Abstract
Cool-season cover crops have been shown to reduce soil erosion and nutrient discharge from maize (Zea maysL.) and soybean [Glycine max(L.) Merr.] production systems. However, their effects on long-term weed dynamics are not well-understood. We utilized five long-term research trials in Iowa to quantify germinable weed seedbank densities and compositions after 10+ years of cover cropping treatments. All five trials consisted of zero-tillage maize-soybean rotations managed with and without the inclusion of a yearly winter rye (Secale cerealL.) cover crop. Seedbank sampling was conducted in the early spring before crop planting at all locations, with three of the five trials having grown a soybean crop the preceding year, and two a maize crop. Two of the trials (both previously soybean) showed significant and biologically relevant decreases (4,070 and 927 seeds m−2, respectively) in seedbank densities in cover crop treatments compared to controls. In another two trials, one previously maize and one previously soybean, no difference was detected in seedbank densities. In the fifth trial (previously maize), there was a significant, but biologically unimportant increase of 349 seeds m−2. All five trials' weed communities were dominated by common waterhemp [Amaranthus tuberculatus(Moq.)], and changes in seedbank composition from cover-cropping were driven by changes in this species. Although previous studies have shown that increases in cover crop biomass are strongly correlated with weed suppression, in our study we did not find a relationship between seedbank changes and the mean amount of cover crop biomass produced over a 10-years period (experiment means ranging from 0.5 to 2.0 Mg ha−1yr−1), the stability of the cover crop biomass production, nor the amount produced going into the previous crop's growing season. We conclude that long-term use of a winter rye cover crop in a maize-soybean system has the potential to meaningfully reduce the size of weed seedbanks compared to winter fallows. However, identifying the mechanisms by which this occurs requires further research into processes such as seed predation and seed decay in cover cropped systems.
Highlights
One-third of the global maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] production comes from the United States (US; Food and Agriculture Organization of the United Nations, 2020)
All trials consisted of two treatments that had been in place for at least 10 years: (1) a maize-soybean rotation with a winter rye cover crop planted in the fall following cash crop harvest and terminated in the spring, and (2) the same rotation without a cover crop
In the three trials with a soybean crop the preceding year, the seedbank density was lower in the cover crop treatment compared to the no-cover treatment by 91 (Central-grain), 927 (Central-silage), and 4,070 (West) seeds m−2, respectively, corresponding to a 17, 61, and 51% reduction
Summary
One-third of the global maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] production comes from the United States (US; Food and Agriculture Organization of the United Nations, 2020). Incorporation of an overwintering rye (Secale cereal L.) cover crop into these systems can significantly reduce soil erosion and nutrient leaching (Strock et al, 2004; Kaspar and Singer, 2011; Kaspar et al, 2012), and may offer additional long-term benefits to the soil (Moore et al, 2014; Basche et al, 2016a,b). Surveys indicate farmers consider cover crops to be a valuable component of an integrated approach to weed management (Arbuckle and Lasley, 2013). Ecologically-based approaches to weed management such as cover crops are becoming more critical as weeds develop herbicide resistance to multiple modes of action (Patzoldt et al, 2005; Price et al, 2011; Bunchek et al, 2020; MacLaren et al, 2020). The effects of over-wintering cover crops on weed dynamics in these systems is not well-understood
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