Know/Define Your Goal(s) When Using Cover Crops
A recent article titled “Cover Crop Biomass Production is More Important than Diversity for Weed Suppression” by MacLaren et al (Crop Sci, Vol. 59, p. 1-16, 2019) provides some interesting points that should be remembered when inserting cover crops into any cropping system. A summary of the conduct of and results from the research reported in this article follow.
• The authors define cover crops as “crops planted for a purpose other than harvest”.
• A trial was designed to explicitly test the role of cover crop diversity (species composition) on weed suppression.
• Eight cover crop mixes that varied in species diversity, functional diversity, and composition, plus a weed fallow or “bare ground” control, were included in the study. Mixes contained either one, four, or eight species. Three mixes contained only cereals, three contained only legumes, and two contained a mix of cereals, legumes, and brassicas. The species differed in their growth forms and root architectures. The various mixes were used to assess the effect of each mix on overall weed biomass. See Tables 1 and 2 in the publication for species composition in each treatment.
• The diversity in growth form within the mixes was expected to create a more complete canopy to restrict light availability to weeds.
• Aboveground biomass of individual cover crops and weeds within each treatment mix was measured at 70 and 120 days after emergence of the cover crops.
• Lower final weed biomass was measured in the cereal and diverse cover crop mixes than in the legume mixes. Weed suppression was similar among all cereal and diverse mixes. None of the legume mixes had fewer weeds than the bare ground control.
• Both total biomass production and the proportion of cereal biomass were associated with reduced weed biomass; i.e., resulted in reduced weed biomass.
• Increasing proportion of brassica biomass in samples had a similar effect to increasing proportion of cereal biomass. Increasing proportion of legume biomass in samples had the opposite effect and was linked to increased weed biomass.
• Higher total cover crop biomass was associated with increased canopy cover by them throughout the season.
• Weed biomass increased most where soil nitrogen (N) was highest and where cover crop canopy was lowest, indicating that N and light availability allowed weeds to be more productive.
• Weed cover in the middle of the cash crop season was higher where weed biomass at the end of the cover crop year was higher, and where cover crop biomass and cereal proportion of the cover crop mix were lower.
• Regression analyses indicated that cover crop biomass was the key to resource uptake and weed suppression, and that early-season N and later-season light availability had the greatest influence on weed biomass.
• These results indicate that cover crops can effectively suppress weed biomass by reducing the availability of resources for weeds, and that it is important to consider the competitiveness of individual cover crop species when designing a cover crop plan to meet a specific goal.
• These results also indicate that diverse cover crop mixes may perform multiple functions with their inclusion, but also may contribute to weed problems if they are composed of non- or poorly-competitive species. They also indicate that cover crop mixes composed mostly or entirely of cereals produced more biomass, captured more resources available for plant growth early in the season, and suppressed more weed biomass.
• Utilizing cover crop mixes that contain only highly competitive species may result in more effective weed suppression than a monoculture, while simultaneously increasing opportunities for these cover crop mixes to perform multiple functions through their diversity (see final note 1) below for termination challenges with cover crop mixes).
TAKE HOME MESSAGE
For any cover crop system to be effective, the makeup of the cover crop–i.e., the species component(s) of the cover crop–must address the issue that is most pressing to a producer–e.g., weed suppression, increased soil organic matter/carbon, erosion prevention, nitrogen scavenging. To do this, the cover crop species or species mix must contain those species that have the capability to address the priority issue(s) that have been established by the producer. For instance, cereal species vs. legume species are more likely to suppress weeds and scavenge nitrogen, and legume vs. cereal species will likely result in more N added to the soil following their destruction.
In the case of weed suppression, which is of paramount importance in the Midsouth where herbicide-resistant pigweed is a pervasive problem, the goal from using cover crops likely will be to aid in or enhance weed control preceding and during the season of soybean production that follows the cover crop. The results reported in the above study certainly indicate that a monoculture of a cereal species is a likely cover crop candidate to accomplish this.
The results reported in this article should be used to complement results from other studies. However, the results presented here do indicate that producers should certainly define their goals from using cover crops so that single or multiple species selected for their operations will in fact accomplish those goals.
Two final notes. 1) Cover crop mixes may present an unwanted challenge for their termination. That is, if the species in the mix are not all at a proper stage for termination at the same time, then some will be terminated too early and some too late. Thus, some of the species may have produced viable seed before termination or recover to set seed afterward, while some species in the mix may not have reached peak biomass production if terminated too early. Thus, it is important to ensure that a multi-species cover crop mix contains species that will be as close as possible to their optimum termination stage at the same time to ensure that they will have each performed their intended function prior to termination. 2) No matter what cover crop mix a producer chooses for any reason or goal, they all will add an additional cost to the producer’s production system that may or may not be recouped in the short or even long term. So not only should a cover crop mix be chosen with a goal in mind, but the cost of achieving that goal by using cover crops must be taken into consideration to ensure that either the intended tangible or intangible result will be achieved to offset the actual cost of the cover crop system.
Composed by Larry G. Heatherly, Mar. 2019, larryheatherly@bellsouth.net