Functional Diversity vs. Species Count in Cover Crop Mixes: Asking the Right Question

Over the last several years, one question has become increasingly common when discussing cover crop mixtures: How many species are in the mix?

Sometimes the conversation begins with a target before the field goals have even been discussed:

“I want a 15-species mix.”

Or:

“Can we make this 20 or 25 species?”

There is nothing inherently wrong with a highly diverse cover crop mix. In the right situation, additional species can broaden functional diversity, increase resilience and provide a wider range of targeted benefits.

The problem occurs when species count becomes the design objective instead of the result of the design process.

Whether the goal is nitrogen fixation, erosion control, pollinator habitat, biomass production, improved water infiltration, compaction management, forage production or a combination of objectives, the desired outcomes should drive species selection.

Functional Diversity vs. Species Diversity

Species diversity describes how many different species are present. Functional diversity asks a more useful agronomic question:

What does each species actually contribute?

Consider a mixture containing several grasses with similar growth habits and rooting characteristics. The species count may be high, but much of the mixture may be performing similar functions.

Now compare that with a smaller mixture containing complementary functional groups:

That mixture may contain fewer species while still providing a broad range of functions.

The goal is not necessarily to minimize species count. Highly diverse mixtures can be appropriate when additional species contribute meaningful differences in rooting characteristics, growth timing, plant architecture, nitrogen contribution, pollinator value, forage characteristics or resilience to environmental variability.

More Species Do Not Automatically Mean More Benefits

One of the easiest mistakes to make in cover crop design is assuming that adding another species automatically adds another agronomic benefit.

Plants growing together are competing for many of the same resources:

• Sunlight
• Soil moisture
• Nutrients
• Rooting space
• Growing-season time

Species do not compete equally.

An aggressive cereal may establish quickly and dominate slower components. A species included at a very low rate may never achieve enough plant density to make a meaningful contribution. A later-than-planned planting date may favor some components while reducing the performance of others.

So a seed tag listing 10, 15 or 20 species does not necessarily mean that all of those species will contribute equally to the established stand.

Seeding Rate Matters as Much as Species Selection

A well-designed cover crop mixture is not simply a list of good species. The rate of each species within the mixture matters.

If an aggressive component is seeded too heavily, it may dominate the stand. If another species is included at too low a rate, there may not be enough plants for that species to deliver the function it was included for in the first place.

That is why mixture design should answer two separate questions:

Simply adding species while continually reducing the amount allocated to each component can eventually produce a mixture where some species are present primarily on the seed tag rather than meaningfully represented in the field.

Planting Date Can Change Which Species Win

A cover crop mix planted early in the fall may perform very differently from the same mixture planted several weeks later.

Different species respond differently to shortening days, cooler temperatures and reduced growing degree accumulation. As planting is delayed, some components may continue to establish aggressively while others contribute much less biomass.

This creates an important practical rule:

There Is No Universally Perfect Cover Crop Mix

Field conditions matter.

Rainfall, irrigation, temperature, soil texture, drainage, fertility, planting method, planting date and the length of the available growing season can all influence establishment and competition within a cover crop.

That becomes especially important in Western agriculture.

A mix intended for an irrigated orchard may face very different constraints from a mixture expected to establish primarily from seasonal rainfall. A vineyard with a narrow planted middle creates different requirements than a broad-acre field crop. Heavy ground with poor infiltration presents different challenges than a well-drained soil.

A mixture can look excellent on paper and still be poorly matched to the field where it is actually planted.

Start With the Goal, Then Select the Species

Think in Functional Groups, Not Just Species Counts

Instead of starting with whether a cover crop should contain 5, 10 or 20 species, determine which functional groups are required to accomplish the field objectives.

Grasses & Cereals

Often used for biomass production, ground cover, fibrous rooting, nutrient scavenging and weed competition.

Legumes

Used primarily where biological nitrogen fixation and high-quality forage are desired.

Brassicas

Can contribute rapid growth, nutrient scavenging and rooting characteristics that differ from many grasses and legumes.

Broadleaf & Flowering Species

Can add different plant architecture, rooting patterns, flowering periods and potential habitat resources.

The proper balance between these groups depends on the field, the crop system and the intended management.

There Is Also an Economic Question

Every additional species has a cost.

As more species are added while maintaining a practical total seeding rate, the amount allocated to each individual species generally becomes smaller.

Eventually, the appropriate question becomes:

If the answer is no, including the species primarily to increase the number printed on the bag may add cost without delivering proportional value.

That does not make complex mixtures a poor choice. It means complexity should have a purpose.

A 15-species mixture designed around complementary functions, appropriate rates and realistic field conditions may be an excellent program.

A 15-species mix developed primarily because “15 sounds better than 8” is a fundamentally different design process.

10 Questions to Ask Before Selecting a Cover Crop Mix

1. What are the primary objectives of the planting?
2. Which functional groups are required to accomplish those objectives?
3. Which species are adapted to the field and growing region?
4. Will those species establish during the expected planting window?
5. Are the component seeding rates high enough to make a meaningful contribution?
6. Could aggressive species overwhelm slower components?
7. How much rainfall or irrigation will be available?
8. How will the cover crop eventually be terminated?
9. Is grazing or forage production part of the management system?
10. Does every species contribute enough value to justify its place in the mix?

The Bottom Line: Design for Function

Cover crop diversity can provide significant value.

But diversity should be intentional.

The objective is not to build the longest ingredient list possible. It is to combine species that complement one another and have a realistic opportunity to perform under the conditions where they will actually be planted.

Sometimes that may mean three or four carefully selected species. In another system, ten or more may be justified.

There is no universal correct number.

Instead of starting with:

“How many species can we put in this mix?”

Start with:

“What do we need this mix to do?”

That is where better cover crop design begins.