Organic Broadcaster

Research evaluates green manures as fertilizer in organic soybean-winter wheat-corn rotation

By  Katja Koehler-Cole

Red clover grows in a winter wheat stand shortly before wheat harvest. Photo submitted

To maintain soil fertility in soybean-winter wheat-corn rotations under organic management, it’s common to apply animal manure after winter wheat harvest in time for next season’s corn. Farmers without livestock can have a difficult time sourcing animal manure. In addition, animal manure can contain weed seeds, and pesticide and antibiotic residues if sourced from a non-organic farm.

Alternatively, farmers can grow a green manure crop between winter wheat harvest in July and corn planting the following May. Green manures are plants grown specifically to enrich the soil with N and organic matter (Cherr et al., 2006) and are most often legumes that fix atmospheric N which can be used by a subsequent crop. However, in drier areas of the Midwest, precipitation might limit green manure growth and N fixation. Growers in these areas are also concerned that green manure soil water use can negatively impact the next crop (Unger and Vigil, 1998).

Research Focus
High dry matter (DM) production is paramount for a green manure as DM production is highly correlated with N fixation and thus the green manure’s ability to meet corn N demand (Peoples et al., 2012). Extending the green manure growing season increases the potential for biomass production, so we wanted species that can be planted in early spring into winter wheat by frost-seeding (broadcasting seed on frozen ground).

We needed green manure species that would not jeopardize wheat yield or obstruct wheat harvest, be winter-hardy to provide winter ground-cover and produce additional DM in the spring, and finally, would suppress weeds. Suitable species include red clover (Trifolium pratense L.), white clover (Trifolium repens L.), alfalfa (Medicago sativa L.) and sweet clover (Melilotus officinalis (L.) Lam.). Red and white clover are shorter species, with less potential to become competitive with wheat than alfalfa and sweet clover. On the other hand, alfalfa and sweet clover are more drought-tolerant, a benefit in areas where dry years are common, such as Eastern Nebraska. All these species have been shown to increase yields of subsequent corn compared to a control (Liebman et al., 2012; Blackshaw et al., 2010).

Research questions
■ How much DM will red clover, white clover, alfalfa, and sweet clover produce when undersown into winter wheat in early spring and killed either in the fall or next spring before corn planting?
■ Will the species of green manure and whether it was mowed influence weed growth in the green manures?
■ How will yields of corn growing after undersown green manures compare to yields of corn growing after cattle manure applications?

Methods
Experiments were conducted on certified organic fields located at the Agricultural Research and Development Center near Mead, Neb. The 30-year mean annual temperature is 50 degrees F and precipitation is 27 inches. All fields were in a soybean-winter wheat-corn rotation.

Green manures were frost-seeded into winter wheat in March 2011, 2012, and 2013, at a rate of 20 lb/acre for red clover (Marathon), sweet clover (variety not stated), and alfalfa (Viking 3200), and 12 lb/acre for white clover (Rivendel). Alfalfa and sweet clover were planted only in 2013. Green manures remained in the field after winter wheat harvest.

Half of the green manure plots were mowed 40 days after winter wheat harvest, and all plant residue was left in place. Green manures were killed either in the fall of the establishment year or spring of the following year by two passes with a disk. Control plots were not undersown and were disked three times to control weeds. To compare corn yield after green manures to corn yield after cattle manure, some control plots were fertilized with cattle manure containing between 100 to 175 lb. total available first-year N per acre.

Plots were established in a randomized design, and were between 30 by 60 feet and 30 by 450 feet, allowing crop management to be carried out with field-sized equipment. To determine DM, all vegetation within a square foot was cut, sorted into green manures and weeds, dried and weighed.

Corn yields were taken in 2012, 2013, and 2014. Corn was harvested with a combine at maturity, weighed on the field and not adjusted for moisture. P-values were significant at α = 0.05.

Results
Figure 1 (on page 12) shows green manure DM yields at each termination time in each year. In 2011, clovers in the winter wheat canopy grew slowly, producing less than 500 lb/acre at winter wheat harvest. Once wheat was harvested and light reached the clover plants, clover growth increased rapidly. In the fall, red clover produced significantly more DM than white clover and mowed clovers produced significantly less DM than unmowed clovers. Unmowed red clover yielded DM of 4,900 lb/acre. Clovers overwintered well, resuming growth early due to warm temperatures in the spring of 2012. At clover termination in April, DM yields were significantly affected by the interaction between type of clover and mowing, with the highest DM yields for mowed red clover (4700 lb/acre) and the lowest DM yields for mowed white clover. Clovers planted in the drought year of 2012 were not able to establish, producing less than 300 lb/acre at fall- or spring-kill.

Green manures undersown in 2013 benefitted from early summer rains, and alfalfa and sweet clover grew as tall as the winter wheat, obstructing wheat harvest. At termination in the fall, however, green manure yields were lower than in 2011, and were significantly impacted only by species, not by mowing. The most productive species, red clover, yielded 3,100 lb/acre. Very cold temperatures and lack of snow-cover in the winter of 2013-2014 probably caused winter-kill of the green manures, with DM yields of less than 800 lb/acre for any green manure species in the spring of 2014.

The amount of N accumulated by green manures was highest in 2011-2012, up to 128 lb/acre, because of the high DM yields in that year. We did not measure how much of this N was from the soil and how much was from fixation, but N fixation rates of approximately 70 percent have been reported (Schipanski and Drinkwater, 2011). Thus, red clover at spring termination in 2011-2012 probably fixed N at 89 lb/acre, but much less N was fixed at the other termination times when green manure DM yields were much lower. Assuming half of the N is mineralized in the year after green manure termination, relatively moderate amounts of N can be expected to become available to the corn.

Weed dry matter production
In 2011, clovers were small at wheat harvest and weeds were able to compete with clover after winter wheat harvest, resulting in overall high weed DM at fall termination (Figure 2). Mowing significantly reduced weed DM in both clover species in the fall. However, in the spring, mowed white clover plots had 2,700 lb/acre of weed DM whereas both mowed and unmowed red clover plots had virtually no weeds. In 2013, green manures grew more rapidly until winter wheat harvest than in previous years, gaining a competitive advantage over weeds and reducing weed DM compared to 2011-2012. At fall-kill in 2013, red clover contained significantly less weed DM than the other green manures. At spring-kill, weed DM was low in all green manures, because the cold temperatures delayed germination and emergence of weeds. We did not identify weed species, but noticed that almost all weeds in white clover stands were volunteer wheat. This could lead to the transfer of virus diseases to newly planted wheat fields if disease vectors take refuge in volunteer winter wheat growing in green manure stands.

Corn yields
As expected, corn yields were higher after cattle manure than after green manures in each year (Figure 3). However, in 2012 and 2013, corn yields after the control were surprisingly high and not significantly different from corn yields after cattle manure. Two main factors limited corn yields during our study: water availability and N availability. In 2012, drought conditions affected corn growth and productivity in all plots, but corn growing in previous clover plots was hit hardest. We estimated that the highly productive red clover stands in 2011-2012 used up to 30 inches of water, based on water use efficiency values from Badaruddin and Meyer (1989). With drought conditions starting in early summer of 2012, soil water was not recharged, resulting in very low corn yields after red and white clover.

In 2013, N availability limited corn yields in the green manure plots because of the failure of the previous year’s clover crop. Yet, corn yields after the control were significantly higher than corn yields after clovers. This is surprising, since control plots did not receive any additional N from either fertilizer or N fixation. Control plots were disked three times between wheat harvest and corn planting, which stimulates mineralization of soil organic matter, and likely increased the availability of N in the control plots.

In 2014, water was not limiting corn growth. The previous year’s green manure used about 11 inches of water, and rainfall during corn growth was about four inches above normal. The yield gap between corn grown after green manures and after cattle manure was much smaller.

Conclusions
Extremely variable precipitation and temperatures are common in Eastern Nebraska and characterized the weather during this study period. Green manure species, especially red clover, that were undersown into winter wheat in this organic soybean-winter wheat-corn rotation yielded high amounts of DM in years with precipitation totals close to normal without affecting wheat yields. The high soil water use of clovers decreased the following year’s corn yield by about 50% in one year. When the green manure stands failed, the following corn yield was reduced due to the lack of available N. The highest corn yields were obtained with cattle manure in each year, because cattle manure had much higher amounts of available N and did not require soil water.

Our results showed that undersown green manures could help producers without access to animal manure. Red clover was the most productive species tested, and accumulated N between 67 and 128 lb /acre in years with high DM production, half of which is available to the next crop. At $76/acre, red clover is cheaper than white clover ($144/acre) or alfalfa ($94/acre), but more expensive than sweet clover ($44/acre). Care must be taken to avoid excessive green manure soil-water use, for example, by killing the green manure in the fall of the establishment year, rather than allowing it to overwinter. A fertilizer to supplement N should be available in case the green manure crop fails.

Red clover also provided the best weed suppression, especially when mowed, while white clover was not competitive with weeds. Alfalfa and sweet clover contained intermediate amounts of weed DM.

Other studies have found that the repeated use of green manures can improve soil water-holding capacity and soil organic matter, and help stabilize yields in drought years. However, farmers might not have the financial freedom to wait several years for this system to work. Future research on increasing the N availability and yields in this type of rotation and region should explore strategies that combine green manuring and animal manuring to balance their advantages and drawbacks.

Katja Koehler-Cole recently graduated with a PhD in Natural Resources from the University of Nebraska-Lincoln and is working as a post-doctoral research associate at the Department of Agronomy & Horticulture at UNL. This study received first place in the poster competition of the Organic Research Forum at the 2015 MOSES Conference. This research had funding from the Organic Agriculture Research and Extension Initiative of the National Institute of Food and Agriculture, USDA.

From the September | October 2015 Issue

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