In a recent Hoosier Ag Today/CCSI Soil Health Podcast, Stephen Meyers of Purdue Horticulture and Dan Perkins of Perkins Good Earth Farm covered spring management of cover crops on vegetable farms. They discussed a variety of methods for terminating: flail mowing and tarping, roller-crimping, tillage, and killing with herbicide.
y pointed out that decisions about when to terminate depend not only on the weather and following crop, but also on the prime purpose of the cover crop which can include:
If fixing or scavenging nitrogen, producing a lot of biomass/residue, or providing blooms for pollinators are important, letting the cover grow longer into the spring will increase benefits.
If protecting soil from erosion by winter winds and rain was the main goal, the cover crop has served its purpose by spring.
They reminded us how important it is to have a back-up plan if the cover crop can’t be killed with the planned method.
The last of the winter cover crops on vegetable farms will likely be terminated in the next few weeks to make room for warm season vegetables. Before terminating is a good time to assess the cover stand and take a look at the roots and soil underneath. We did that at the Pinney Purdue Ag Center the second Monday in May. ‘Hazlett’ winter rye was seeded after soybeans in late September at 100 lb./A. The soils are predominantly well drained Tracy sandy loams. By early April there was a healthy stand of well-tillered plants (Fig. 1).
By May the rye was 2 to 3 ft. tall and in the boot stage (Fig. 2). Notice that in both Fig. 1 and Fig. 2 that the stands are thick and even.
If the goal of the cover crop is a mat of residue to retain soil moisture and block weeds from germinating into the season, you want a nice thick even stand of plants.
Also in Fig. 2, note that in addition to the stand being thick and even, it is all at the same growth development stage. We are planning to terminate some of the rye in Fig. 2 with a roller-crimper, so consistent growth stage is more important. Occasionally with a “Variety Not Stated” rye (VNS) there will be a mix of varieties with different growth habits and maturity characteristics, further complicating an already challenging termination timing issue. With a chemical termination plan, a consistent growth stage is less important because the rye will respond favorably at most growth stages to common grass herbicide programs. While you are evaluating the stand you also want to scout for any problem weeds. If there are problem weeds present, account for that in your termination plan or weed control strategy for the season.
Looking into the soil, rye roots were plentiful, and had penetrated through a slightly compacted layer about 4 to 5 inches below the surface (Fig. 3, upper arrows).
We saw earthworms and rye roots growing down earthworm channels. Some of the many benefits of cover crops are that roots will:
find existing channels
hold existing channels and connected pores open
create new paths for crop roots to utilize as cover crop roots decay
Dropping the soil block gently on the ground allowed it to split at natural break points. About 8 to 10 inches deep a natural break was visible associated with a second compacted layer and change in soil color from light brown to orangey-brown (Fig. 3, lower arrows). This break represents the division between the plow layer, or Ap horizon, and the subsoil, or B horizon. The soil was quite wet after a 2.5-inch rain two nights previous, and by using the ‘ribbon test’ we could see the difference in soil texture between surface and subsoil (Fig. 4).
One thing we are sometimes concerned about with that horizontal fracturing and compaction layer is root restriction. There was no obvious sign of this in the cover crop, and good soil aggregation and granular structure are visible. Under normal circumstances, we would not suspect that a root restricting layer will develop, especially with all of the root channels and connected pores intact, but still want to be mindful of field conditions during heavy traffic times to avoid creating a problem.
When we compare Fig. 3 and Fig. 5, we see several differences in soil structure. These are the same soils with the same cover crop planted at the same time. The only difference is one tillage pass has been done in Fig. 5. Clear differences in soil structure can be seen when comparing the spade depth of soil from the right side of Fig. 5 to the same spade depth of soil from Fig. 3. The soil in the tilled area (Fig. 5) is much less friable and shows less granular structure. In addition, we see clearly defined horizontal fracturing at several layers with larger, less open, soil structure in between. These layers in Fig. 5 we might be more concerned about in less than ideal conditions, especially at or right after planting when the small crop roots would have to punch their way through these structures. At least one more tillage pass will be needed to break up these layers in the tilled treatment to prepare those plots for planting. This soil will be more susceptible to degradation by heavy traffic and rain that will continue to break down the positive structure built by the cover crop roots.
This rye will be part of the no-till pumpkin and sweet corn plots this season. Rye in some plots was treated with glyphosate the second week of May, so it will be killed by planting time scheduled for the second week of June. In the next group of plots, rye was tilled in mid-April and additional tillage will fully work it in by planting time. The third group of plots are scheduled to be roller-crimped just before or just after planting sweet corn or pumpkins.
Watch this site for more updates during the season, and for information about the Vegetable Field Day at Pinney Purdue, planned for the evening of August 10, when you are invited to join us and see how the season winds up.
Extension Specialist, Dept. of Horticulture and Landscape Architecture, Purdue University
Conservation Agronomist, Conservation Cropping Systems Initiative (CCSI) and Purdue Extension