Refining Cover Crops' Seeding Rates
In a Cover Crops White Paper on this website, information about recommended seeding rates for selected cover crop (CC) species that can be used in the Midsouth is provided (MP568, Univ. of Ark.). All recommended seeding rates are presented as lb. of seed per acre. For cereal rye, the recommended rates range from 35 to 60 lb./acre, depending on method of seeding.
A recent article titled “Seed size variability has implications for achieving cover cropping goals” by Lounsbury et al. (Agric. Environ. Lett. 2022;7:e20080, https://doi.org/10.1002/ael2.20080) provides information that indicates that a seeding rate based on mass-based units (e.g. lb./acre) may not be the best option for selecting a seeding rate for a CC species such as cereal rye. Pertinent points from that article follow.
• Cover crops are used and managed to provide ecosystem services such as prevention of off-season erosion, recycling of nutrients, suppression of weeds, and sequestration of carbon in the soil.
• Guidance for the appropriate seeding rate for a CC species remains imprecise, especially since most recommendations are based on mass-based units such as lb. of seed/acre. This is in contrast to seeding rate recommendations for commodity crops such as corn and soybean that are usually based on achieving a target plant density.
• In the research described in the above-linked article, seed in 27 lots of commercially available winter/cereal rye were counted and weighed to determine seeds/lb. in each of the lots. Germination rate was used to calculate the number of live seed that were sown on a given area using a mass-based seeding rate.
• Results from the research indicated that rye seed counts were highly variable among lots, ranging from 13,000 to nearly 23,000 seeds/lb., which is a nearly two-fold difference.
• Because seed size of a CC species can vary significantly among seed lots, a mass-based seeding rate can lead to a wide range of in-field plant densities of the chosen CC. Thus, using a mass-based unit to describe CC seeding rate is likely to conceal information about seed size differences among seed lots of a selected CC species. This may compromise the intended outcome that is likely based on achieving a certain plant density of a particular CC species.
• The authors contend that metrics such as live seed sown per unit area to achieve a targeted plant density of the CC would improve seeding rate recommendations to allow the desired outcome from using the CC.
• The authors concluded that their results show that a first step toward improving seeding rate recommendation for CC’s is to acknowledge that CC seeding rates based on a mass-based unit may be confounded by highly variable plant densities that will result from the variability in number of CC seed that are planted. This will likely impact the intended result from using any of the myriad CC species listed in the above MP568 publication from the Univ. of Ark.
• Finally, the authors concluded that to fully realize the intended ecosystem service(s) provided by a chosen CC species, it is important to refine recommended CC seeding rates so that they include density-based metrics such as live seeds per unit of sown area to ensure that the desired in-field plant density is achieved.
The results from this research with cereal rye CC (which arguably is the best grass CC species to use to achieve the greatest weed suppression and produce the most residue) show that it is likely just as important to know the number of live seed of a CC species to be planted to an area as it is to know the same information for a planted commodity crop. Further research is needed to understand the relationships between seed size and cover crop performance as related to the initially defined goal(s) of using a particular CC species–i.e. will the chosen seeding rate for a selected CC species provide the number of plants necessary to accomplish the intended goal? This issue should be considered when deciding on whether or not to use a particular CC species to achieve any of the ecosystem services listed above.
Takehome Message. To achieve the optimum result from using any CC species, the seeding rate used should take into account the number of viable seed per mass-unit of the CC seed lot so that the final plant density is sufficient to accomplish the intended goal from using that CC species.
Composed by Larry G. Heatherly, Oct. 2022, larryh91746@gmail.com