A nurse crop is one tool for reducing erosion in potato fields.
A spring cover crop planted at potato planting time – called a nurse crop – offers a way to reduce soil erosion during the weeks between potato planting and hilling. But what is the optimal way to grow a nurse crop to benefit the soil, the potato crop, and the grower’s bottom line?
Researchers and growers in New Brunswick and Maine are working on answering that question.
“The best part of the soil is lost when it erodes – you lose your organic matter, your fines [clay particles] and your productive A horizon,” explains John Jemison, an extension professor of soil and water quality at the University of Maine.
Potato production systems in New Brunswick and Maine are at risk of soil erosion for many months due to bare soils and intense rainfall events. Given the short growing season, growers typically till the soil in the fall before the potato year of their rotation because they want to start planting as soon as possible in the spring. Fall tillage leaves the soil exposed to snowmelt and rainfall through late fall, winter and early spring. After potato planting, the soil remains bare for about three more weeks until the crop emerges and then partially covered for another three or four weeks until the potato canopy fills in.
“The Potato Belt of New Brunswick has fairly steep slopes, so we have a lot of soil erosion. There are estimates that 20 tonnes per hectare of soil can be lost in just one year when we have bare soil on steep slopes,” notes Josée Owen with the Fredericton Research and Development Centre of Agriculture and Agri-Food Canada (AAFC).
Such serious erosion losses are contributing to declining soil health, which is associated with stagnant or decreasing potato yields in the region.
A nurse crop is one tool for reducing erosion in potato fields. Generally, the term nurse crop refers to a cover crop seeded when another crop is planted to offer some protective benefit, such as reducing the force of wind or rain on delicate seedlings. In potato systems, it means a crop planted right around potato planting that emerges quickly to cover the soil and reduce spring soil erosion.
Agronomist Gilles Moreau, who recently retired from McCain Foods Canada in New Brunswick, has been a pioneer in the use of nurse crops in potatoes. Over several years, he worked with growers at multiple locations to conduct on-farm, field-scale trials to test various nurse crop options, like different seeding rates, crop types and termination methods, and he measured potato yields and calculated the economic costs and benefits of nurse cropping.
Moreau also inspired both Jemison and Owen to conduct small-plot nurse crop trials to complement his on-farm trials. “On-farm trials are often very strong in using farm-scale machinery and working in a real agricultural landscape, and they provide a lot of information about the practicability of the cropping practice,” explains Owen. “Small-plot trials allow a more in-depth, scientific look at the effects of different treatments.”
Maine Research in 2016
“With a nurse crop, we approach soil quality from the angle of ‘let’s protect what we have.’ When we get heavy, torrential rains after the potato seed has been planted, we can have a fair amount of erosion loss and have seed come to the surface,” says Jemison.
He has been interested in ways to protect and enhance soil quality in potato rotations for many years, so he was intrigued by Moreau’s nurse crop trials. Jemison started his small-plot study in 2016 in Maine, and he is continuing and expanding his nurse crop research in 2017 and 2018.
“To me, nurse crops seem like a nice way to get at least two full weeks or even three weeks of soil protection,” he says. “But nurse cropping comes with a lot of questions, which we are looking at in our research. Our goal is to answer four questions: How densely should we plant the nurse crop? Is there a difference in species to use as a nurse crop? How long do we run the nurse crop? Do we need to kill it before we hill it, or can we just hill it to get rid of it?”
He is hoping to find nurse cropping practices that will give good soil protection, require minimal input costs, maintain or boost potato yields, and perhaps provide other benefits for the potato crop.
“Growers are concerned about their soil resource, but they are also concerned about their business bottom line. Most growers are hesitant to take on any major change in their production system that requires an investment [of time and money] without seeing a definite benefit right away. So nurse crops have to make sense for them,” says Jemison.
He adds, “On the other hand, we have 15 or 20 farmers who are using nurse crops right now and we would like to see that number grow.”
Jemison’s trials are taking place on the university’s research farm near Orono in central Maine. The farm’s land is very flat with little risk of soil erosion, so the treatments are focusing on factors that might affect competition between the nurse crop and the potato crop.
In 2016, Jemison and his team compared winter rye sown at 100 and 200 pounds per acre, annual ryegrass at 20 pounds per acre and a check treatment with no nurse crop. For winter rye, they compared growing the nurse crop for three versus four weeks, and then terminating it either by killing it with a herbicide prior to one-pass hilling or by just hilling it. They collected data on such factors as nurse crop biomass and potato yields.
They broadcast the nurse crop seed by hand and then planted the potatoes with their potato planter. Jemison notes that Moreau’s field-scale trials used a more efficient nurse crop seeding system: a drop seeder dropped the nurse crop seed on the ground during the finish cultivation, just before potato planting.
Jemison found that annual ryegrass grew more slowly and produced significantly lower biomass than winter rye. So he does not recommend annual ryegrass as a nurse crop for potatoes, and he isn’t continuing with annual ryegrass in his nurse crop trials.
As expected, winter rye at the higher seeding rate had significantly higher plant numbers and biomass than at the lower seeding rate.
When the nurse crop was grown for three weeks, Jemison’s team was able to terminate it just by hilling it. However, when it was grown for four weeks, they needed to spray it before hilling it to terminate it. The rye was too tall after four weeks to be buried by hilling; as a result, it continued to grow and competed with the potato crop.
Last year was very dry in central Maine so potato yields in the trial were very low. Although there was no yield benefit from having a nurse crop, there was also no yield penalty as long as the nurse crop was completely terminated.
Next Steps for the Maine Trials
Jemison’s 2016 nurse crop trial was done on a shoestring budget. Recently he received a small grant from the USDA’s Sustainable Agriculture Research and Education program to continue the research in 2017 and 2018. He is collaborating with Lakesh Sharma, an assistant professor of extension and sustainable agriculture at the university. Sharma’s site, which is near Presque Isle in northern Maine, isn’t as flat as the Orono site, so it might allow the researchers to assess the effect of the nurse crops on erosion.
In 2017, Jemison is comparing winter rye at 100, 200 and 300 pounds per acre, oats at 200 pounds per acre and a check with no nurse crop. The rye plots seeded at 100 pounds per acre will be terminated after either three or four weeks, while the other rye plots and the oat plots will be terminated after three weeks. The oat plots and rye plots at 100 and 200 pounds will be terminated by hilling, or by spraying and then hilling. The rye plots at 300 pounds will be terminated by spraying and then hilling.
Winter rye is considered a good option for nurse crops because it tends to germinate and cover the ground fairly quickly. However, some growers think winter rye is a little harder to kill than oats. So Jemison wants to compare the two crop types. He believes other cereals, such as barley, would likely also work as nurse crops. One possibility might be for farmers to simply use their own saved seed from whatever small grain cereals they are growing so they can minimize nurse crop costs.
The study will be collecting data on things like nurse crop biomass, the time it takes for the nurse crops to reach full canopy cover, effectiveness of different termination practices, potato yields and quality, and production economics.
He adds, “I would also like to answer the question: Is there a positive microbiological benefit from a nurse crop? My hope is that in growing a nurse crop, not only are we protecting the soil, but we might also be stimulating microbial activity [because the nurse crop’s root exudates could provide food for the microbes]. So when you incorporate the nurse crop, there would be a flush of microbial activity to feed on the dying nurse crop. That could help release more nutrients, which could help improve the early growth of the potato crop.”
Although he doesn’t have funding to conduct microbial evaluations in 2017, Jemison hopes to infer the effects of microbial activity by measuring early season potato growth.
“If we can say to a grower, ‘The nurse crop is protecting your soil for two weeks and the potatoes are getting to canopy closure a week earlier, so you have three weeks of extra protection and it costs only X amount of money,’ then more growers would be interested,” notes Jemison.
Owen’s nurse crop project
Owen started exploring nurse crop options in 2013-2014, after meeting with Moreau at one of his field trials. She started a more formal three-year project in April 2015, with funding from AAFC’s AgriInnovation Program. The project is one activity under a major industry-led study, with Potatoes New Brunswick as the study’s proponent.
Owen and her team are experimenting with various nurse crop types, such as winter rye, oat, field pea and buckwheat. They are evaluating the crop types in terms of factors like seed costs, time to emergence and canopy closure, competition with the potato crop, and effects on soil properties like nitrogen level and organic matter content, and effects on water infiltration.
“If rain is coming down very quickly, the water doesn’t necessarily have time to percolate into and through the soil. So it runs off over the soil surface, carrying soil particles away,” she explains. “When a crop is growing, the roots going down into the soil create channels where the water can penetrate more quickly. So a cover crop might get more water into the soil and less water going across the surface.”
Owen’s team plants the potatoes and then seeds the nurse crop right after. They are comparing broadcasting versus drilling for seeding the nurse crops.
“We thought there might be some advantages to broadcasting in terms of being able to pair up the seeding operation with another operation, like a secondary tillage operation or a broadcast fertilizer application,” she notes. “And we also wondered if having the seeds more randomly scattered might be better than seeds in rows in terms of the water runoff.”
For termination practices, they are assessing hilling alone versus spraying and then hilling.
Along with collecting data on things like nurse crop biomass, potato yields, soil moisture levels and soil organic matter levels, Owen’s team is calculating nurse crop input costs. She says they are not attempting the much more complex determination of how much it costs not to have a nurse crop, which would require calculating such soil erosion impacts as long-term crop productivity losses and water quality degradation.
Owen highlights a few of the initial results from her study. “One of the things we figured out very quickly was that buckwheat is not a viable nurse crop in a potato cropping system. It is an excellent soil cover, but it is so fast-growing and aggressive that it is far too competitive with the potato crop. So we excluded buckwheat from our future experiments.”
So far, drilling seems to be better than broadcasting because drilling produces a much more uniform crop cover. The jury is still out on which termination option is best, but she expects to have more definitive results after the final field season this year.
What about fall cover crops?
Both Jemison and Owen are doing some research on fall cover crops for soil protection during the fall, winter and early spring.
“Whenever we can work in a cover crop, it is a positive to protect the soil and to provide a food source for soil microorganisms,” says Jemison. He is looking for options that provide as much soil protection and soil organic matter as possible without hampering potato production.
He explains that adding a cover crop after potato harvest is not always practical. “Our growers want to get the best size profile they can with their potatoes. So they tend to grow their potatoes late enough into the fall that there is not much time for a cover crop to be seeded and become established after harvest. If they grow a shorter-season potato variety, they could plant winter rye in the fall, but by planting time in the following spring, the rye cover crop would be quite tall. If they were to spray it in mid-May, it might be knee-high, and that would be almost too much biomass to break down to effectively go into some other crop.”
Most Maine potato growers have a two- or three-year rotation, usually potatoes followed by barley, followed by either one other crop or potatoes. Jemison thinks the best option for adding a cover crop is probably in the barley year. So starting in 2017, he and Sharma are conducting a two-year study at their two locations to examine various ways of managing barley in the first year, and followed by potatoes in the second year.
Jemison’s trials include seven different barley treatments. Three treatments involve barley under-seeded with annual ryegrass and then different ryegrass termination options including fall tillage followed by spring harrowing to prepare the field for potatoes, fall spraying followed by spring harrowing and spring tillage followed by spring harrowing.
The other four treatments involve barley followed by different cover crop and soil management options, including: fall tillage, followed by fall-seeded oats that are winterkilled, then spring harrowing; fall tillage, followed by fall-seeded winter rye, then spring spraying and harrowing; fall tillage and no cover crop, then spring harrowing; and, no fall tillage and no cover crop, then spring tillage and spring harrowing.
Jemison wants to see which option provides the easiest to transition into the potato crop because that is a crucial issue for growers.
Owen is experimenting with an innovative way to add a cover crop in the fall of the potato year of the rotation. She and her team are broadcasting cover crop seed at vine-kill, about two weeks before the potato harvest. They are assessing how well different cover crop species grow and how they affect things like soil organic matter levels.
Keys to resilient cropping systems
Owen’s take-home message to growers is: “Cover your soil whenever possible.” Even though there is a small input cost to cover cropping, soil erosion has productivity costs that add up year after year.
She emphasizes the value of increasing soil organic matter and biodiversity by adding other crops to the two-year potato-barley rotations that are common in New Brunswick. “We know there is a benefit to having increased biodiversity. Some of it has to do with the soil microbial communities. If we introduce a cover crop, we are introducing a third crop. If that cover crop is a triple mix, then we have five species instead of two in the same period of time.”
Jemison notes strategies such as using cover crops are important to creating healthier, more resilient soils that are able to withstand changing weather patterns. He has surveyed Maine growers about weather trends, and most growers think the weather is getting more variable. Their experiences are in line with the trends shown in climate data.
“The climate projections are predicting more variable weather – more intensive rain when we get it and more gaps without rain,” he notes.
To enable a cropping system to better withstand this drought/deluge variability, it is crucial to maintain and improve soil quality. Better soil quality contributes to better soil structure, increased water infiltration rates, less runoff and erosion, improved soil water-holding capacity, and more productive crops that provide greater soil cover and return more residues to the soil.
“As a soil and water quality specialist, I think the most important thing is for growers to build soil quality any way they can, whether that is through applying organic amendments like compost, lengthening the crop rotation between potato years, adding a nurse crop to protect the soil for a little while, [adding a forage crop to the rotation if there is a local market for forages,] reducing tillage activity, or moving from two hillings to one hilling per year,” says Jemison.
“Each little thing is incremental. You can add one thing this year, something else next year, and some other thing a couple of years after that. Growers need to be thinking about this from a five-year, 10-year, 20-year window perspective.”