Interseeding red clover (Trifolium pratense L.) or alfalfa (Medicago sativa L.) into winter cereals in the North Central USA can provide forage and a green manure crop. We hypothesize that winter cereal canopy traits such as leaf area index (LAI) and whole plant dry matter (DM) influence interseeded legume establishment and productivity, yet the effect of canopy traits on resource competition in interseeded systems is not well understood. This study was conducted from 2005 to 2007 to evaluate the impact of diverse cereal canopy traits on the establishment of frost-seeded legume intercrops. In March, red clover and alfalfa were frost-seeded into three winter wheat (Triticum aestivum L.) and three triticale (X Triticosecale Wittmack) varieties selected based on LAI, plant height, DM, and maturity date. Across three growing seasons, the cereals produced a range of LAI from 2.1 to 6.2 and whole plant DM at harvest of 817 to 2029 g m^-2. Legume densities were influenced by cereal in one year and legume DM was influenced by cereal in two years. Alfalfa and red clover densities were similar, yet DM production was 42% higher in red clover 40 d after harvest. The presence of a legume intercrop did not affect grain yield or yield components, but reduced weed densities and DM 40 d after grain harvest.

Linear models to predict legume densities were developed using cereal LAI, intercepted photosynthetically active radiation (IPAR), cereal species, and legume origin. Soil water content was assumed not to contribute to legume mortality because soil water content did not exceed the permanent wilting point for an extended duration during the two growing seasons when data were collected to develop the models. When maximum cereal LAI > 4.1 and IPAR exceeded 90%, the models predicted post-harvest legume densities within five plants m^-2 or 2 to 11% of observed densities. When maximum cereal LAI < 4.1 and IPAR remained below 90%, estimates were less accurate, predicting densities of 16 and 24 plants m^-2 or 16 and 21 % less than observed.

An additional study reported herein evaluated the effect of intensive tillage (IT), moderate tillage (MT), and no-tillage (NT) with and without compost amendment in a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.]-wheat/red clover rotation on frost-seeded red clover establishment. Higher grain yield in IT corresponded to lower red clover densities at wheat harvest in one of three years. Red clover plant densities at wheat harvest were higher under NT and MT compared with IT in one year and were 41% lower with compost 40 d after wheat harvest of the same year. Red clover shoot DM at wheat harvest and 40 d after harvest averaged 70% higher when grown without compost in one year. Wheat and red clover under MT consistently performed equal to or greater than NT or IT. Producers should achieve similar red clover densities and DM using this intercrop in reduced tillage systems. However, they must evaluate the tradeoff between the positive effect of compost on corn and soybean in the rotation and the potentially negative residual effect of compost on red clover DM production. When selecting winter cereals for this intercrop, producers must also give attention to varieties known to produce maximum LAI values above 5.6 because of their potential to reduce legume productivity.