On-farm research shows benefits of planting “green” with soybeans into a cereal rye cover crop

As more Iowa farmers add cover crops to their fields, they are turning to cereal rye – for good reason. Not only does it hold soil in place, suck up leftover nitrates and help with weed suppression, cereal rye can be aerially seeded before corn harvest in the fall (or drilled after harvest) and is hardy enough to withstand even some of Iowa’s tougher winters.

One question farmers often ask is when to kill the cereal rye. When planting it ahead of soybeans, many farmers play it safe by terminating the rye two to three weeks before soybean planting, hoping to avoid impacts on yield.

But results of on-farm research by Practical Farmers of Iowa reveal there are actually benefits to letting that rye grow. Two years of data from an ongoing study shows that waiting to terminate the rye had no impact on soybean yield – but late termination can confer other benefits. Bigger rye plants have a better ability to scavenge nitrates as they are produced in the soil. Once terminated, the plants are also more effective as a mulch for weeds in and around the cash crops.

In the study, two farmers compared the effects of terminating a cereal rye cover crop two to three weeks prior to seeding soybeans versus terminating the cover crop within five days of seeding soybeans.

They found it did not matter when the cover crop was terminated – soybean yields were equivalent. The research – “Cereal Rye Cover Crop Termination Date Ahead of Soybeans, 2016 Update” – is available for free online at practicalfarmers.org/farmer-knowledge/research-reports.

Jeremy Gustafson, of Boone, is one of the farmers conducting the research. He says that planting into such a thick patch of grass can be intimidating at first, but the results allayed his fears. “I learned not to be scared to plant into the cover crop even if it’s taller than the toolbar on my planter,” Jeremy says, meaning the cover crop could be 2 to 3 feet tall, or more, in places.

On his farm, Jeremy found that the beans planted into the later-terminated rye actually yielded better than the beans planted into earlier-terminated rye. The result surprised him because, at the start of the year, the soybeans from the late-terminated strip were noticeably shorter – 6 to 8 inches – than the soybeans terminated earlier. “Visually, don’t let the beans dictate what you think the yields are going to be,” Jeremy says.

The farmers involved in the research also found that terminating the cereal rye just before soybean planting resulted in a thick mulch that persisted on the soil surface, making it harder for weed seeds to germinate. There was also an economic benefit: Thanks to the bigger rye, the farmers were able to use less herbicide.

Jeremy says that, apart from the acres he devoted to the study – which he’ll be repeating this year – he has about 300 acres with rye on it that will be going into soybeans. On those acres, he will likely spray just once, either within a day before or after soybean planting, to kill the cover crop.

“We’ll scout, but if we have as good of growth on the rye as we did last year, we probably won’t have to spray a second time.” He adds that the amount of weed suppression you get depends on the quality of the stand that’s established in the fall. “We seed rye with an airplane in the fall, which is convenient,” Jeremy says. “But then sometimes you get patches where there’s no rye.”