Relative maturities and row spacing effect on establishment of interseeded cover crops into soybean

An experiment was conducted investigating the relative maturity and row spacing effect on establishment of intereseeded cover crops in soybean. This experiment used 2017 growing season data from North Dakota State University’s NW22 experiment station located near Fargo, ND and NDSU’s Casselton experiment site located to the west of Casselton, ND. The experimental design is randomized complete block with a factorial design and four replicates per environment. Treatments included soybean relative maturity, soybean row spacing, and cover crop type. Soybean relative maturities are early (0.5) or late (0.9) and plot row spacing’s are either narrow (30.5 cm) or wide (61 cm) with all soybean plots planted at a seeding rate of 469,300 live seeds per ha^-1. Cover crop treatments include none, camelina, and rye with camelina planted at a rate of 10 kg live seeds per ha^-1 and rye at 67 kg live seeds per ha^-1. Cover crops were planted when soybean reached the R6 growth stage; the dates varied based on relative maturity of the two soybean varieties, respectively.

Eight plots in each replicate represented growth of each cover crop without soybean competition for light, soil water, and nutrients. Upon R6 stage of early and late maturing soybean and before planting of cover crops, specific maturity soybean group designated plots have the soybean plants removed. These plots were designated as ‘No Soybean’ and plots where soybean plants were not disturbed are designated ‘Soybean’.

Plant canopy coverage readings were taken using the Canopeo application, which records canopy coverage as a percentage of green plant tissue compared to visible soil or residue. These readings were recorded at three different dates for ‘No Soybean’ plots (additional readings were taken before harvest of ‘Soybean’ plots) and twice for ‘Soy’.

The canopeo results are indicated in Table 13. Significant differences were observed between all treatments. For cover crops interseeded (Soy), the early maturing soybean variety which reached the R6 stage (the stage determined when cover crops would be interseeded) on 18 August produced nearly half the cover crop canopy coverage compared with the late maturing variety which reached its R6 stage on 24 August. This reduction in canopy coverage could be attributed to uneven emergence of cover crops at 18 August interseeding. At this time, soil moisture was adequate, but no sufficient rainfall was received following planting. This resulted in germination of cover crops only where successful seed to soil contact was achieved. For the late intereseeded cover crops, top soil moisture levels were dry, but a significant rainfall event of 34.6 mm occurred on 1 September resulting in uniform germination and strong root development increasing the plants ability to scavenge nutrients and water. At NW22 location in Fargo, ND, a total of 101.89 mm of precipitation was accumulated following both seeding dates. Following cover crop planting, cover crop stand counts were taken one week following first emerged plant which resulted in significant differences between camelina and rye (Table 14). This difference is due to camelina's small seedling size while rye as it has the ability to tiller. In addition cover crop biomass samples were taken before first killing frost and soybean yield are provided in Table 14.

A second experiment to determine the effect of soybean maturities in cover crops growth was conducted. In 2016, a randomized complete block with split-plot arrangement was started. Winter camelina and cereal rye were interseeded into established soybean. Four varieties were used including 0.4, 0.5, 0.8, and 0.9 soybean maturities. All cover crops were interseeded into established soybean at the R7 growth stage (beginning of senescence) of the 0.4 variety, using direct and broadcast seeding methods. Two cover crop seeding rates were used, 100 and 75 percent of recommend rates, with winter camelina cultivar ‘Joelle’ seeded at 6 and 4.5 pounds per acre and cereal rye cultivar ‘Rymin’ seeded at 60 and 45 pounds per acre. Data collected included soybean yield, cover crop canopy coverage percentage in late fall of 2016 before first snow and before termination in the spring of 2017 and cover crop biomass production in 2017. Following termination of cover crops, hard red spring wheat was planted in the previous year’s soybean/cover crop plots. Data was collected from wheat plots to analysis effects of cover crops on wheat growth and yield.

Table 3 shows combined cover crop data of both broadcast and direct seeding method. Cover crop canopeo is expressed as a percentage. Readings were taken before first snowfall and before termination in following spring (2017). Cereal rye produced significantly greater percent cover in both readings and had additional growth in the spring. The spring winter camelina cover reading was 50% lower the fall reading. This is primarily attributed to camelina small plant size in the fall, which resulted in high winter mortality. Cereal rye produced nearly four times the biomass compared with that of winter camelina. Cover crops interseeded in early-maturing varieties produced significantly greater coverage in the fall, but the differences became non-significant in the following spring observation. Although canopy coverage was non-significant, there was a relationship of soybean variety and cover crop biomass with the cover crops following the 0.4 variety producing significantly more biomass than the 0.9 variety. Since all cover crops were planted at the R7 stage of the 0.4 variety, the light competition in later maturating soybean varieties resulted in less biomass and canopy coverage.

Soybean yield results are located in Table 15. A yield difference was observed between cover crop treatments and the check. Disruption to soybean plant roots during seeding of cover crops may have caused yield differences, as check plots where undisrupted during planting of cover crops. Significant yield differences were observed among difference soybean varieties, respectively. Wheat canopeo is a metric of canopy coverage of wheat crop following cover crop trials in 2016. Wheat was planted directly into cover crop trial plots from 2016 following termination of cover crops (10 days prior to planting of the wheat). Rye produced over three times the biomass of camelina which directly resulted in poor wheat stand and canopy coverage for 2017 wheat crop. Despite this reduction in stand, no yield differences were observed between treatments. Wheat yields trended negatively across soybean maturities, with 0.9 variety having the highest corresponding wheat yield, 2661 kg ha^-1, and 0.4 resulting in the lowest, 2473 kg ha^-1. This is attributed to increased cover crop coverage with earlier maturing varieties and increased biomass. These increased metrics are attributed to increased light available, as the earlier maturing variety achieved senescence early in the growing season.