land equivalent ratios (LERs) greater than one, indicating complementarity, likely a result of resource partitioning. Visualizing competition using partial LERs allowed us to see consistency in different mixtures across the site-years. Intercrops, particularly the four-species mixture, exhibited greater stability in yields across environments. The pearl millet–sorghum sudangrass–sunn hemp intercrop had the greatest evenness and displayed the most partial LERs above 0.33, suggesting that species selected for annual intercrops should have similar monoculture growth rates to minimize asymmetric competition.