What We've Learned: Cover Cropping into Standing Soybeans
Drone seeding hairy vetch into soybeans in Lancaster County on September 11, 2024. Photo credit: Heidi Reed, Penn State Extension.
Pennsylvania farmers often struggle to establish cover crops other than cereal rye after soybean harvest. Broadcast interseeding (BI) cover crops into standing soybeans could increase cereal rye growth or allow farmers to diversify cover crop species.
The objectives for this Pennsylvania Soybean Board funded trial were approached in a stepwise manner in three distinct phases: (1) determine cover crop species viable for BI into standing soybeans, (2) compare BI to post soybean harvest seeding, and (3) establish an optimum window for BI selected species.
Twenty-three site years in 11 counties were included over the trial. For phase one, nine species were BI into standing soybeans using a variety of equipment targeting leaf yellowing. Plots were arranged in a randomized complete block with three to four replications. Phase two followed a similar design but added a post soybean harvest seeding comparison. In phase three, cereal rye or hairy vetch was interseeded with a drone on three dates per site, then compared with post-harvest seeding; sites served as replicates, with no within-site replication.
Cover crop density and groundcover were measured in the fall, and again within three days of spring cover crop termination. Cover crop biomass was also measured before spring termination. All phases included commercial and research farms.
Results
Phase one revealed that cereal rye and clovers were the most and least successful species to use for this method, respectively. This seeding method also resulted in generally very low biomass (less than 1,000 pounds per acre of dry matter) at all but 1 site-year, which reached 6,000 pounds per acre of dry matter. Two of nine site-years were considered failures across species. Selected illustrative data are shown below (Figure 1).
In phase two, BI increased (Figure 2), decreased (Figure 3), or had no impact on cover crop density and biomass over post-harvest seeding. The benefits of BI into standing soybeans increased as post-harvest seeding and spring termination dates got later. The penalty of late seeding (into October) was noted.Â
In phase three, the greatest benefit to drone BI cereal rye over post-harvest seeding was quicker fall groundcover, as there was minimal impact on biomass production or spring groundcover (Figures 4-5). Hairy vetch followed a similar trend for fall groundcover, but the benefit of more groundcover for broadcast interseeding continued through cover crop termination in the spring (Figure 6). Earliest interseeded hairy vetch (late August) also produced significantly more biomass than hairy vetch drill seeded after harvest (Figure 7).Â
| (Per acre) | Aerial | Ground Broadcast | Drill |
|---|---|---|---|
| Median custom rate* | $15 | $10.50 | $21.60 |
| Recommended seeding rate | 56-112 pounds | 56-112 pounds | 30-56 pounds |
| Total cost | $33-$51 | $28.50-$46.50 | $31.24-$39.60 |
Budget comparison assuming cereal rye costs $18 per 56 pound bag, or $0.32 per pound and a custom operator is used. *Custom operator rates based on values provided in most recently available Maryland/Delaware, Ohio, and Pennsylvania surveys.Â
Conclusions
- Broadcast interseeding resulted in inconsistent establishment, both in site-to-site variability and patchiness within sites.
- Timely rainfall was vital for success
- Broadcast interseed cereal rye in late-September and at least six weeks before soy harvest
- Target early-September for hairy vetch seeding (only tested in southeast PA)
- Do not interseed within one month of soybean harvest
- Benefits of broadcast interseeding increase the later post-harvest seeding and spring termination dates get
- Broadcast interseeding hairy vetch increased spring biomass compared to post-harvest seeding
- Broadcast interseeding rye did not increase spring biomass compared to post-harvest seeding
- Weeds produced 2-3x dry matter as cover crops at almost half of site-years; this is a major obstacle to widespread adoption
When does it make sense to broadcast interseed into soybeans?
- Quick fall groundcover is the priority
- You plan to delay cover crop termination or plant green
- Non-weedy fields
- You plan to plant cereal rye, wheat, hairy vetch, or annual ryegrass into double crop soybeans, or when soybeans will be harvested very late
- Irrigated fields
For more information, or if you are interested in participating in future trials, please reach out to Heidi Reed at hreed@psu.edu or 717-472-8108.
