Organic Broadcaster

Research explores potential benefits of mixing cover crops

By Angela Tran, University of Nebraska-Lincoln

This was the winning poster entry in the Organic Research Forum at the 2014 MOSES Conference. A total of 52 research posters were presented through the forum.

The potential benefits of using cover crops are wide ranging and well documented. The potential benefits of using cover crop mixtures, however, have been less thoroughly explored. As more farmers consider using multi-species cover crop mixtures—also referred to as cover crop cocktails—the benefits of their use warrant investigation.

While there are many reasons set forth for the use of diverse cover crop mixtures over a single species of cover crop, they all seem to boil down to three general reasons—customization, stabiliza­tion, and enhancement of their performance.

Customization

Cover crops vary widely in their ability to per­form a variety of functions. Consequently, cover crops may be mixed in order to combine their be8nefits, mitigate their detriments, and create results intermediate to those that we would have had if we had used monocultures.

Stabilization

Cover crops also vary widely in their ability to tolerate and thrive in different environmental conditions. Therefore, multiple species might be used instead of one species in order to reduce the risk of the cover crop not performing as desired.

Performance Enhancement

The “enhancement” suite of reasons encom­passes every reason that suggests that the per­formance of a cover crop mixture will exceed the sum performance of its constituent parts in a positive way.

Measuring Performance

Inherent in each of these justifications is the idea of cover crop performance, but given the wide variety of purposes for which cover crops are used, how are we to measure cover crop performance?

In order to explore the question, “How does cover crop mixture diversity affect cover crop performance?” we need to identify the desired out­comes of the cover crop as well as ways of measur­ing the degree to which those outcomes are met.

It is my hope to explore this question at the University of Nebraska-Lincoln with regard to biomass production, weed suppression, soil fertil­ity, soil moisture, and biodiversity over the next two years. However, for the purposes of this arti­cle we will focus primarily on biomass production.

How does cover crop mixture diversity affect cover crop biomass production?

This is the starting point of this project because there is much in the ecological literature to suggest a positive link between plant mixture diversity and plant mixture productivity. The diversity-pro­ductivity hypothesis states that greater diversity will on average lead to greater total productivity (Tilman, 2001). Thus, we hypothesized that the more diverse the cover crop mixture, the more productive on average.

This was also the starting point because many of the positive functions of cover crops are directly related to their biomass production. For example, the more biomass the cover crop can produce, typically the more nitrogen fixation in the case of legumes, the less soil erosion, and the greater the weed suppression.

Methods

Twenty cover crop treatments (Table 1) were planted in nine fields across eastern and south­eastern Nebraska in the late summer and early fall of 2013. Cover crops were broadcast after small grains harvest, into maturing soybeans and corn, and after seed corn harvest.

Seeding rates used in mixtures were propor­tional to full rates used in monocultures—e.g., in three-way mixes, each species was seeded at one-third the full rate.

Species-specific cover crop and weed dry bio­mass was taken prior to killing frosts. Mixture diversity was measured as the number of species and functional groups in a mixture. Functional groups defined were cool season grasses, cool sea­son legumes, and brassicas.

Of the nine sites planted, four sites were not sampled for biomass due to limited cover crop establishment. Of the remaining five sites sampled, the data presented here represents the site with the most substantial cover crop growth. This site was a harvested wheat field in Hooper, Neb. The treatments were planted on Aug. 31 and sampled on Oct. 31, 2013.

Results

Figure 1. Average species specific biomas (±SEM of total cover crop biomass) for each treatment.

When planted in monoculture and in mixtures, the brassicas tended to put on the most biomass, followed by the grasses, and then distantly followed by the legumes, which were relatively slow growing (Figure 1). Within that general trend, however, the rankings of each of the species in terms of biomass production varied by site.

Increasing the number of species or functional groups in a mixture increased the average bio­mass production of the cover crop—“average” being the key term. When compared to averages of its constituent parts, more diverse mixtures pro­duced more biomass than less diverse mixtures. However, particular monocultures outperformed particular mixtures.

Figure 2. Relationship between cover crop biomass and weed biomass.

Increasing the number of species in a mixture without increasing the number of functional groups did not increase average biomass produc­tion. In relating cover crop biomass to cover crop function, cover crop biomass appeared to be a driving factor in determining weed suppression (Figure 2).

Preliminary Conclusions

While it appears that increasing mixture func­tional diversity has a positive effect on biomass production and increasing mixture species diver­sity alone has a neutral effect on biomass produc­tion, not enough is known at this point about the mechanisms driving these patterns to draw any definite conclusions.

Are we observing simple compensation—i.e., one species stepping in to fill in the void of an unsuccessful species? Or are we actually observ­ing some sort of enhancement—i.e., one species actually improving the success of another?

Also, does increasing the diversity of a mixture at some point become redundant? For example, the brassica species behaved remarkably alike. What impact does adding another brassica species have once you already have one in the mixture?

As usual, research leaves us with more ques­tions than answers, but we keep plugging along.

Angela Tran is a graduate research fellow in the Depart­ment of Agronomy and Horticulture at the University of Nebraska-Lincoln. North Central SARE funded this project.

Reference:

Tilman, D. 2001. Functional Diversity. p.109-120 In S. A. Levin (ed.) Encyclopedia of biodiversity. Academic Press: San Diego, CA.

From the July | August 2014 Issue

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