The fallow period between soybean harvest and corn planting in a crop rotation is highly vulnerable to sediment and nutrient losses, but soybean harvest is often so late that there isn’t enough time to reliably establish a cover crop.
If a cover crop is established, options are typically limited to cereal rye or winter wheat. For many, the financial risk of a failed cover crop stand outweighs the potential benefits.
One planting strategy that can expand the cover crop seeding window for farmers is broadcasting seed into standing soybeans.
Success with this type of broadcast seeding using fertilizer spreaders has been noted by some farmers, and it can buy additional weeks for cover crop growth, but best practices have not been established for this area.
Research into Broadcast Seeding
In 2020 the Pennsylvania Soybean Board On-Farm Network added a study to evaluate the feasibility of this practice in five locations.
Extension educator Heidi Reed says sites included three cooperating farms (Montgomery, Potter, and York counties) and the Penn State research farms in Landisville and Rock Springs.
Nine cover crop species were included at research farms: cereal rye, winter wheat, annual ryegrass, rape, forage radish, balansa clover, crimson clover, red clover, and hairy vetch. Cooperating sites chose their desired species.
No-cover control plots were also included at all sites, and species “treatments” were replicated four times at each location.
Broadcast seeding method varied by location from hand seeding to commercial scale equipment.
Targeted seeding was after the R6 stage and before leaf drop. R6 marks the full seed stage where a pod contains a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf.
Seeding occurred at leaf drop at the Landisville and Potter County sites. Seeding dates ranging from Sept. 22 to Oct. 6, 2020, and farmers gained between 16 and 56 days of cover crop growth by using this method compared to drilling or broadcasting cover crops after soybean harvest.
On average across sites, cereal rye and winter wheat performed better than all other cover crop species, accumulating the most biomass, highest density, and most ground cover.
Plots with cereal rye or wheat also had lower weed biomass compared to other cover crop species or the no cover crop control.
However, biomass was still low compared to drilling after soybean harvest, even considering the earlier planting date.
Rye and wheat ranged from as low as 800 pounds per acre in Montgomery County to 2,000 pounds per acre at Landisville; the only exception was the moderate 6,000 pounds per acre of rye in York.
Considering the generally low biomass of the covers, we were not surprised to see little influence on soil nitrate.
Across sites, annual ryegrass, hairy vetch and rape did not have as successful an establishment or as much biomass as cereal rye and wheat, but they performed better than clovers.
Unfortunately, all clover species failed to overwinter at all cooperator sites, and balansa clover did not emerge at all at Rock Springs. However, crimson clover was among the best-performing species at Landisville.
We were encouraged that we were able to seed 16 to 56 days earlier than we would have if waiting until after soybean harvest — but our average broadcast date was still the last week of September, later than the recommended date for most legumes.
This resulted in low success with those species and moderate success with species like cereal rye and wheat, which would have likely established well with a drill after harvest anyway.
Thus, in 2022, we will target earlier-planted and earlier-maturing soybean fields that may be hitting maturity (our R6 leaf yellowing window) in late August or early September. This will hopefully facilitate better establishment of species other than rye and wheat.