Cover Crop Benefits Depend on Biomass: How Much Do You Need?

By Andrew McGuire

Cover crop biomass matters. Biomass is the measure of additional energy that cover crops put into the biological field-soil system. It tells us how protected the soil surface is from wind and rain. It shows how much sunlight, water, and nutrients the crop took away from weeds. And it indicates amount of time with living roots. Because of these relationships, most of the benefits of cover crops are related to biomass production (Wagg et al. 2021).

Erosion control, soil benefits, or weed suppression, what do you want?

If biomass is so important, how much do you need? That depends on which benefit you want. Research has estimated “minimum biomass thresholds” for various cover crop purposes. These are not tipping points where below a certain level you get no benefits and above you get full benefits, but rather ballpark numbers where a judgement has been made about a desired level of benefit. It’s a rough cost-benefit analysis based on best estimates as to how much biomass is needed to see the benefits that you might expect. They are targets, not guarantees of success.

Biomass for erosion control

Erosion is a primary reason for planting cover crops. Cover crops, both shoots and roots (see Figure 2), provide a physical barrier to wind, rain, and overland water flow, thereby reducing erosion. For erosion, the good news is any biomass is better than none, and more is better up to a point. Rather than biomass, percent soil cover is more often used for erosion control evaluations, with 60% cover being the minimum standard set by NRCS. This corresponds to about 1,420 lb/acre of biomass (1.5 Mg/ha) for cool season small grain crops. A soil cover of 50-70% (890-1,780 lb/acre, 1-2 Mg/ha) will reduce erosion by about 80% (Prabhakara et al., 2015).

A few important details:

• For short growing windows with low biomass potential, consider increasing your seeding rate. This will increase percent of soil covered which is an important factor for erosion control.
• With their taller growth, higher biomass, and more durable residues, grass crops provide better erosion control than legumes and other broadleaves.
• Fields on steeper slopes or in high‐rainfall areas may need higher biomass to protect soil from erosion.
• Extreme weather events, whether from wind or rain, will require more biomass to protect the soil.

Biomass for soil benefits

Erosion control is priority, because you can’t improve a soil that is eroding. When that is covered, then moving on to improving soils requires more biomass.

Cover crop biomass improves soils through rooting, providing microbes with root exudates, scavenging nutrients, and providing biomass for organisms and for soil organic matter. At the lowest level, a biomass of >890 lb/acre (~1 Mg/ha) provided effective nitrate retention (Hively et al., 2009) The same level was needed to obtain positive effects of cover crop nitrogen in the following cash crop (Blanco-Canqui et al. 2025; De Notaris et al., 2018; De Notaris et al., 2025). Below this, cover crops reduced yields of following cash crops, probably due to nitrogen immobilization, or provided no nitrogen benefits.

Increasing soil organic matter (SOM) is often a goal of using cover crops in regenerative agriculture, and many improvements in other soil properties follow from increased SOM. This, however, requires even more biomass. A review of cover crop research in the US 21 found that >1,780 lb/acre (2 Mg/ha) is needed for consistent increases in soil organic matter (Blanco-Canqui, 2022). And one year won’t do it. This study found that 5+ years of this higher level of cover crop biomass was needed to consistently increase SOM. Why? Because 90% of biomass is used by soil organisms, leaving only 10% to go to SOM. This is also why practices such as no-till and perennial crop rotation are often needed in addition to cover cropping to increase soil organic matter.

Biomass for weed control

Cover crops suppress weeds by taking resources away from them: sunlight, water, and nutrients. Some cover crops also produce chemicals that suppress weeds (allelopathy). As with erosion control and soil health, more cover crop provides better weed control (Osipitan et al., 2019), but for practical levels of suppression, there are minimum thresholds.

Meta-analyses have found biomass threshold levels for various regions:

• Midwest US: >4,460 lb/acre (5 Mg/ha) was needed for a 75% reduction in weed biomass by cover crops (Nichols et al., 2020).
• Southeast US: ~5,890 lb/acre (6.6 Mg/ha) was needed for 50% weed suppression (Weisberger et al., 2023).

In Pennsylvania, Finney et al. (2016) found a biomass threshold of ~4,460 lb/acre (5 Mg/ha) for near 100% control of weeds. At the top end of biomass needs are organic systems. In both the NE and SE US, >7,140 lb/acre (8 Mg/ha) was needed for full-season control of weeds in both the living cover crop and the following cash crop (Mirsky et al., 2012, 2013).

Several studies found cover crop mixtures performed the same as the best monocultures for suppressing weeds (Bybee-Finley et al., 2017; Finney et al., 2016; MacLaren et al. 2019; Smith et al., 2020). Whether mixture or monoculture, cover crop biomass was the best predictor of weed suppression (MacLaren et al., 2019)

Gerhards et al. (2024) found that most cover crop species required 2,680 lb/acre (3 Mg/ha) or more to provide effective weed control, but that the needed biomass can be less if it can also control weeds with allelopathy. There are other important factors affecting weed control with cover crops as biomass is not always related to degree of control (Rouge et al., 2022; Leskovšek et al., 2025)

Table 1. Minimum Biomass Thresholds for Cover Crops by Purpose.

Managing cover crops for biomass production

There are two approaches to managing cover crops for biomass production:

1. Determine your available growing window from your cash crop season and adjust management for the biomass that window can produce. The cover crop’s growing window will determine its biomass potential, which will determine what benefits you can expect.
2. Choose your cover crop goal erosion control, soil improvement, or weed suppression and adjust your growing window to obtain the needed biomass.

Either way, you will end up with one or more goals for your cover crop, a growing window, and an estimated biomass target.

Because biomass thresholds and benefits are related to the physical biomass itself, they should not vary much by region, climate, soil type, or cover crop species. 2,000 lb/acre of biomass is the same whether in the arid West or humid East. What will vary is how your biomass target is achieved, which depends on many factors:

• Growing window temperatures and precipitation
• Species selection
• Seeding rate
• Soil nutrient status

To manage it, measure it

Both cover crop approaches (growing window or goal) require that you estimate the biomass production for a given growing window. Getting exact numbers for this is difficult, but there are a few sources that can help.

Except in the southern US or mild coastal regions, many growing windows that only include late fall and early spring will produce only enough biomass for some erosion control but no other goals. Planting in late summer or early fall, or termination in late spring, will produce higher biomass levels for both erosion and soil health goals. The highest biomass levels needed for weed suppression will require both early planting in late summer and late termination in late spring.

Depending on where you farm, there may be some cover crop calculators you can use for planning: Nebraska, Eastern US, and the 25 Eastern and Southern states covered by the Cover Crop Species Selector. Finally, check with your Extension office. They may have some local data. Measuring cover crop biomass in the field is also an option. There’s the common cut and weigh methods (video here), but several research projects have worked to make it easier to estimate cover crop biomass. Eric Brennan of the USDA-ARS has developed “a simple, fast, farmer-friendly & robust method” based on the length of the main stem of a cereal cover crop. Here is his video showing the method. An Iowa State University Extension video shows how to use a visual method to estimate cereal rye biomass. And finally, there is a high-tech method using 3D imaging and geostatistics.

Manage for the expected biomass

Level 1: for a low biomass cover crop with a single goal of providing as much erosion control as possible

• Plant as early as possible, terminate as late as possible.
• Choose a cover crop species that is both well suited to your short growing window and has inexpensive seed.
• Increase seeding rate as much as economically feasible.

Level 2: For both erosion control and soil benefits

• All of the above plus,
• Consider using a two-species cover crop mixture of a legume with a non-legume.
• Consider fertilizing the cover crop if nutrient deficiencies could limit biomass production.

Level 3: For control erosion, soil benefits, and weed suppression

• All of the above plus,
• Use a monoculture grass or a legume-grass cover crop mix for best suppression of weeds by both the living cover crop and its residues after termination.

As with much of farming, this is an optimization process of the growing window length, temperature, sunlight, soil nutrient levels, water supply, and planting species, with several important tradeoffs:

• Cash crop season vs. cover crop growing window length vs. biomass/benefits.
• Biomass/benefits vs. water use by cover crop.

Cover crop biomass provides the benefits

It’s no surprise that biomass is key to cover crop benefits. In ecology, biomass productivity is often used as a measure of ecosystem health or function. Biomass is the physical protection of the soil, the energy that drives all the biology, and the competitive suppressor of weeds. If you want benefits from cover crops, focus on biomass production.