Jarrod Miller, Amy Shober, & Mark VanGessel
The benefits of cover crops to the following corn crop can include additional nitrogen (N) or weed suppression, but maximizing these benefits requires later termination to build greater biomass. These N and weed control characteristics are especially appealing this season as input costs are relatively high while supplies are relatively low. However, growers should take the time to estimate the additional costs of allowing a more robust cover crop to accumulate this spring, as surface residues reduce proper seed placement as well as limit seed to soil contact. This article will discuss the management of cover crops for both maximizing N benefits as well as weed suppression in the following corn crop.
Maximizing Cover Crop Biomass
Estimating the amount of N that could be available requires knowledge about 1) total cover crop biomass and 2) cover crop C:N ratio, both of which are affected by termination timing. The longer a cover crop is allowed to grow, the greater the amount of cover crop biomass will accumulate. For an NRCS-sponsored research project, we grew rye and rye mixed with clover and vetch for three seasons (Figure 1) in small plots at the Carvel Research Center in Georgetown, DE with two termination timings: 1) two weeks prior (early) to corn planting and 2) at corn planting (late). The later burndown at planting is also known as “planting green”. The later termination of rye (yellow), rye/clover (blue), rye/vetch (green) allowed for more cover crop growth and associated biomass (Figure 1). In some cases, cover crop biomass doubled by waiting two weeks, as evidenced with the rye/clover mix in 2019. The 2019 rye/clover biomass was 3 Mg/ha (3.3 ton/ac) at early termination compared to nearly 7 Mg/ha (7.7 ton/ac) when terminated late. In 2021, the rye biomass nearly tripled in total weight with late termination to nearly 6 Mg/ha (6.6 ton/ac) when compared to the 2 Mg/ha (2.2 ton/ac) at early termination. While we consistently saw biomass bumps from later termination in any given year, we also saw a large difference in total biomass across all study years. For example, cover crops planted in 2020 benefited from a much warmer winter which allowed for much greater biomass production.
Maximizing Cover Crop Nitrogen
As total biomass, or weight, of cover crops increases, more N also accumulates in the biomass, regardless of the species (Figure 2). Even the weeds that germinated in the no-cover plots accumulated more total N with greater biomass. Mixes containing rye and legumes (i.e., clover and vetch) had a higher concentration of total N than rye, as these legumes can fix atmospheric nitrogen. As such, the mixes contained significantly more total N in the biomass, especially with later termination (Figure 2). For example, the total N content (2021 season) of the late terminated rye/clover mix was almost 200 kg/ha (178 lbs/acre), while the total N content of the late terminated rye/vetch mix was roughly 225 kg/ha (200 lb/ac); both these late terminated cover crop had more the double the total N content of their early terminated counterparts. However, we still need to consider the potential for this cover crop N to be available to the corn crop that will be planted behind it.Later Termination and C:N Ratios
While total N in the cover crop is important, we must consider that not all that total N will be available to the subsequent corn crop during the growing season due to the carbon (C) to N ratio (C:N ratio) of the cover crop biomass (Figure 3). As cover crops mature (or any crop for that matter), they accumulate more C compared to N and the C:N ratio increases. In general, residues with a C:N ratio less than 20 (20 parts C to 1 part N; black line Figure 3) will release N to the soil (mineralization), while biomass with a C:N ratio above 30 (red line, Figure 3) is more likely to tie N up in the soil (immobilization). The biomass of cereal crops, like wheat and rye, will typically have a C:N ratio that is higher than 30 once the crop matures past the flag leaf or pre-boot state (Feekes 8) because there is more stem than leaf biomass present past this stage. While rye is preferred as a cover crop for its ability to scavenge N from the soil over the winter, allowing for later termination of a rye cover crop can ultimately result in residue with N that will not quickly mineralize to supply N to the subsequent cash crop. This was the case for the late-terminated rye cover in 2020, where waiting two additional weeks to terminate the crop resulted in the C:N ratio rising above 30, so the biomass was less likely to release any N to the soil early in the corn season (Figure 3). However, we did not see the same trend in biomass C:N ratio for late-terminated rye in 2021, because plots were terminated a bit earlier in the season before rye had reached the flagleaf stage. The good news was that planting a legume with rye resulted in an increase in total biomass N while biomass C:N ratios remained below 20.
Choosing Termination Timing
Choosing termination timing this spring is more difficult due the higher costs of both fertilizer and herbicides adding to management decisions. Our typical factors of field conditions, managing cover crop residues when planting, and slug pressure remain in play as well. Ultimately the decision is up to each operator, but keep in mind the results of this local cover crop N study if you may be looking to lower fertilizer costs.
Weed suppression is most effective when cover crops have >5 Mg/ha (5.5 tons/ac, Finney et al., 2016 ), which was achieved in all cover crop plots in 2020 and late terminated rye/legume mixtures in 2019 and 2021 (Figure 1). Similar to total available N, more biomass is better in this case, and terminating cover crops later into April or early May is the best way to achieve this.
Another consideration in weed suppression could be the C:N ratio. While a high cover crop C:N is not ideal for providing N to the following corn crop, it will slow the breakdown of the cover crop residues. The longer those residues remain in the field, the more mulch they can provide, shading out potential weed growth. In this case, rye is the most likely candidate to remain in the field, having the highest C:N compared to most other treatments (Figure 3).
All weed management decisions will overlap with burndown considerations. While cover crop biomass is lower earlier in the spring, you may be considering saving money and lowering the rate. However, weather and environmental conditions may not be ideal for plant susceptibility or herbicide efficacy. The full rate is also needed when planting green, particularly if cereals are at the boot stage, to make sure no plant survives. Another potential situation would be poor cover crop performance, so that even if you terminate later, reduced biomass allows some weeds to germinate in the spring. In this case you would still need the full rate to kill both the cover crop and any germinating weeds.
Final Thoughts and Effects on Yields
We only observed reduced corn stands during one season with late termination but did observe slightly lower corn yields in two out of three years with late termination. While the full rate of herbicide should be used no matter when you terminate the cover crop, later termination may reduce weed germination and save you on applications during the corn growing season. Later termination of any stand mixed with a legume can certainly provide an additional amount of N to the corn crop, most likely around the maximum growth period (corn V6), and adjustments could be made on the expected N contributions.
Finney, D.M., White, C.M. and Kaye, J.P. (2016), Biomass Production and Carbon/Nitrogen Ratio Influence Ecosystem Services from Cover Crop Mixtures. Agronomy Journal, 108: 39-52. https://doi.org/10.2134/agronj15.0182
This article was written with a project supported through the USDA-NRCS.