Dr. Heather Darby grew up on a dairy farm in northwest Vermont. After earning graduate degrees in Wisconsin and Oregon, she returned to her home state to work at University of Vermont Extension as a specialist in agronomy and soils.

One of her areas of study is how to successfully utilize cover crops in the management of farms in the cold climates of upper New England and northeastern New York.

Recently, Darby participated in a webinar in which she detailed some of what she and the farming community in her region have learned about cover crops over the past 20 years. She highlighted the results of a multi-year study into best practices for interseeding cover crops, funded in part by a NRCS Conservation Innovation Grant (CIG).

Because the region is so cold, it can be hard to grow corn for grain. Some farming regions in Vermont receive fewer than 2,000 growing degree days annually. As a consequence, much of the corn grown in the region ends up as silage.

Because the goal in silage is tonnage, minimal post-harvest biomass remains in the field. When winter cover crops are not planted to follow corn silage, fields can exhibit exposed soil throughout winter – a risk to soil health and water quality.

When Darby first started working with Vermont farmers 20 years ago, one of her first goals was to get them to plant winter cover crops.

Because of the long winters in the northern latitudes, cover crops must be planted early to achieve a successful stand – which means harvesting corn in September.

“One of the biggest things we started with is variety selection,” she recalled. They started testing 80- to 100-day corn. Conventional wisdom was that longer-maturity varieties would have larger yields. There is some truth to that, Darby noted, but those varieties also run the risk of not fully maturing up north due to the short growing season.

Getting a silage crop in Vermont harvested in September has other advantages. Weather is typically better in September than later in autumn. Growing earlier-maturing corn gives farmers the opportunity not only to plant cover crops but also to spread manure (when relevant) and to perform autumn tillage (when relevant).

Referring to past studies, Darby said the best time to plant winter rye in Vermont is from the third week of September to the end of September. In October, “even though the rye may survive, it just doesn’t provide adequate cover.”

At first, Vermont farmers were broadcasting winter cover crops. Over time, Darby advocated for using grain drills to get better seed-to-soil contact. Today, with the cost of seed being so high, drilling seed can also lead to cost savings (compared to broadcasting).

When tillage radishes became a popular management tool earlier this century, farmers in the region realized they couldn’t plant them in October and expect to get much benefit, due to the length of time it takes radishes to mature and the fact that fall frosts will kill them.

That’s when the farming community started focusing on interseeding cover crops into standing corn, the focus of some of Darby’s recent studies.

“We don’t get much growth when oats or ryegrass or radishes are planted past Labor Day,” Darby said. “There’s much more spring residue when the cover crop is planted the third week of August than the third week of September.”

Cover crops in cold climates a decades-long study for Darby

Highboys were used in the study of interseeding cover crops. Submitted photo

There are a number of challenges to interseeding into standing corn. First, fields of corn silage are typically planted at fairly high density to maximize yield. That density can prohibit the successful establishment of interseeded cover crops. Also, in conventional scenarios cover crops are seeded not long after an herbicide application, so there could be some residual effect from the herbicide.

August is also typically a dry month, so there’s question of whether there will be enough soil moisture for the interseeded crops to germinate. Then there’s the question of how to get the cover crop at the right seed depth. (For organic operations, Darby noted, interseeding cover crops typically comes immediately after a final cultivation, so the ground is worked up enough to get decent soil-to-seed contact.)

Despite numerous years of practicing interseeding – with application methods including aerial seeding and a drill interseeder – the success rate was highly variable.

Farmers were urging Darby to study ways to improve the success rate. “That is not a conversation I could have had 20 years ago,” she said, “but farmers today want to be successful with interseeding.”

So, Darby received a CIG to study ways to modify the way Vermont farmers grow corn to enhance the probability of success of interseeded cover crops.

Thirty farms participated in the study, with the trial fields covering more than 1,500 acres. They used corn varieties with upright leaves and 95- to 97-day maturities. They planted at 30,000 (rather than 32,000 – 34,000) plants/acre. They also utilized an herbicide program to minimize any impact on the interseeded cover crop. They used skip rows by blocking spaces on the discs in the corn planter. They used the test fields to compare results with those from control fields.

Interseeding took place about a week after the last spraying, when the corn was between V3 and V4. The study showed they were able to get better cover crop establishment without sacrificing yield.

Two key factors in increased interseeded cover crop success were the use of slightly lower populations of corn silage and the use of the right variety of corn. “There are some varieties better adapted to interseeding than others,” Darby said. “We are still looking into this.”

Darby, who is the seventh generation on her family farm, publishes results of her corn silage performance trials online. You can search for them or consult with your local Extension agent to gather and use those data.

Even though this study had some success, there was still variability in how well an interseeded cover crop became established, likely in part due to factors of moisture and sunlight. Soil moisture, for example, was clearly a limiting factor, as during two dry years the interseeded crops were clearly less successful.

Sunlight also affects the establishment success of an interseeded cover crop. Darby recently wrapped up a study where corn silage was planted on 15-, 30-, 36-, 40- and 60-inch rows. “There is more light as row spacing gets wider,” she said, “which leads to more cover crop growth.”

However, there is a drag on silage yield. When going from 30- to 60-inch rows, studies showed a three to six ton (or more) yield drop.

Perhaps the biggest success from Darby’s years of studying cover crops in Vermont has been the realization that managing cover crops as a potentially harvestable forage not only improves the flexibility of the feeding program of northern dairy farmers but also improves the soil moisture and soil organic matter on those farms.

Farmers used to worry about cover crops getting too aggressive, inhibiting spring tillage and planting. Today, where double crops are managed, farmers are grazing them or putting them up as stored feed.

“Once we actually start treating a cover crop like a crop, the outcome is much better,” Darby said.

by Karl H. Kazaks