Fertility and Nutrients
Less fertilizer, bigger yields
By Carolyn King
Adding winter wheat to your corn-soybean rotation increases corn and soybean yields and reduces
reliance on fertilizer nitrogen. Those benefits are thanks mostly to improved soil quality that develops in rotations with wheat.
These advantages are clearly demonstrated in the findings from long-term crop rotations at Elora and Ridgetown. The two studies have somewhat different research designs, but the value of rotations that include wheat is clear.
Dr. Bill Deen from the University of Guelph is leading the rotational study at Elora. Established in 1980 on silt-loam soil, this study includes eight rotations: continuous corn (CCCC); continuous alfalfa (AAAA); corn-corn-alfalfa-alfalfa (CCAA); corn-corn-soybean-soybean (CCSS); corn-corn-soybean-wheat (CCSW); corn-corn-soybean-wheat underseeded to red clover (CCSWrc); corn-corn-oats-barley (CCOB); and corn-corn-oats underseeded to red clover-barley underseeded to red clover (CCOrcBrc). All of these rotations include two tillage treatments: no till and conventional till.
Dr. Dave Hooker, from the University of Guelph’s Ridgetown Campus, is leading the Ridgetown study, which was started in 1995 by Doug Young of Ridgetown Campus. This site has a clay-loam soil. Its rotations include continuous corn (CC), corn-soybean (CS), continuous soybean (SS), soybean-wheat (SW), corn-soybean-wheat (CSW), and corn-soybean-wheat underseeded to red clover (CSWrc). The underseeded treatments are a recent addition for which there are three years of data. For each rotation, there are two tillage systems: 1) no-till for soybean and wheat, and fall strip till for corn; and 2) a conventional till treatment.
In the Elora study, all the rotations use the same nitrogen (N) rate. The rate is intended to be non-limiting, so it’s typically about 150 kilograms of nitrogen per hectare, applied as a side-dress.
“At Elora we’ve found that when we have wheat, or wheat plus red clover, or alfalfa in a rotation – essentially when we make the rotation more complex – our yield potential for corn increases. So we get higher corn yields with the same nitrogen rate when we have a more diverse rotation,” explains Deen.
In the Ridgetown study, the corn plots include four nitrogen treatments: 0, 60, 120, and 180 kg/ha of applied nitrogen. So Hooker can calculate the optimum economic rates of nitrogen for corn yields in the different rotations.
Based on the three years of data, wheat or wheat underseeded to red clover has a strong yield benefit for corn. The CSWrc corn yields were about 10 bu/ac higher than corn yields in the CSW rotation, and about 30 bu/ac higher than the corn yields in the CS rotation. Soybean yields were consistently 4 to 6 bu/ac higher in a CSW rotation compared to those in a CS rotation.
Those higher corn yields in the CSW and CSWrc rotations required less applied nitrogen.
For the Ridgetown data averaged over the three years and across the tillage treatments, the optimum economic rates for nitrogen fertilizer on corn were as follows: 110 kg N/ha for CSWrc; 130 kg N/ha for CSW; and 155 kg N/ha for CS. And for CC, the optimum nitrogen rate was higher than 180 kg/ha, the maximum nitrogen rate used in the study.
So, to maximize corn yields in the CC or CS rotations, you have to apply quite a bit more nitrogen than you do in the CSW and CSWrc rotations.
Another interesting aspect of the Ridgetown fertilizer data is the corn yield with no applied nitrogen. “In the corn-soybean rotation, when we apply no nitrogen, our five-year average corn yield is 95 bu/ac. But in the corn-soybean-wheat rotation with no nitrogen applied, our corn yield is 135 bu/ac. So there’s a 40-bushel yield difference due to rotation alone,” says Hooker. “And it is all credited to the better soil structure that wheat produces in the rotation.”
Better soil, better yields, better nitrogen use
“There is evidence in the literature in general that rotation complexity is associated with higher yields and with better soil quality. We see that at the Elora trial,” notes Deen.
“Our continuous corn and corn-soybean rotations tend to be associated with poorer soil structure and poorer average corn yields… The corn-corn-soybean-soybean rotation is the treatment with the poorest soil structure by a number of measures, the lowest soil organic matter and the lowest corn yields.”
The CCSS rotation has the poorest soil structure because soybeans produce the lowest amounts of organic matter of all the crops grown at Elora. “It’s the addition of organic matter that determines the soil structural properties that we desire,” Deen emphasizes.
A higher soil organic matter content promotes the formation of small, stable soil aggregates, contributing to a well-drained, porous soil with good moisture-holding capacity. Also, this improved soil structure makes it easier for roots to move through the soil to access water and nutrients, including nitrogen. These conditions all help the crop plants to be healthier, less stressed and more productive.
The Elora data demonstrate that the highest organic matter levels tend to be associated with the more complex rotations. One reason for that is the wheat in some of these more complex rotations. Wheat’s
fibrous root system contributes a substantial amount of organic matter to the soil. In addition, having winter wheat in the rotation allows growers to underseed red clover. Red clover is a great source of
organic matter for the soil. It’s also efficient at fixing nitrogen, providing about 45 to 82 kg/ha of nitrogen as a plowdown crop, depending on conditions.
“Probably more importantly, if you have wheat or wheat with red clover in the rotation, it elevates the productivity of all the other crops in the rotation, which also adds organic matter. So the whole rotation becomes more productive in terms of organic matter returns to the system,” says Deen.
Similarly, in the Ridgetown study, the CS rotation has the lowest soil organic matter and the poorest soil structure, while the rotations that include wheat have much better soil quality characteristics.
“In the corn-soybean-wheat system, the soil has a higher proportion of finer, more stable aggregates, producing a better structured soil with higher amounts of organic matter, compared to rotations without wheat. Wheat has a deep, fibrous root system which helps build soil structure compared to soils where
wheat is not grown, such as a corn-soybean rotation,” explains Hooker.
This improved soil structure increases the corn and soybean yields in the CSW rotation. As well, it makes the corn in this rotation less reliant on nitrogen fertilizer. With better soil structure, the corn develops a better root system that is better able to seek out nutrients already in the soil, and the improved soil moisture and aeration conditions make for a healthier plant able to take better advantage of the available nitrogen.
“The soil structure is so much better that the crop doesn’t need as much nitrogen fertilizer to produce high yields,” says Hooker.
The Elora data also show that other diverse rotations also provide similar types of benefits. For example, replacing the two years of soybean in the CCSS rotation with oats and barley, for the CCOB and CCOrcBrc rotations, resulted in a very good boost to corn yields.
Deen also points out that the improved soil structure helps to increase corn yields in dry years and wet years. “In a droughty year, a soil with good soil structure holds more water. In addition, the corn plant’s root exploration of the soil is better, so the plant can access more water. As a result corn yields will be better.”
In a year with above-average rainfall, a soil with poor structure is more likely to become saturated, which can reduce crop health, performance and yields. As well, nitrogen is lost more readily from a saturated soil. “So if you have a good rotation, you’ll have good soil structure, your crop will be happier, and you’ll lose less nitrogen out of that system,” says Deen.
Credit wheat for yield boosts
Hooker emphasizes that growers should give credit where credit is due when it comes to wheat in the rotation. “A lot of growers look at corn, soybean and wheat crops individually, as if they are not influenced by one another. But the increased corn and soybean yields [in the corn-soybean-wheat rotation] should be credited to the wheat enterprise. We’re encouraging growers to use that approach to make decisions on whether to grow wheat or not,” says Hooker.
Similarly, when you’re deciding whether to include red clover, he recommends including the extra yield benefit – as well as the nitrogen credit – that red clover provides. Hooker notes, “In the [Ontario Ministry of Agriculture and Food] Agronomy Guide, the nitrogen credit with a good crop of red clover is around 80 kg/ha. Although there is no corn yield increase for red clover included in the Agronomy Guide, our data show quite consistently a 10-bu/ac increase when red clover is included.”
He adds, “The economics are really in favour of wheat because without wheat in the rotation we couldn’t underseed to red clover. So the whole system relies heavily on wheat in the rotation.”
Overall, a more diverse rotation that includes crops like wheat with deep, fibrous root systems will promote better soil quality, which contributes to higher yields of both corn and soybean, with less reliance on fertilizer nitrogen