There has been a lot of discussion lately about the benefits of using cover crops. They protect soil from erosion, scavenge important crop nutrients and provide a green manure that helps to feed the soil microbial community and improve soil quality over time.
However, because cover crops are growing between cash crop seasons, cover crop management decisions can influence the success of the cash crop and sometimes can cause problems for cash crop establishment. One potential impact is through allelopathy.
Allelopathy is when one plant produces chemicals that inhibit the growth of another plant. Based on laboratory studies, we know that grasses we use as cover crops - oats, barley and rye, produce allelopathic chemicals that have can inhibit the growth of other plants. However, problems are usually not widespread. But this year it seems that allelopathy is contributing to issues.
There have been many reports of new seedings failing this spring across southern Wisconsin. These reports are not just coming from cover cropped fields, and the dry and cold spring has certainly created less than ideal conditions all new seedings.
However, there have been many reports of failed stands after a rye cover crop. One field in particular that was planted half to a barley cover crop and half to a rye cover crop gave us the biggest indication that allelopathy is contributing to stand failure. The part of the field that had been barley had a decent alfalfa stand but where the rye had been there was almost no alfalfa and almost no weeds.
Allelopathy can also inhibit weeds. So why did this happen? Here are several possible reasons:
The cold spring
Although most of the rye had been sprayed in late March, the cold weather led to a slow kill on the rye so there was a slow release of alleopathic chemicals.
The cold soils had low biological activity so there was a slower breakdown of the allelopathic chemicals in the soil.
Although conservation tillage is ideal for retaining residue, it leads to shallow soil mixing and shallow dispersion of the rye residue so the allelopathic chemicals remain concentrated in the upper few inches of soil.
In one field that had this problem, half the field was field cultivated and half was vertical tilled. The side that was field cultivated had a much better alfalfa stand than the side that was vertical tilled. The other solution to this problem may be to not till at all. There have been several reports of fields were alfalfa was no till planted into a rye cover crop and they had no issues.
The dry spring
Allelopathic chemicals can be washed through the soil profile with adequate rain but we have been on the dry side this spring. Rye is also known to reduce spring soil moisture which may have led to dry conditions and possibly created more stress for new seedings.
Seed size & placement
Alfalfa is a small seeded crop and small seeded crops are more vulnerable to allelopathic chemicals. Cover crops like rye are probably better used before larger seeded crops like corn or soybeans. Deeper seed placement, by using a no-till drill, also helps by placing the crop seed out of the zone where the allelopathic chemicals are concentrated and more moisture is present.
Fields that had been planted with a Brillion seemed to have suffered more this year because they were more shallowly planted. Although allelopathy likely played a role in the new seeding issues this year, it is always good to keep in mind that
cover crops that overwinter, like rye, always have the possibility of changing spring soil conditions causing planting issues. They dry out the soil more and can create physical impediments to planting. Good management can help alleviate these issues.
Every year that a farmer uses cover crops provides more opportunities to adjust their management decisions to maximize positive impacts and minimize negative impacts on their cash crops. Understanding things like allelopathy can help to guide those decisions. Farmers can also learn from each other's experiences to shorten their cover crop learning curve.
Johnson is the Dane County UW-Extension crops and soils agent. This article oringinally ran in Forward Farmer, the official publication of YPF, Inc.