Cover crops may have potential to lower yields

Cover cropping has gained considerable traction in recent years, driven by its potential to improve soil health, reduce erosion and runoff.

In the United States, there were 50 percent more cover crops planted in 2017 than in 2012.

However, recent research has shown that cover cropping could lower crop yields and lead to negative environmental impacts caused by expanded cultivation necessary to make up for those yield losses.

Research has shown that cover crops can affect the productivity of primary crops but much depends on the environment, the methods of implementation, and land use practices from farm to farm.

To understand the complications of cover cropping and in what ways it makes a difference to crop yield, researchers at Stanford University in California, undertook the first large-scale, field-level analysis of yield impacts on corn and soybean from cover cropping across the U.S. corn belt.

“There are a lot of policies that have been recently enacted at state and federal levels to promote cover cropping,” said David Lobell, the Gloria and Richard Kushel director of the Center on Food Security and the Environment, and professor in Earth System Science at Stanford.

“So, a natural question was how these policies were affecting agriculture. We know some things from experiments but there is often a difference between how things work in a controlled environment and how they work in the real world. Agriculture is a very tricky business to get right, and things typically don’t work out as planned.”

According to the media release, from 2019 to 2020 the research team used satellite imagery to look over 20 million acres of farmland in Iowa, Indiana, Missouri, Ohio, Illinois, and Michigan. That equated to about 90,000 fields.

They analyzed every field that had grown cover crops for at least three years, then compared them to fields that had not been cover cropped. About five percent of agricultural fields in the study area were cover cropped in 2017, up from 2.6 percent in 2012.

On average, fields that had been cover cropped saw yield declines of 5.5 percent for corn and 3.5 percent for soybeans.

According to the report, the mean yield loss for corn was consistent with results from experimental and modelling studies, especially given that the main cover crop species used in the study area had been cereal rye and annual ryegrass. Previous independent studies had found that non-legume cover crops generally resulted in greater yield losses.

Financially, those results translated into a loss of about $40 per acre for corn and $20 per acre for soybeans. The losses, when added to the various costs of undertaking cover cropping on a year-by-year basis can make adoption of the practice challenging.

The researchers, though, recognized the influence of other factors affecting growth.

“The presence of a cover crop is just one of many factors that influence yield,” said Lobell. “The weather is obviously a key factor as well as other management practices. We were focused here on the effects of cover crops and how these effects interact with other aspects of management and soil.”

In their report, researchers did not have data on cover crop species or timing and therefore they looked at the yield impacts on cover cropping as currently practiced overall. They found that the reasons for drops in yield included low nitrogen, low water in the soil, and oxygen depletion especially during a wet spring.

They wrote in the report that non-legume cover crops immobilize a substantial fraction of soil nitrogen and the release of nitrogen after termination of the cover crop is often too slow to avoid some nitrogen stress for the primary crop in the current year.

A second mechanism is that cover crops can compete for water and, for corn yield losses, this can be exacerbated when cover crops are used prior to dry growing seasons. Precipitation in April is an important determinant for subsequent growth, and therefore yield.

A third mechanism is lack of oxygen in the soil during the early growing season, especially during wet springs. The thought is that the activity of soil microbes stimulated by the cover crop consumes oxygen at the expense of the primary crop.

“These are three reasons that experiments suggest can lead to yield losses from cover cropping and we find some evidence for all three,” he said. “There are also likely other things happening. For example, one that we are looking at now is whether cover crops lead to a delay in sowing date that would reduce yield potential.”

They estimated an average corn yield loss of 5.5 percent on fields where cover crops were used for three or more years compared with fields that did not adopt cover cropping.

Corn losses were larger on fields with better soil ratings, cooler mid-season temperatures, and lower spring rainfall.

For soybeans, the average yield loss was 3.5 percent but larger impacts occurred on fields with warmer June temperatures, lower spring and late-season rainfall, and, to some extent, better soils.

“One of the main risks we find is that, if spring moisture is low, having had a cover crop will expose the farmer to more risk of low yields,” said Lobell. “This is an important downside that needs to be factored into policy. Some anecdotal evidence from the last year suggests cover crops help when springs are soaked, though we did not pick that up in our analysis. Figuring out these interactions could help farmers try to adapt the system to climate change.”

To mitigate the problem, ensuring that the cover crop is removed with enough lead time before planting primary crops could result in higher summer crop yields. While many farmers use rye as a cover crop, good alternatives are clover, hairy vetch, or other legumes would also possibly result in higher primary crop yields. Farmers in areas with less susceptibility to water stress might be less likely to suffer yield penalties.

Lobell said that they have received messages both from farmers and other researchers.

“In general, they have said that our results jibe with their experience.”

Despite the findings, the researchers emphasize that cover crops could still prove beneficial to farmers. It might be that benefits take some time to kick in and farmers, over time, will become better at implementation.

Next, researchers plan to look at how farmers can adjust their cover crop species and when to remove it before seeding the primary crop to conserve water, nitrogen and oxygen.

“This is something we’d like to look at more closely, and many others are also doing interesting work in this area,” said Lobell.

“My guess is that moving away from rye as the most common cover crop will be helpful, and possibly also earlier termination. In general, there are a lot of knobs to turn, and our message is not to get rid of the entire idea of cover crops, but to work on implementing as smartly as possible.”

Cover cropping is not for every farmer everywhere in Canada. While it may be suitable for some provinces, it may not easily apply across Western Canada. Some challenges include poor cover crop establishment, a late harvest of the primary crop that delays sowing seed for the winter cover crop, the costs of seed, labour, and time associated with the practice, and extreme winter conditions that delay growth.

The research was published in the journal Global Change Biology.

Tags: cover crop, corn, United States, soybean, Canada, yields, crop, rye, productivity, loss, cover cropping