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The Perennial Challenge of Reduced-Tillage Organic Production

Posted: September 28, 2009

A team of Penn State researchers are investigating how different organic feed and forage production systems can be managed to deal with perennial weed populations without compromising soil quality.
Canada thistle grows in a reduced tillage plot (on the left) at the Rock Springs, PA site. (Photo by Rich Smith)

Canada thistle grows in a reduced tillage plot (on the left) at the Rock Springs, PA site. (Photo by Rich Smith)

From earlier research, we learned that reducing tillage for the duration of the three-year organic transition period can lead to large populations of perennial weeds. That result led our team to ask whether, following certification, organic production systems can be managed to deal with perennial weed and soil quality issues that may develop during the transition period. To address this question, our team established a new study in 2007 at the site of the previous organic transition study at the Russell E. Larson Agricultural Research Center near Rock Springs, PA. The experiment includes four organic management systems, each differing in crop rotation and tillage intensity.

Figure 1
Figure 1. Abundance of perennial weeds in four organic cropping systems.The abundance of perennial weeds, represented here by box plots showing the spread in the data, in our four experimental organic cropping systems in July 2009. During the previous transition experiment, Systems 2 and 4 were managed with full tillage and therefore had lower perennial weed densities (at the end of the transition period in 2007) compared to systems 1 and 3. However, since organic certification, Systems 2 and 4 have been managed with a reduced-tillage strategy to build soil organic matter (while systems 1 and 3 have been aggressively cultivated), which appears to be leading to a growing perennial weed problem.

While the study is ongoing, some interesting data are beginning to emerge. Perennial weeds (mainly Canada thistle and field bindweed) are able to respond rather quickly to changes in tillage regime. In July 2009, we assessed the abundance of weeds growing in each of the four experimental organic cropping systems that had been established in 2007. We found that the abundance of perennial weeds in the two experimental systems that incorporated tillage was relatively low, while the abundance of perennial weeds in systems that reduced tillage was quite high (see Figure 1). This pattern is interesting because those systems in which tillage is currently being used were reduced-tillage systems during the previous organic transition study (from 2004-2006). During that study, those systems were disturbed less frequently and intensively than the current treatments, and they subsequently developed large populations of perennial weeds by the end of the study (2006). What this suggests is that yes, we can reduce perennial weed populations by altering tillage and cover cropping, and this reduction can occur relatively rapidly (1-2 yrs). Interestingly, the converse may also be true. The systems which had low perennial weed populations at the end of the transition experiment, and in which we subsequently reduced tillage following organic certification, have now developed high perennial weed populations.

It is interesting to note that the abundance of perennial weeds in the moderately disturbed treatment is more variable from plot to plot than in the least disturbed treatment (indicated in the graph by the large spread of data encompassed by the box plot). As of now, the abundance of perennial weeds in the four systems mirrors very closely the degree of tillage within each of the four experimental cropping systems. Whether these results hold for the remainder of the study, or not, remains to be seen.

By Rich Smith, Penn State Weed Ecology Lab